SYLLABUS
1. OOPS PARADIGM
Programming language, Object-Oriented Programming, Object-Oriented Languages,
Object-based programming languages, Object-oriented programming languages.
2. INTRODUCTION TO C++
Basic concept of oops-Objects, Classes, Encapsulation, Data Abstraction, Inheritance,
Polymorphism, Dynamic Binding, Message Passing, Brief History of C++
3. Data Types & Variables
Structure of a C++ program., Comments, Variables,Identifiers,Data types. Declaration of
variables, Initialization of variables, Scope of variables, Constants
4. Operator and control structures
Types of Operators. Priority of Operators. Control
5. Array and pointer
Arrays. Initializing arrays, Strings, Pointers. Pointers and arrays ,Dynamic Memory.
6. structures and union
Structures.,User defined data types.
7. Functions
Functions . Default values in arguments
8. classes and objects
Introduction to class, Class Definition, Classes and Objects, Access specifiers – Private,
Public and Protected. Member functions of the class.
9. Constructor and destructor
Constructors, Overloading Constructors, Destructor
10. Function Overloading
Function overloading, Precautions to be taken while overloading functions. Static Class
Members, Static Member Functions, Friend Functions
11. Operator Overloading
Introduction to Operator Overloading. ,Operator Overloading Fundamentals.
Implementing the operator functions.
12. Inheritance
Reusability.,Inheritance concept-singleinheritance.Using the derived class,Constructor
and destructor in derived class, Object initialization and conversion., Nested classes
(Container classes).\,Multilevel inheritance., Multiple inheritance., Hybrid Inheritance.
Virtual base class.
13. Abstract and virtual function
Abstract class, Virtual function. Pure virtual function
14. Templates and exception handling
Templates. Exception handling, Advanced
15. File Input Output
Input/Output with files. Open a file, closing a file
REFERENCES:
1
1. Herbert Schildt, “C++ the Complete Reference “, III edition, TMH 1999
2. Balagurusamy, Entrepreneurial “Object Oriented programming with C++”, TMH
3. Barkakatin “objects oriented programming in C++” PHI 1995
2
SYALLABUS
Introduction to OOPS,Data types and variables, Operators and control structures,Array and pointer,Structures and union,Functions,Classes and
Objects
Constructor and Destructor Functions,Function Overloading,Operator
Overloading,Inheritance,Abstract and virtual function,Templates and exception
handling,File handling
3
TABLE OF CONTENTS
Unit1 Introduction to OOPS
Programming language
Object-oriented programming paradigm
Object-Oriented Programming
Object-Oriented Languages
Object-based programming languages
Object-oriented programming languages.
Basic concept of oops
Objects
Classes
Encapsulation
Data Abstraction
Inheritance
Polymorphism
Dynamic Binding
Message Passing
Fits of OOP
Application of OOPS
Brief History of C++
Unit 2-Data Types & Variables
2.1
Structure of a C++ program.
2.2
Comments
2.3
Variables
2.4
Identifiers
2.5
Data types.
2.6
Declaration of variables
2.7
Initialization of variables
2.8
Scope of variables
2.9
Constants
Unit3-Operator and control structures
3.1
Types of Operators.
3.2
Priority of Operators.
3.3
Communication through console.
3.3.1
Output
3.3.2
input
3.4
Control Structures.
3.4.1
Conditional structure
3.4.2
Repetitive structures or loops
3.4.3
Bifurcation of control and jumps
3.4.4
The selective Structure: switch
Unit4-Array and pointer
4.1
Arrays.
4
4.1.1
Initializing arrays
4.1.2
Access to the values of an Array
4.1.3
Multidimensional Arrays
4.1.4
Arrays as parameters
4.2
Strings
4.2.1
1Initialization of strings
4.2.2
Assigning values to strings
4.2.3
Converting strings to other types
4.2.4
Functions to manipulate strings
4.3
Pointers.
4.4
Pointers and arrays
4.5
Dynamic Memory.
Unit 5-structures and union
5.1
Structures.
5.1.1
Poniters to structures
5.1.2
Nesting structures
5.2
User defined data types.
5.2.1
Typedef
5.2.2
Union
5.2.3
Num
Unit-6-Functions
6.1
Functions .
6.2
Default values in arguments
6.3
Void Functions
6.4
Call by value and reference
6.5
Passing Reference to Functions.
6.6
Returning References from Functions
6.7
Inline function
6.8
Recursive function
6.9
Prototyping function
Unit7-classes and objects
7.1
Introduction to class.
7.2
Class Definition
7.3
Classes and Objects
7.4
Access specifiers – Private, Public and Protected.
7.5
Member functions of the class.
7.6
Passing and returning objects.
7.7
Pointers to objects.
7.8
Array of objects.
7.9
The special ‘this’ pointer
7.10 self test
Unit8 : Constructor and destructor
8.1
Constructors
8.1.1
Syntax rules for writing constructor functions
8.1.2
Different ways of calling contructor
8.2
Overloading Constructors
8.3
Destructor
8.4
Self test
5
Unit9- Function Overloading
9.1
Function overloading
9.2
Precautions to be taken while overloading functions.
9.3
Static Class Members
9.4
Static Member Functions
9.5
Friend Functions
9.6
Friend for Overloading Operators
9.7
Granting friendship to another class
9.8
Granting friendship to a member function of another class
Unit10-. Operator Overloading
10.1 Introduction to Operator Overloading.
10.2 Operator Overloading Fundamentals.
10.3 Implementing the operator functions.
10.4 Rules for overloading the operators.
10.5 Pointer oddities (assignment) and Operator Overloading.
10.6 Overloading the Extraction and Insertion Operators
10.7 Conversion functions.
10.7.1 Conversion from basic to user-defined variable.
10.7.2 Conversion from User-Defined to Basic data type
10.7.3 Conversion Between Objects of Different Classes
10.7.4 Conversion function in the Destination Class
10.8 Table for Type Conversions
10.9 Self Test
Unit 11. Inheritance
11.1
Reusability.
11.2
Inheritance concept- single inheritance.
11.2.1
Private derivation
11.2.2
Public derivation
11.2.3
The Protected Access
11.2.4
Summary of derivation
11.2.5
Table of derivation and access specifiers
11.3
Using the derived class
11.4
Constructor and destructor in derived class.
11.5
Object initialization and conversion.
11.6
Nested classes (Container classes).
11.7
Multilevel inheritance.
11.8
Multiple inheritance.
11.9
Hybrid Inheritance.
11.10
Virtual base class.
Unit 12- Abstract and virtual function
12.1
Abstract class.
12.2
Virtual function.
12.3
Pure virtual function
12.4
self test
Unit-13 Templates and exception handling
13.1
Templates.
13.1.1 Function template
13.1.2 Class templates
13.1.3 Template specialization
6
13.1.4 Parameter values for templates
13.1.5 Templates and multiple -file project
13.2
Exception handling
13.2.1 Exception not caught
13.2.2 Standard exception
13.3
Advanced class type-casting.
13.3.1 Reinterpret cast
13.3.2 Static cast
13.3.3 Dynamic cast
13.3.4 Tonst_cast
13.3.5 Typeid
13.4
Preprocessor directives.
Unit 14- File Input Output
14.1
Input/Output with files.
14.2
Open a file
14.3
Closing a file
14.4
Methods of Input and Output Classes
14.5
Text mode files
14.6
State flags
14.7
Binary files
14.8
Buffers and Synchronization
14.9
I/O Manipulation
7
UNIT 1
INTRODUCTION TO OOPS
Contents
1.1
Programming language
1.2
Object-oriented programming paradigm
1.2.1 Object-Oriented Programming
1.2.2 Object-Oriented Languages
1.2.3 Object-based programming languages
1.2.4 Object-oriented programming languages.
1.3
Basic concept of oops
1.3.1 Objects
1.3.2 Classes
1.3.3 Encapsulation
1.3.4 Data Abstraction
1.3.5 Inheritance
1.3.6 Polymorphism
1.3.7 Dynamic Binding
1.3.8 Message Passing
1.4
Benefits of OOP
1.5
Application of OOPS
1.6
Brief History of C++
1.1 Programming Languages
So, what exactly is a programming language? As a loose definition, a programming language is a
tool used by a programmer to give the computer very specific instructions in order to serve some
purpose for the user. A program is like a recipe. It outlines exactly the steps needed to create
something or perform a certain task.
Computers do exactly what they are told, no more, no less. When writing a program, a
programmer must outline every possible step and scenario that could occur.
The first programming languages that emerged, were assembly languages. These languages are
exactly the instruction set of a specific processor. These languages are very low-level and hard to
understand. For example, say we wanted to add two numbers, 3 and 4 and get a result:
in C++:
in
assembly:
Int a = 3 + 4;
ldl 3, R1
ldl 4, R2
addl R1,
R2, R3
8
The version in C++ is easier to understand and simpler to write. Programmers write their code in a
high level language and then use a compiler to translate their code into an assembly language and
then into a machine language that will run on the machine they are using.
Programs consist of algorithms. An algorithm is just a well-outlined method for completing a task.
•
ask the user for the first number
•
ask the user for the second number
•
add the two numbers
•
display the result on the screen
This high-level abstraction is not actual code. However, it does express the ideas of a program,
and is called pseudo-code. Often, programmers will design their programs in pseudo-code, and
then use this to write their actual code.
So, why is there more than one programming language? It may seem that a standard language
should be agreed on, since all languages are translated using a compiler anyways. However,
languages are often designed with a specific use in mind, and some are better than others for
dealing with certain problems. So if a programmer is capable of writing a compiler (which is a very
complex piece of software) then they can design and create a language.
The most important thing to remember about programming languages is that they are only an
abstraction! Programming languages were created so developers could express their ideas on a
higher level than a computer can understand. Once a user has a good concept of how computers
work, and has learned a few computer languages, it becomes much easier to pick up new
languages.
A programming language is a tool used by programmers in order to specifically outline a series of
steps that a computer is to take in a certain instance. High-level programming languages allow a
programmer to express ideas on an abstract level, and forces the compiler to worry about the lowlevel implementation details. This allows for faster development of applications, since applications
are easier to write. There are even fourth generation languages emerging as viable programming
languages. Recall that machine code is considered first generation, assembly languages are
second generation, compiled languages are third generation. Fourth generation languages are
actually code-generating environments, such as Microsoft's Visual Basic. These fourth generation
languages allow programmers to express their ideas visually, and the environment then writes the
code to implement these ideas.
1.2 OBJECT-ORIENED PROGRAMMING PARADIGNM
The major motivating factor in the invention of object-oriented approach is to remove some of the
flaws encountered in the procedural approach. OOP treats data as a critical element in the
program development and does not allow it to flow freely around the system. It ties data more
closely to the functions that operate on it, and protects it from accidental modification from
outside functions. OOP allows decomposition of a problem into a number of entities called object
and then builds data and functions around these objects. The organization of data and functions
in object-oriented programs is shown in fig. The data of an object can access the functions of
other objects.
9
Object A
Object B
Data
Data
Functions
Functions
Object C
Functions
Data
Some of the striking features of object-oriented programming are:
•
Emphasis is on data rather than procedure.
•
Programs are divided into what are known as objects
•
Data structures are designed such that they characterize the objects.
•
Functions that operate on the data of an object are tied together in the data structure.
•
Data is hidden and cannot be accessed by external functions.
•
Objects may communicate with each other through functions.
•
New data and functions can be easily added whenever necessary.
•
Follows bottom-up approach in program design.
Object-oriented programming is the most recent concept among programming paradigms and still
means different things to different people. It is therefore important to have a working definition of
object-oriented programming before we proceed further. We define “object-oriented programming
as an approach that provides a way of modularizing programs by creating partitioned memory
area for both data and functions that can be used as template for creating copies of such modules
on demand.” Thus, an object is considered to be a partitioned area of computer memory that
stores data and set of operations that can access that data. Since the memory partitions are
independent, the objects can be used in a variety of different programs without modifications.
1.2.1
Object-Oriented Programming
Since object –oriented programming (OOP) drove the creation of ++, it is necessary to understand
its foundational principles. OOP is a powerful way to approach the job of programming.
10
Programming methodologies have changed grammatically since the invention of the computer,
primarily to accommodate the increasing complexity of programs. For example, when computers
were first invented, programming was done by toggling in the binary machine instructions using
the computer’s front panel. As long as programs grew, assembly language was invented so that a
programmer could deal with larger, increasingly complex programs, using symbolic
representations of the machine instructions. As program continued to grow, high-level languages
were introduced that gave the programmer more tools with which to handle complexity. The first
widespread language was, of course, FORTRN. Although FORTRON was a very impressive first
step, it is hardly a language that encourages clear, easy-to-understand programs.
The 1960s gave birth to structured programming. This is the method encouraged by languages
such as and Pascal. The use of structured languages made it possible to write moderately complex
programs fairly easily. Structured languages are characterized by their support for stand-alone
subroutines, local variables, rich control constructs, and their lack of reliance upon the GOTO.
Although structured languages are a powerful tool, even they reach their limit when a project
becomes too large.
Consider this: At each milestone in the development of programming, techniques and tools were
created to allow the programmer to deal with increasingly greater complexity. Each step of the
way, the new approach took the best elements of the previous methods and moved forward. Prior
to the invention of OOP, many projects were nearing (or exceeding) the point where the structured
approach no longer worked. Object-oriente4d methods were created to help programmers break
through these barriers.
Object-oriented programming took the best ideas of structured programming and combined them
with several new concepts. The result was a different way of organizing a program. In the most
general sense, a program can be organized in of two ways: around its code (what is happening) or
around its data (who is being affected). Using only structured programming techniques, programs
are typically organized around code. This approach can be thought of as “code acting on data”.
For example, a program written in a structured language such as is defined by its functions, any
of which may operate on any type of data used by the program.
Object-oriented programs work the other way around. They are organized around data, with the
key principle being “data controlling access to code.” In an object-oriented language, you define
the data and the routines that are permitted to act on that data. Thus, a data type defines
precisely what sort of operations can be applied to that data.
To support the principles of object-oriented programming, all OOP languages have three traits in
common: encapsulation, polymorphism, and inheritance.
1.2.2 Object-Oriented Languages
Object-oriented programming is not the right way of any particular language. Like structured
programming, OOP concepts can be implemented using languages such as C and Pascal .
However, programming becomes clumsy and may generate confusion when the programs grow
large. A language that is specially designed to support the OOP concepts makes it easier to
implement them.
11
The languages should support several of the OOP concepts to claim that they are object-oriented.
Depending upon the features they support, they can be classified into the following two
categories:
1.2.3 Object-based programming languages,
Object-based programming is the style of programming that primarily supports encapsulation and
object identity. Major features that are required for object-based programming are:
•
Data encapsulation
•
Data hiding and access mechanism
•
Automatic initialization and clear-up of objects
•
Operator overloading
Languages that support programming with objects are said to be object-based programming
languages. They do not support inheritance and dynamic binding. Ad is a typical object-based
programming language.
1.2.4 Object-oriented programming languages.
Object-oriented programming incorporates all of object-based programming features along with
two additional features, namely, inheritance and dynamic binding. Object-oriented programming
can therefore be characterized by the following statement:
Object based features + inheritance + dynamic binding
Languages that support these features include C++, Smalltalk, Object Pascal and Java . There are
a large number of object-oriented programming languages. Table lists some popular general
purpose OOP languages and their characteristics.
Characteristi
cs
Simul
a
*
Smalltal
k
*
Objectiv
e
c
C++
Ada
**
Object
Pascal
Turbo
Pasca
l
Ffecl
*
Java
*
Binding
(early or late)
Both
Late
Both
Both
Arly
Late
Early
Earl
y
Both
Poor
Poor
Poor
Diffi
cult
No
No
Pro
mise
d
-----
-----
Polymorphis
m
Data Hiding
Concurrency
Inheritance
Multiple
Inheritance
No
No
No
Garbage
Collection
No
12
No
Persistence
No
Promise
d
No
Genericity
NO
No
No
NO
Object
Libraies
Like
3 GL
No
No
No
No
Som
e
supp
ort
No
Not
muc
h
As seen from Table, all languages provide for polymorphism and data hiding. However , many of
them do not provide facilities for concurrency, persistence and generosity. Eiffel, Ad and C++
provide generic facility which is an important construct for supporting reuse.
However, persistence(a process of storing objects) is not full supported by any of them. In
Smalltalk, though the entire current execution state can be saved to disk, yet the individual
objects cannot be saved to an external file.
Commercially, C++ is only 10 years old, Smalltalk and Objective C13 years old, and Java only 5
years old. Although Similar has existed for more than two decades, it has spent most of its life I n
a re search environment. The field is so new, however ,that it should not be judged too harshly.
Use of a particular language depends on characteristics and requirements of an application,
organizational impact of the choice, and reuse of the existing programs. C++ has now become the
most successful , practical, general purpose OOP language, and is widely used in industry today.
1.3 Basic concept of Object-oriented programming
It is necessary to understand some of the concepts used extensively in object-oriented
programming. These include:
1.3.1. Objects:
Objects are the basic run-time entities in an object-oriented system. They may represent a person,
a place, a bank account, a table of data or any item that the program has to handle. They may
also represent user-defined data such as vectors, time and lists. Programming problem is
analyzed in term of objects and the nature of communication between them. Program objects
should be chosen such that they match closely with real-world objects. Objects take up space in
memory and have an associated address like a record in Pascal, or a structure in .
When a program is executed, the objects interact by sending messages to one another. Foe
example, if” customer” and “account” are two objects in a program, then the customer object may
send a message to the account object requesting for the bank balance. Ach object contains data
and code to manipulate the data. Objects can interact without having to know details of each
other’s data or code. It is sufficient to know the type of message accepted, and the type of
response return by the objects. Although different authors represent them differently, fig. below
shows two notations that are popularly used in object-oriented analysis and design.
13
STUDENT
Object:STUDEN
T
Total
DATA
Name
Date-of-birth
Marks
Average
Display
FUNCTIONS
Total
Average
Display
1.3.2. Classes:
We just mentioned that objects contained that objects contain data, and code to manipulate that
data. The entire set of data and code of an object can be made a user-defined data type with the
help of a class. In fact, objects are variables of that class type. Once a class has been defined , we
can create any number of objects belonging to that class. Each object is associated with the data
of type class with which they are created. A class is thus a collection of objects of similar type.
Foe example, mango, apple and orange are member of the class fruit. Classes are user-defined
data types and behave like the built-in types of a programming language. The syntax is used to
create an object is no different than the syntax used to create an integer object in C. If fruit has
been defined as a class, then the statement: Fruit mango; Will create an object mango belonging
to the class fruit.
1.3.3 Encapsulation:
The wrapping of data and functions into a single unit
Data encapsulation is the most striking feature of a
outside world, and those functions, which are wrapped
provide the interface between the object’s data and the
direct access by the program is called data hiding .
(called class) is known as encapsulation.
class. The data is not accessible to the
in the class, can access it. These function
program. This insulation of the data from
1.3.4 Data Abstraction:
Abstraction refers to the act of representing essential features with out including the background
details or explanations. Classes use the concept of abstraction and are defined as a list of
abstract attributes such as size, weight and cost and functions to operate on these attributes.
Sometimes, these are called data members because they hold information. The functions that
operate on these data are called methods or member functions.
1.3.5 Inheritance
14
Inheritance is the process by which objects of one class acquire the properties of objects of
another class. It supports the concept of hierarchical classification. For example, the bird ‘robin’
is a part of the class ‘flying bird’ which is again a part of the class ‘bird’. The principle behind this
sort of division is that each derived class shares common characteristics with the class from
which it is derived as illustrated in figure.
In OOP, the concept of inheritance provides the idea of reusability. This means that we can add
additional features to an existing class without modifying it. This is possible by deriving a new
class from the existing one. The new class will have the combined features of both of the classes.
The real appeal and the power of the inheritance mechanism is that it a allows the programmer to
reuse a class that is almost, but not exactly, what he wants, and to tailor the class in such a way
that it does not introduce any
Bird
Attributes
Feathers
Lay eggs
Flying Bird
Non-flying Bird
Attributes
…………
…………
Attributes
………………
………………..
Robin
Attributes
…………
…………
…………
Swallow
Attributes
…………
…………
.
Penguin
Attributes
……….
……….
Kiwi
Attributes
………..
…………
Undesirable side-effects into the rest of the classes.
Note that each sub-class defines only those features that are unique to it. Without the use of
classification, each class would have to explicitly include all its features.
1.3.6 Polymorphism:
Polymorphism is another important OOP concept. Polymorphism, a Greek term, means the ability
to take more than one form. An operation may exhibit different behaviors in different instances.
The behavior depends upon the type of data used in the operation. For example, consider the
15
operation of addition. For two numbers, the operation will generate a sum. If the operands are
strings, then the operation would produce a third string by concatenation. The process of making
an operator to exhibit different behaviors in different instances is known as operator overloading.
Figure below illustrates that a single function name can be used to handle different number and
different types of arguments. This is something similar to a particular word having several
different meanings depending on the context. Using a single function name to perform different
types of tasks is known as function overloading.
Shape
Draw ()
Circle object
Box object
Triangle object
Draw(circle)
Draw(box)
Draw(triangle)
Polymorphism plays an in important role in allowing objects having different internal structures
to share the same external interface. This means that a general class of operations may be
accessed in the same manner even though specific actions associated with each operation may
differ. Polymorphism is extensively used in implementing inheritance.
1.3.7 Dynamic Binding:
Binding refers to the linking of a procedure call to the code to be executed in response to the call.
Dynamic binding (also known as late binding) means that the code associated with a given
procedure call is not known until the time of the call at run-time. It is associated with
polymorphism and inheritance. A function call associated with a polymorphism reference depends
on the dynamic type of that reference.
Consider the procedure “draw” in the above figure. By inheritance, every object will have this
procedure. Its algorithm is, however, unique to each object and so the draw procedure will be
refined in each class that defines the object. At run-time, the code matching the object under
current reference will be called.
1.3.8 Message Passing:
An object-oriented program consists of a set of objects that communicate with each other. The
process of programming in an object-oriented language, therefore, involves the following basic
steps:
1. Creating classes that define objects and their behavior,
16
2. Creating objects from class definitions, and
3. Establishing communication among objects.
Objects communicate with one another by sending and receiving information much the same way
as people pass message to one another. The concept of message passing makes it easier to talk
about building systems that directly model or simulate their real-world counterparts.
A message for an object as a request for execution of a procedure, and therefore will invoke a
function (procedure) in the receiving object that generates the desired result. Message passing
involves specifying the name of the object, the name of the function(message) and the information
to be sent. Example:
Employee. salary(name);
object
information
message
Objects have a life cycle. They can be created and destroyed. Communication with an object is
feasible as long as it is alive.
1.4 Benefits of OOP
OOP offers several benefits to both the program designer and the user. Object-orientation
contributes to the solution of many problems associated with the development and quality of
software products. A new technology promises greater programmer productivity, better quality of
software and lesser maintenance cost. The principal advantages are:
•
Through inheritance, we can eliminate redundant code and extend the use of existing
•
Classes.
•
We can build programs from the standard working modules that communicate with one
another, rather than having to start writing the code from scratch. This leads to saving of
development time and higher productivity.
•
The principle of data hiding helps the programmer to build secure programs that cannot
be invaded by code in other parts of the program.
•
It is possible to have multiple instances of an object to co-exist without any interference.
•
It is possible to map objects I the problem domain to those in the program.
•
It is easy to partition the work in a project based on objects.
•
The data-centered design approach enables us to capture more details of a model in
implemental form
•
Object-oriented systems can be easily upgraded from small to large systems.
17
•
Message passing techniques for communication between objects makes the interface
descriptions with external systems much simpler.
•
Software complexity can be easily managed.
While it is possible to incorporate all these features in an object-oriented system, their importance
depends on the type of the project and the preference of the programmer. There are a number of
issues that need to be tackled to reap some of the benefits stated above. For instance, object
18
libraries must be available for reuse. The technology is still developing and current products may
be superseded quickly. Strict controls and protocols need to be developed if reuse is not to be
compromised.
Developing software that is easy to use makes it hard to build. It is hoped that the object-oriented
programming tools would help manage this problem.
1.5 Application of OOPS
OOP has become one of the programming buzzwords today. There appears to be a great deal of
excitement and interest among software engineers in using OOPS . Applications of OOP are
beginning to gain importance in many areas. The most popular application of object –oriented
programming, up to now, has been in the area of user interface design such as windows.
Hundreds of windowing systems have been developed, using the OOP techniques.
Real –business systems are often much more complex and contain many more objects with
complicated attributes and methods. OOP is useful in these types of applications because can
simplify a complex problem. The promising areas for application of OOP includes.
•
Real-time systems
•
Simulation and modeling
•
Object-oriented databases
•
Hypertext, hypermedia and expert text
•
AI and expert systems
•
Neural networks and parallel programming
•
Decision support and office automation systems
•
CIM/CAM/CAD systems
The object-oriented paradigm sprang from the language, has matured into design, and has
recently moved into analysis. It is believed that the richness of OOP environment will enable the
software industry to improve not only the quality of software systems but also its productivity.
Object-oriented technology is certainly going to change the way the software engineers think,
analyze, design and implement future systems.
1.6 Brief History of C++
The C++ Programming Language is basically an extension of the C Programming Language. The C
Programming language was developed from 1969-1973 at Bell labs, at the same time the UNIX
operating system was being developed there. C was a direct descendant of the language B, which
was developed by Ken Thompson as a systems programming language for the fledgling UNIX
operating system. B, in turn, descended from the language BCPL which was designed in the
1960s by Martin Richards while at MIT.
In 1971 Dennis Ritchie at Bell Labs extended the B language (by adding types) into what he called
NB, for "New B". Ritchie credits some of his changes to language constructs found in Algol68,
although he states "although it [the type scheme], perhaps, did not emerge in a form that Algol's
19
adherents would approve of" After restructuring the language and rewriting the compiler for B,
Ritchie gave his new language a name: "C".
In 1983, with various versions of C floating around the computer world, ANSI established a
committee that eventually published a standard for C in 1989.
In 1983 Bjarne Stroustrup at Bell Labs created C++. C++ was designed for the UNIX system
environment, it represents an enhancement of the C programming language and enables
programmers to improve the quality of code produced, thus making reusable code easier to write.
Why Program in C++?
So what is so special about C++? Why should you use C++ to develop your applications? First,
C++ is not the best language to use in every instance. C++ is a great choice in most instances, but
some special circumstances would be better suited to another language.
There are a few major advantages to using C++:
1. C++ allows expression of abstract ideas
C++ is a third generation language that allows a programmer to express their ideas
at a high level as compared to assembly languages.
2. C++ still allows a programmer to keep low-level control
Even though C++ is a third generation language, it has some of the "feel" of an
assembly language. It allows a programmmer to get down into the low-level
workings and tune as necessary. C++ allows programmers strict control over
memory management.
3. C++ has national standards (ANSI)
C++ is a language with national standards. This is good for many reasons. Code
written in C++ that conforms to the national standards can be easily integrated
with preexisting code. Also, this allows programmers to reuse certain common
libraries, so certain common functions do not need to be written more than once,
and these functions behave the same anywhere they are used.
4. C++ is reusable and object-oriented
C++ is an object-oriented language. This makes programming conceptually easier
(once the object paradigm has been learned) and allows easy reuse of code, or parts
of code through inheritance.
5. C++ is widely used and taught
C++ is a very widely used programming language. Because of this, there are many
tools available for C++ programming, and there is a broad base of programmers
contributing to the C++ "community".
20
UNIT 2
DATA TYPES & VARIABLES
Contents
2.1
Structure of a C++ program.
2.2
Comments
2.3
Variables
2.4
Identifiers
2.5
Data types.
2.6
Declaration of variables
2.7
Initialization of variables
2.8
Scope of variables
2.9
Constants
2.1 Structure of a C++ program.
Probably the best way to start learning a programming language is with a program. So here is our
first program:
// my first program in C++
#include <iostream.h>
int main ()
{
cout << "Hello World!";
return 0;
}
The above code shows the source code for our first program, which we can name, for example,
hiworld.cpp. The way to edit and compile a program depends on the compiler you are using.
Depending on whether it has a Development Interface or not and on its version. Consult section
compilers and the manual or help included with your compiler if you have doubts on how to
compile a C++ console program.
The previous program is the first program that most programming apprentices write, and its
result is the printing on screen of the "Hello World!" sentence. It is one of the simpler programs
that can be written in C++, but it already includes the basic components that every C++ program
has. We are going to take a look at them one by one:
// my first program in C++
This is a comment line. All the lines beginning with two slash signs (//) are considered comments
and do not have any effect on the behavior of the program. They can be used by the programmer
21
to include short explanations or observations within the source itself. In this case, the line is a
brief description of what our program does.
#include <iostream.h>
Sentences that begin with a pound sign (#) are directives for the preprocessor. They are not
executable code lines but indications for the compiler. In this case the sentence #include
<iostream.h> tells the compiler's preprocessor to include the iostream standard header file. This
specific file includes the declarations of the basic standard input-output library in C++, and it is
included because its functionality is used later in the program.
int main ( )
This line corresponds to the beginning of the main function declaration. The main function is the
point by where all C++ programs begin their execution. It is independent of whether it is at the
beginning, at the end or in the middle of the code - its content is always the first to be executed
when a program starts. In addition, for that same reason, it is essential that all C++ programs
have a main function.
main is followed by a pair of parenthesis () because it is a function. In C++ all functions are
followed by a pair of parenthesis () that, optionally, can include arguments within them. The
content of the main function immediately follows its formal declaration and it is enclosed between
curly brackets ({}), as in our example.
cout << "Hello World";
This instruction does the most important thing in this program. cout is the standard output
stream in C++ (usually the screen), and the full sentence inserts a sequence of characters (in this
case "Hello World") into this output stream (the screen). cout is declared in the iostream.h header
file, so in order to be able to use it that file must be included.
Notice that the sentence ends with a semicolon character (;). This character signifies the end of
the instruction and must be included after every instruction in any C++ program (one of the most
common errors of C++ programmers is indeed to forget to include a semicolon ; at the end of each
instruction).
return 0;
The return instruction causes the main() function finish and return the code that the instruction
is followed by, in this case 0. This it is most usual way to terminate a program that has not found
any errors during its execution. As you will see in coming examples, all C++ programs end with a
sentence similar to this.
Therefore, you may have noticed that not all the lines of this program did an action. There were
lines containing only comments (those beginning by //), lines with instructions for the compiler's
preprocessor (those beginning by #), then there were lines that initiated the declaration of a
function (in this case, the main function) and, finally lines with instructions (like the call to cout
<<), these last ones were all included within the block delimited by the curly brackets ({}) of the
main function.
22
The program has been structured in different lines in order to be more readable, but it is not
compulsory to do so. For example, instead of
int main ( )
{
cout << " Hello World ";
return 0;
}
we could have written:
int main ( ) { cout << " Hello World "; return 0; }
in just one line and this would have had exactly the same meaning.
In C++ the separation between instructions is specified with an ending semicolon (;) after each
one. The division of code in different lines serves only to make it more legible and schematic for
humans that may read it.
Here is a program with some more instructions:
// my second program in C++
Hello World! I'm a C++ program
#include <iostream.h>
int main ()
{
cout << “Hello World! ";
cout << "I'm a C++ program";
return 0;
}
In this case we used the cout << method twice in two different instructions. Once again, the
separation in different lines of the code has just been done to give greater readability to the
program, since main could have been perfectly defined thus:
int main () { cout << " Hello World! "; cout << " I'm to C++ program "; return 0; }
We were also free to divide the code into more lines if we considered it convenient:
int main ()
{
cout <<
"Hello World!";
cout
<< "I'm a C++ program";
return 0;
}
23
And the result would have been exactly the same than in the previous examples.
Preprocessor directives (those that begin by #) are out of this rule since they are not true
instructions. They are lines read and discarded by the preprocessor and do not produce any code.
These must be specified in their own line and do not require the include a semicolon (;) at the end.
2.2 Comments
Comments are pieces of source code discarded from the code by the compiler. They do nothing.
Their purpose is only to allow the programmer to insert notes or descriptions embedded within
the source code.
C++ supports two ways to insert comments:
// line comment
/* block comment */
The first of them, the line comment, discards everything from where the pair of slash signs (//) is
found up to the end of that same line. The second one, the block comment, discards everything
between the /* characters and the next appearance of the */ characters, with the possibility of
including several lines.
We are going to add comments to our second program:
/* my second program in C++
with more comments */
Hello World! I'm a C++ program
#include <iostream.h>
int main ()
{
cout << "Hello World! ";
// says Hello
World!
cout << "I'm a C++ program"; // says I'm a
C++ program
return 0;
}
If you include comments within the source code of your programs without using the comment
characters combinations //, /* or */, the compiler will take them as if they were C++ instructions
and, most likely causing one or several error messages.
2.3 Variables
The usefulness of the "Hello World" programs shown in the previous section are something more
than questionable. We had to write several lines of code, compile them, and then execute the
resulting program just to obtain a sentence on the screen as result. It is true that it would have
24
been much faster to simply write the output sentence by ourselves, but programming is not
limited only to printing texts on screen. In order to go a little further on and to become able to
write programs that perform useful tasks that really save us work we need to introduce the
concept of the variable.
Let's think that I ask you to retain the number 5 in your mental memory, and then I ask you to
also memorize the number 2. You have just stored two values in your memory. Now, if I ask you
to add 1 to the first number I said, you should be retaining the numbers 6 (that is 5+1) and 2 in
your memory. Values that we could now subtract and obtain 4 as the result.
All this process that you have made is a simile of what a computer can do with two variables. This
same process can be expressed in C++ with the following instruction set:
a = 5;
b = 2;
a = a + 1;
result = a - b;
Obviously this is a very simple example since we have only used two small integer values, but
consider that your computer can store millions of numbers like these at the same time and
conduct sophisticated mathematical operations with them.
Therefore, we can define a variable as a portion of memory to store a determined value.
Each variable needs an identifier that distinguishes it from the others, for example, in the
previous code the variable identifiers were a, b and result, but we could have called the variables
any names we wanted to invent, as long as they were valid identifiers.
2.4. Identifiers
A valid identifier is a sequence of one or more letters, digits or underline symbols ( _ ). The length
of an identifier is not limited, although for some compilers only the 32 first characters of an
identifier are significant (the rest are not considered).
Neither spaces nor marked letters can be part of an identifier. Only letters, digits and underline
characters are valid. In addition, variable identifiers should always begin with a letter. They can
also begin with an underline character ( _ ), but this is usually reserved for external links. In no
case they can begin with a digit.
Another rule that you have to consider when inventing your own identifiers is that they cannot
match any key word of the C++ language nor your compiler's specific ones since they could be
confused with these. For example, the following expressions are always considered key words
according to the ANSI-C++ standard and therefore they must not be used as identifiers:
asm, auto, bool, break, case, catch, char, class, const, const_cast, continue,
default, delete, do, double, dynamic_cast, else, enum, explicit, extern, false,
float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator,
private, protected, public, register, reinterpret_cast, return, short, signed,
sizeof, static, static_cast, struct, switch, template, this, throw, true, try,
typedef, typeid, typename, union, unsigned, using, virtual, void, volatile,
wchar_t
Additionally, alternative representations for some operators do not have to be used as identifiers
since they are reserved words under some circumstances:
25
and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq
Your compiler may also include some more specific reserved keywords. For example, many
compilers which generate 16 bit code (like some compilers for DOS) also include far, huge and
near as key words.
Very important: The C++ language is "case sensitive", that means that an identifier written in
capital letters is not equivalent to another one with the same name but written in small letters.
Thus, for example the variable RESULT is not the same as the variable result nor the variable
Result.
2.5.Data types
When programming, we store the variables in our computer's memory, but the computer must
know what we want to store in them since storing a simple number, a letter or a large number is
not going to occupy the same space in memory.
Our computer's memory is organized in bytes. A byte is the minimum amount of memory that we
can manage. A byte can store a relatively small amount of data, usually an integer between 0 and
255 or one single character. But in addition, the computer can manipulate more complex data
types that come from grouping several bytes, such as long numbers or numbers with decimals.
Next you have a list of the existing fundamental data types in C++, as well as the range of values
that can be represented with each one of them:
DATA TYPES
Name
Bytes* Description
Range*
char
1
character or integer 8 bits length.
signed: -128 to 127
unsigned: 0 to 255
short
2
integer 16 bits length.
signed: -32768 to 32767
unsigned: 0 to 65535
long
4
integer 32 bits length.
signed:-2147483648 to
2147483647
unsigned: 0 to 4294967295
Int
*
Integer. Its length traditionally depends
on the length of the system's Word type,
thus in MSDOS it is 16 bits long,
whereas in 32 bit systems (like Windows See short, long
9x/2000/NT and systems that work
under protected mode in x86 systems) it
is 32 bits long (4 bytes).
float
4
floating point number.
3.4e + / - 38 (7 digits)
double
8
double precision floating point number.
1.7e + / - 308 (15 digits)
long
double
10
long double precision floating point
number.
1.2e + / - 4932 (19 digits)
26
bool
1
wchar_t 2
Boolean value. It can take one of two
values: true or false NOTE: this is a type
recently added by the ANSI-C++
true or false
standard. Not all compilers support it.
Consult
section
bool
type
for
compatibility information.
Wide character. It is designed as a type
to store international characters of a
two-byte character set. NOTE: this is a wide characters
type recently added by the ANSI-C++
standard. Not all compilers support it.
* Values of columns Bytes and Range may vary depending on your system. The values included
here are the most commonly accepted and used by almost all compilers.
In addition to these fundamental data types there also exist the pointers and the void parameter
type specification, that we will see later.
2.6. Declaration of variables
In order to use a variable in C++, we must first declare it specifying which of the data types above
we want it to be. The syntax to declare a new variable is to write the data type specifier that we
want (like int, short, float...) followed by a valid variable identifier. For example:
int a;
float mynumber;
Are valid declarations of variables. The first one declares a variable of type int with the identifier
a. The second one declares a variable of type float with the identifier mynumber. Once declared,
variables a and mynumber can be used within the rest of their scope in the program.
If you need to declare several variables of the same type and you want to save some writing work
you can declare all of them in the same line separating the identifiers with commas. For example:
int a, b, c;
27
declares three variables (a, b and c) of type int , and has exactly the same meaning as if we had
written:
int a;
int b;
int c;
Integer data types (char, short, long and int) can be signed or unsigned according to the range of
numbers that we need to represent. Thus to specify an integer data type we do it by putting the
keyword signed or unsigned before the data type itself. For example:
unsigned short NumberOfSons;
signed int MyAccountBalance;
By default, if we do not specify signed or unsigned it will be assumed that the type is signed,
therefore in the second declaration we could have written:
int MyAccountBalance;
with exactly the same meaning and since this is the most usual way, few source codes include the
keyword signed as part of a compound type name.
The only exception to this rule is the char type that exists by itself and it is considered a different
type than signed char and unsigned char.
Finally, signed and unsigned may also be used as a simple types, meaning the same as signed
int and unsigned int respectively. The following two declarations are equivalent:
unsigned MyBirthYear;
unsigned int MyBirthYear;
To see what variable declaration looks like in action in a program, we are going to show the C++
code of the example about your mental memory proposed at the beginning of this section:
// operating with variables
4
#include <iostream.h>
int main ()
{
// declaring variables:
int a, b;
int result;
// process:
a = 5;
b = 2;
a = a + 1;
result = a - b;
// print out the result:
cout << result;
28
// terminate the program:
return 0;
}
Do not worry if something about the variable declarations looks a bit strange to you. You will see
the rest in detail in coming sections.
2.7. Initialization of variables
When declaring a local variable, its value is undetermined by default. But you may want a
variable to store a concrete value the moment that it is declared. In order to do that, you have to
append an equal sign followed by the value wanted to the variable declaration:
type identifier = initial_value ;
For example, if we want to declare an int variable called a that contains the value 0 at the
moment in which it is declared, we could write:
int a = 0;
Additionally to this way of initializating variables (known as c-like), C++ has added a new way to
initialize a variable: by enclosing the initial value between parenthesis ():
type identifier (initial_value) ;
For example:
int a (0);
Both ways are valid and equivalent in C++.
2.8. Scope of variables
All the variables that we are going to use must have been previously declared. An important
difference between the C and C++ languages, is that in C++ we can declare variables anywhere in
the source code, even between two executable sentences, and not only at the beginning of a block
of instructions, like happens in C.
Anyway, it is recommended under some circumstances to follow the indications of the C language
when declaring variables, since it can be useful when debugging a program to have all the
declarations grouped together. Therefore, the traditional C-like way to declare variables is to
include their declaration at the beginning of each function (for local variables) or directly in the
body of the program outside any function (for global variables).
29
Global variables can be referred
to anywhere in the code, within
any function, whenever it is after
its declaration.
The scope of the local variables is
limited to the code level in which
they are declared. If they are
declared at the beginning of a
function (like in main) their scope
is the whole main function. In the
example above, this means that if
another function existed in
addition to main(), the local
variables declared in main could
not be used in the other function
and vice versa.
In C++, the scope of a local variable is given by the block in which it is declared (a block is a group
of instructions grouped together within curly brackets {} signs). If it is declared within a function it
will be a variable with function scope, if it is declared in a loop its scope will be only the loop, etc...
In addition to local and global scopes there exists external scope, that causes a variable to be
visible not only in the same source file but in all other files that will be linked together.
2.9. Constants: Literals.
A constant is any expression that has a fixed value. They can be divided in Integer Numbers,
Floating-Point Numbers, Characters and Strings.
•
Integer Numbers
1776
707
-273
They are numerical constants that identify integer decimal numbers. Notice that to express a
numerical constant we do not need to write quotes (") nor any special character. There is no doubt
that it is a constant: whenever we write 1776 in a program we will be referring to the value 1776.
In addition to decimal numbers (those that all of us already know) C++ allows the use as literal
constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an
octal number we must precede it with a 0 character (zero character). And to express a
hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the
following literal constants are all equivalent to each other:
// decimal
75
// octal
0113
0x4b
// hexadecimal
All of them represent the same number: 75 (seventy five) expressed as a radix-10 number, octal
and hexdecimal, respectively.
30
•
Floating Point Numbers:-They express numbers with decimals and/or exponents. They
can include a decimal point, an e character (that expresses "by ten at the Xth height",
where X is the following integer value) or both.
3.14159 // 3.14159
6.02e23 // 6.02 x 1023
1.6e-19 // 1.6 x 10-19
3.0
// 3.0
These are four valid numbers with decimals expressed in C++. The first number is PI, the second
one is the number of Avogadro, the third is the electric charge of an electron (an extremely small
number) -all of them approximated- and the last one is the number 3 expressed as a floating point
numeric literal.
•
Characters and strings:-There also exist non-numerical constants, like:
'z'
'p'
"Hello world"
"How do you do?"
The first two expressions represent single characters, and the following two represent strings of
several characters. Notice that to represent a single character we enclose it between single quotes
(') and to express a string of more than one character we enclose them between double quotes (").
When writing both single characters and strings of characters in a constant way, it is necessary to
put the quotation marks to distinguish them from possible variable identifiers or reserved words.
Notice this:
x
'x'
x refers to variable x, whereas 'x' refers to the character constant 'x'.
Character constants and string constants have certain peculiarities, like the escape codes. These
are special characters that cannot be expressed otherwise in the source code of a program, like
newline (\n) or tab (\t). All of them are preceded by an inverted slash (\). Here you have a list of
such escape codes:
\n
Newline
\r
carriage return
\t
Tabulation
\v
vertical tabulation
\b
Backspace
\f
page feed
\a
alert (beep)
\'
single quotes (')
\"
double quotes (")
\?
question (?)
\\
inverted slash (\)
For example:
31
'\n'
'\t'
"Left \t Right"
"one\ntwo\nthree"
Additionally, you can express any character by its numerical ASCII code by writing an inverted
slash bar character (\) followed by the ASCII code expressed as an octal (radix-8) or hexadecimal
(radix-16) number. In the first case (octal) the number must immediately follow the inverted slash
(for example \23 or \40), in the second case (hexacedimal), you must put an x character before
the
number
(for
example
\x20
or
\x4A).
Constants of string of characters can be extended by more than a single code line if each code line
ends with an inverted slash (\):
"string expressed in \
two lines"
You can also concatenate several string constants separating them by one or several blankspaces,
tabulators, newline or any other valid blank character:
"we form" "a single" "string" "of characters"
•
Defined constants (#define)
You can define your own names for constants that you use quite often without having to resort to
variables, simply by using the #define preprocessor directive. This is its format:
#define identifier value
For example:
#define PI 3.14159265
#define NEWLINE '\n'
#define WIDTH 100
They define three new constants. Once they are declared, you are able to use them in the rest of
the code as any if they were any other constant, for example:
circle = 2 * PI * r;
cout << NEWLINE;
In fact the only thing that the compiler does when it finds #define directives is to replace literally
any occurrence of the them (in the previous example, PI, NEWLINE or WIDTH) by the code to
which they have been defined (3.14159265, '\n' and 100, respectively). For this reason, #define
constants are considered macro constants.
The #define directive is not a code instruction, it is a directive for the preprocessor, therefore it
assumes the whole line as the directive and does not require a semicolon (;) at the end of it. If you
include a semicolon character (;) at the end, it will also be added when the preprocessor will
substitute any occurence of the defined constant within the body of the program.
Declared constants (const)
32
With the const prefix you can declare constants with a specific type exactly as you would do with
a variable:
const int width = 100;
const char tab = '\t';
const zip = 12440;
In case that the type was not specified (as in the last example) the compiler assumes that it is
type int.
33
UNIT 3
OPERATOR AND CONTROL STRUCTURES
Contents
3.1
3.2
3.3
3.4
Types of Operators.
Priority of Operators.
Communication through console.
3.3.1
Output
3.3.2
input
Control Structures.
3.4.1
Conditional structure
3.4.2
Repetitive structures or loops
3.4.3
Bifurcation of control and jumps
3.4.4
The selective Structure: switch
Introduction
Once we know of the existence of variables and constants we can begin to operate with them. For
that purpose, C++ provides the operators, which in this language are a set of keywords and signs
that are not part of the alphabet but are available in all keyboards. It is important to know them
since they are the basis of the C++ language.
3.1 Different types of operators
•
Assignation (=).
The assignation operator serves to assign a value to a variable.
a = 5;
Assigns the integer value 5 to variable a. The part at the left of the = operator is known as lvalue
(left value) and the right one as rvalue (right value). lvalue must always be a variable whereas the
right side can be either a constant, a variable, the result of an operation or any combination of
them.
It is necessary to emphasize that the assignation operation always takes place from right to left
and never at the inverse.
a = b;
34
assigns to variable a (lvalue) the value that contains variable b (rvalue) independently of the value
that was stored in a at that moment. Consider also that we are only assigning the value of b to a
and that a later change of b would not affect the new value of a.
For example, if we take
int a, b;
a = 10;
b = 4;
a = b;
b = 7;
this code (with the evolution of the variables' content in green color):
// a:? b:?
// a:10 b:?
// a:10 b:4
// a:4 b:4
// a:4 b:7
Will give us the result that the value contained in a is 4 and the one contained in b is 7. The final
modification of b has not affected a, although before we have declared a = b; (right-to-left rule).
A property that C++ has over other programming languages is that the assignation operation can
be used as the rvalue (or part of an rvalue) for another assignation. For example:
a = 2 + (b = 5);
is equivalent to:
b = 5;
a = 2 + b;
That means: first assign 5 to variable b and then assign to a the value 2 plus the result of the
previous assignation of b (that is 5), leaving a with a final value of 7. Thus, the following
expression is also valid in C++:
a = b = c = 5;
Assigns 5 to the three variables a, b and c.
•
Arithmetic operators ( +, -, *, /, % )
The five arithmetical operations supported by the language are:
+ addition
- subtraction
* multiplication
/ division
% module
Operations of addition, subtraction, multiplication and division should not suppose an
understanding challenge for you since they literally correspond with their respective mathematical
operators.
The only one that may not be known by you is the module, specified with the percentage sign (%).
Module is the operation that gives the remainder of a division of two integer values. For example,
35
if we write a = 11 % 3;, the variable a will contain 2 as the result since 2 is the remainder from
dividing 11 between 3.
•
Compound assignation operators (+=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=)
A feature of assignation in C++ that contributes to its fame of sparing language when writing are
the compound assignation operators (+=, -=, *= and /= among others), which allow to modify the
value of a variable with one of the basic operators:
value += increase; is equivalent to value = value + increase;
a -= 5; is equivalent to a = a - 5;
a /= b; is equivalent to a = a / b;
price *= units + 1; is equivalent to price = price * (units + 1);
and the same for all other operations.
•
Increase and decrease.
Another example of saving language when writing code are the increase operator (++) and the
decrease operator (--). They increase or reduce by 1 the value stored in a variable. They are
equivalent to +=1 and to -=1, respectively. Thus:
a++;
a+=1;
a=a+1;
are all equivalent in its functionality: the three increase by 1 the value of a.
Its existence is because in the first C compilers the three previous expressions produced different
executable code according to which one was used. Nowadays this type of code optimization is
generally done automatically by the compiler.
A characteristic of this operator is that it can be used both as a prefix or as a suffix. That means it
can be written before the variable identifier (++a) or after (a++). Although in simple expressions
like a++ or ++a they have exactly the same meaning, in other operations in which the result of the
increase or decrease operation is evaluated as another expression they may have an important
difference in their meaning: In case that the increase operator is used as a prefix (++a) the value is
increased before the expression is evaluated and therefore the increased value is considered in the
expression; in case that it is used as a suffix (a++) the value stored in a is increased after being
evaluated and therefore the value stored before the increase operation is evaluated in the
expression. Notice the difference:
Example 1
B=3;
A=++B;
// A is 4, B is 4
Example 2
B=3;
A=B++;
// A is 3, B is 4
36
In Example 1, B is increased before its value is copied to A. While in Example 2, the value of B is
copied to A and B is later increased.
•
Relational operators ( ==, !=, >, <, >=, <= )
In order to evaluate a comparison between two expressions we can use the Relational operators.
As specified by the ANSI-C++ standard, the result of a relational operation is a bool value that can
only be true or false, according to the result of the comparison.
We may want to compare two expressions, for example, to know if they are equal or if one is
greater than the other. Here is a list of the relational operators that can be performed in C++:
== Equal
!= Different
> Greater than
< Less than
>= Greater or equal than
<= Less or equal than
Here you have some examples:
(7 == 5) would return false.
(5 > 4) would return true.
(3 != 2) would return true.
(6 >= 6) would return true.
(5 < 5) would return false.
of course, instead of using only numberic constants, we can use any valid expression, including
variables. Suppose that a=2, b=3 and c=6,
(a == 5)
would return false.
(a*b >= c)
would return true since (2*3 >= 6) is it.
(b+4 > a*c) would return false since (3+4 > 2*6) is it.
((b=2) == a) would return true.
Be aware. Operator = (one equal sign) is not the same as operator == (two equal signs), the first is
an assignation operator (assigns the right side of the expression to the variable in the left) and the
other (==) is a relational operator of equality that compares whether both expressions in the two
sides of the operator are equal to each other. Thus, in the last expression ((b=2) == a), we first
assigned the value 2 to b and then we compared it to a, that also stores value 2, so the result of
the operation is true.
In many compilers previous to the publication of the ANSI-C++ standard, as well as in the C
language, the relational operations did not return a bool value true or false, rather they returned
an int as result with a value of 0 in order to represent "false" and a value different from 0
(generally 1) to represent "true". For more information, or if your compiler does not support the
bool type, consult the section bool type.
37
•
Logic operators ( !, &&, || ).
Operator ! is equivalent to boolean operation NOT, it has only one operand, located at its right,
and the only thing that it does is to invert the value of it, producing false if its operand is true
and true if its operand is false. It is like saying that it returns the opposite result of evaluating its
operand. For example:
!(5 == 5) returns false because the expression at its right (5 == 5) would be true.
!(6 <= 4) returns true because (6 <= 4) would be false.
!true
returns false.
!false
returns true.
Logic operators && and || are used when evaluating two expressions to obtain a single result.
They correspond with boolean logic operations AND and OR respectively. The result of them
depends on the relation between its two operands:
First
Second
result result
Operand Operand
a && b a || b
a
b
true
true
true
true
true
false
false
true
false
true
false
true
false
false
false
false
For example:
( (5 == 5) && (3 > 6) ) returns false ( true && false ).
( (5 == 5) || (3 > 6)) returns true ( true || false ).
•
Conditional operator ( ? ).
The conditional operator evaluates an expression and returns a different value according to the
evaluated expression, depending on whether it is true or false. Its format is:
condition ? result1 : result2
if condition is true the expression will return result1, if not it will return result2.
7==5 ? 4 : 3
returns 3 since 7 is not equal to 5.
7==5+2 ? 4 : 3 returns 4 since 7 is equal to 5+2.
5>3 ? a : b
returns a, since 5 is greater than 3.
a>b ? a : b
returns the greater one, a or b.
Bitwise Operators ( &, |, ^, ~, <<, >> ).
38
Bitwise operators modify variables considering the bits that represent the values that they store,
that means, their binary representation.
op asm Description
& AND Logical AND
|
OR
Logical OR
^
XOR Logical exclusive OR
~
NOT Complement to one (bit inversion)
<< SHL Shift Left
>> SHR Shift Right
For more information about binary numbers and bitwise operations, consult Boolean logic.
•
Explicit type casting operators
Type casting operators allows you to convert a datum of a given type to another. There are several
ways to do this in C++, the most popular one, compatible with the C language is to precede the
expression to be converted by the new type enclosed between parenthesis ():
int i;
float f = 3.14;
i = (int) f;
The previous code converts the float number 3.14 to an integer value (3). Here, the type casting
operator was (int). Another way to do the same thing in C++ is using the constructor form:
preceding the expression to be converted by the type and enclosing the expression between
parentheses:
i = int ( f );
Both ways of type casting are valid in C++. And additionally ANSI-C++ added new type casting
operators more specific for object oriented programming.
sizeof()
This operator accepts one parameter, that can be either a variable type or a variable itself and
returns the size in bytes of that type or object:
a = sizeof (char);
This
will
return
1
to
a
because
char
is
a
one
byte
long
type.
The value returned by sizeof is a constant, so it is always determined before program execution.
Other operators
Later in the tutorial we will see a few more operators, like the ones referring to pointers or the
specifics for object-oriented programming. Each one is treated in its respective section.
39
3.2 Priority of operators
When making complex expressions with several operands, we may have some doubts about which
operand is evaluated first and which later. For example, in this expression:
a=5+7%2
we may doubt if it really means:
a = 5 + (7 % 2) with result 6, or
a = (5 + 7) % 2 with result 0
The correct answer is the first of the two expressions, with a result of 6. There is an established
order with the priority of each operator, and not only the arithmetic ones (those whose preference
we may already know from mathematics) but for all the operators which can appear in C++. From
greatest to lowest priority, the priority order is as follows:
Priority Operator
Description
Associativity
1
::
Scope
Left
2
() [ ] -> . sizeof
Left
++ --
increment/decrement
~
Complement to one (bitwise)
!
Unary NOT
&*
Reference and Dereference (pointers)
(type)
Type casting
+-
Unary less sign
4
*/%
arithmetical operations
Left
5
+-
arithmetical operations
Left
6
<< >>
bit shifting (bitwise)
Left
7
< <= > >=
Relational operators
Left
8
== !=
Relational operators
Left
9
&^|
Bitwise operators
Left
10
&& ||
Logic operators
Left
11
?:
Conditional
Right
12
= += -= *= /= %=
Assignation
>>= <<= &= ^= |=
Right
13
,
Left
3
Right
Comma, Separator
Associativity defines -in the case that there are several operators of the same priority level- which
one must be evaluated first, the rightmost one or the leftmost one.
40
All these precedence levels for operators can be manipulated or become more legible using
parenthesis signs ( and ), as in this example:
a = 5 + 7 % 2;
might be written as:
a = 5 + (7 % 2); or
a = (5 + 7) % 2;
According to the operation that we wanted to perform.
So if you want to write a complicated expression and you are not sure of the precedence levels,
always include parenthesis. It will probably also be more legible code.
3.3 Communication Through Console
The console is the basic interface of computers, normally it is the set composed of the keyboard
and the screen. The keyboard is generally the standard input device and the screen the standard
Output device.
In the iostream C++ library, standard input and output operations for a program are supported by
two data streams: cin for input and cout for output. Additionally, cerr and clog have also been
implemented - these are two output streams specially designed to show error messages. They can
be redirected to the standard output or to a log file.
Therefore cout (the standard output stream) is normally directed to the screen and cin (the
standard input stream) is normally assigned to the keyboard.
By handling these two streams you will be able to interact with the user in your programs since
you will be able to show messages on the screen and receive his/her input from the keyboard.
3.3.1 Output (cout)
The cout stream is used in conjunction with the overloaded operator << (a pair of "less than"
signs).
cout << "Output sentence"; // prints Output sentence on screen
// prints number 120 on screen
cout << 120;
// prints the content of variable x on screen
cout << x;
The << operator is known as insertion operator since it inserts the data that follows it into the
stream that precedes it. In the examples above it inserted the constant string Output sentence,
the numerical constant 120 and the variable x into the output stream cout. Notice that the first of
the two sentences is enclosed between double quotes (") because it is a string of characters.
Whenever we want to use constant strings of characters we must enclose them between double
quotes (") so that they can be clearly distinguished from variables. For example, these two
sentences are very different:
cout << "Hello";
// prints Hello on screen
41
cout << Hello;
// prints the content of Hello variable on screen
The insertion operator (<<) may be used more than once in a same sentence:
cout << "Hello, " << "I am " << "a C++ sentence";
This last sentence would print the message Hello, I am a C++ sentence on the screen. The utility
of repeating the insertion operator (<<) is demonstrated when we want to print out a combination
of variables and constants or more than one variable:
cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode;
If we suppose that variable age contains the number 24 and the variable zipcode contains 90064
the output of the previous sentence would be:
Hello, I am 24 years old and my zipcode is 90064
It is important to notice that cout does not add a line break after its output unless we explicitly
indicate it, therefore, the following sentences:
cout << "This is a sentence.";
cout << "This is another sentence.";
will be shown followed in screen:
This is a sentence.This is another sentence.
even if we have written them in two different calls to cout. So, in order to perform a line break on
output we must explicitly order it by inserting a new-line character, that in C++ can be written as
\n:
cout << "First sentence.\n ";
cout << "Second sentence.\nThird sentence.";
produces the following output:
First sentence.
Second sentence.
Third sentence.
Additionally, to add a new-line, you may also use the endl manipulator. For example:
cout << "First sentence." << endl;
cout << "Second sentence." << endl;
would print out:
First sentence.
Second sentence.
42
The endl manipulator has a special behavior when it is used with buffered streams: they are
flushed. But anyway cout is unbuffered by default.
You may use either the \n escape character or the endl manipulator in order to specify a line
jump to cout. Notice the differences of use shown earlier.
3.3.2 Input (cin).
Handling the standard input in C++ is done by applying the overloaded operator of extraction (>>)
on the cin stream. This must be followed by the variable that will store the data that is going to be
read. For example:
int age;
cin >> age;
Declares the variable age as an int and then waits for an input from cin (keyborad) in order to
store it in this integer variable.
cin can only process the input from the keyboard once the RETURN key has been pressed.
Therefore, even if you request a single character cin will not process the input until the user
presses RETURN once the character has been introduced.
You must always consider the type of the variable that you are using as a container with cin
extraction. If you request an integer you will get an integer, if you request a character you will get
a character and if you request a string of characters you will get a string of characters.
// i/o example
#include <iostream.h>
Please enter an integer value: 702
The value you entered is 702 and its double
is 1404.
int main ()
{
int i;
cout << "Please enter an integer value: ";
cin >> i;
cout << "The value you entered is " << i;
cout << " and its double is " << i*2 << ".\n";
return 0;
}
The user of a program may be one of the reasons that provoke errors even in the simplest
programs that use cin (like the one we have just seen). Since if you request an integer value and
the user introduces a name (which is a string of characters), the result may cause your program
to misoperate since it is not what we were expecting from the user. So when you use the data
input provided by cin you will have to trust that the user of your program will be totally
cooperative and that he will not introduce his name when an interger value is requested. Farther
ahead, when we will see how to use strings of characters we will see possible solutions for the
errors that can be caused by this type of user input.
43
You can also use cin to request more than one datum input from the user:
cin >> a >> b;
is equivalent to:
cin >> a;
cin >> b;
In both cases the user must give two data, one for variable a and another for variable b that may
be separated by any valid blank separator: a space, a tab character or a newline.
3.4 Control Structures
A program is usually not limited to a linear sequence of instructions. During its process it may
bifurcate, repeat code or take decisions. For that purpose, C++ provides control structures that
serve to specify what has to be done to perform our program.
With the introduction of control sequences we are going to have to introduce a new concept: the
block of instructions. A block of instructions is a group of instructions separated by semicolons
(;) but grouped in a block delimited by curly bracket signs: { and }.
Most of the control structures that we will see in this section allow a generic statement as a
parameter, this refers to either a single instruction or a block of instructions, as we want. If we
want the statement to be a single instruction we do not need to enclose it between curly-brackets
({}). If we want the statement to be more than a single instruction we must enclose them between
curly brackets ({}) forming a block of instructions.
3.4.1 Conditional structure: if and else
It is used to execute an instruction or block of instructions only if a condition is fulfilled. Its form
is:
if (condition) statement
where condition is the expression that is being evaluated. If this condition is true, statement is
executed. If it is false, statement is ignored (not executed) and the program continues on the next
instruction after the conditional structure.
For example, the following code fragment prints out x is 100 only if the value stored in variable x
is indeed 100:
if (x == 100)
cout << "x is 100";
If we want more than a single instruction to be executed in case that condition is true we can
specify a block of instructions using curly brackets { }:
if (x == 100)
{
44
cout << "x is ";
cout << x;
}
We can additionally specify what we want that happens if the condition is not fulfilled by using
the keyword else. Its form used in conjunction with if is:
if (condition) statement1 else statement2
For example:
if (x == 100)
cout << "x is 100";
else
cout << "x is not 100";
prints out on the screen x is 100 if indeed x is worth 100, but if it is not -and only if not- it prints
out x is not 100.
The if + else structures can be concatenated with the intention of verifying a range of values. The
following example shows its use telling if the present value stored in x is positive, negative or none
of the previous, that is to say, equal to zero.
if (x > 0)
cout << "x is positive";
else if (x < 0)
cout << "x is negative";
else
cout << "x is 0";
Remember that in case we want more than a single instruction to be executed, we must group
them in a block of instructions by using curly brackets { }.
3.4.2 Repetitive structures or loops
Loops have as objective to repeat a statement a certain number of times or while a condition is
fulfilled.
The while loop.
Its format is:
while (expression) statement
And its function is simply to repeat statement while expression is true.
For example, we are going to make a program to count down using a while loop:
// custom countdown using while
#include <iostream.h>
int main ()
{
int n;
Enter the starting number > 8
8, 7, 6, 5, 4, 3, 2, 1, FIRE!
45
cout << "Enter the starting number > ";
cin >> n;
while (n>0) {
cout << n << ", ";
--n;
}
cout << "FIRE!";
return 0;
}
When the program starts the user is prompted to insert a starting number for the countdown.
Then the while loop begins, if the value entered by the user fulfills the condition n>0 (that n be
greater than 0 ), the block of instructions that follows will execute an indefinite number of times
while the condition (n>0) remains true.
All the process in the program above can be interpreted according to the following script:
beginning in main:
•
1. User assigns a value to n.
•
2. The while instruction checks if (n>0). At this point there are two possibilities:
o
true: execute statement (step 3,)
o
false: jump statement. The program follows in step 5..
•
3. Execute statement:
cout << n << ", ";
--n;
(prints out n on screen and decreases n by 1).
•
4. End of block. Return Automatically to step 2.
•
5. Continue the program after the block: print out FIRE! and end of program.
We must consider that the loop has to end at some point, therefore, within the block of
instructions (loop's statement) we must provide some method that forces condition to become false
at some moment, otherwise the loop will continue looping forever. In this case we have included -n; that causes the condition to become false after some loop repetitions: when n becomes 0, that
is where our countdown ends.
Of course this is such a simple action for our computer that the whole countdown is performed
instantly without practical delay between numbers.
The do-while loop.
Format:
do statement while (condition);
Its functionality is exactly the same as the while loop except that condition in the do-while is
evaluated after the execution of statement instead of before, granting at least one execution of
statement even if condition is never fulfilled. For example, the following program echoes any
number you enter until you enter 0.
// number echoer
#include <iostream.h>
Enter number (0 to end): 12345
You entered: 12345
46
int main ()
{
unsigned long n;
do {
cout << "Enter number (0 to end): ";
cin >> n;
cout << "You entered: " << n << "\n";
} while (n != 0);
return 0;
}
Enter number (0 to end): 160277
You entered: 160277
Enter number (0 to end): 0
You entered: 0
The do-while loop is usually used when the condition that has to determine its end is determined
within the loop statement, like in the previous case, where the user input within the block of
intructions is what determines the end of the loop. If you never enter the 0 value in the previous
example the loop will never end.
The for loop.
Its format is:
for (initialization; condition; increase) statement;
And its main function is to repeat statement while condition remains true, like the while loop. But
in addition, for provides places to specify an initialization instruction and an increase instruction.
So this loop is specially designed to perform a repetitive action with a counter.
It works the following way:
1. Initialization is executed. Generally it is an initial value setting for a counter varible. This is
executed only once.
2. Condition is checked, if it is true the loop continues, otherwise the loop finishes and statement
is skipped.
3. Statement is executed. As usual, it can be either a single instruction or a block of instructions
enclosed within curly brackets { }.
4. Finally, whatever is specified in the increase field is executed and the loop gets back to step 2.
Here is an example of countdown using a for loop.
// countdown using a for loop
#include <iostream.h>
int main ()
{
for (int n=10; n>0; n--) {
cout << n << ", ";
}
cout << "FIRE!";
return 0;
}
10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!
47
The initialization and increase fields are optional. They can be avoided but not the semicolon signs
among them. For example we could write: for (;n<10;) if we want to specify no initialization and no
increase; or for (;n<10;n++) if we want to include an increase field but not an initialization.
Optionally, using the comma operator (,) we can specify more than one instruction in any of the
fields included in a for loop, like in initialization, for example. The comma operator (,) is an
instruction separator, it serves to separate more than one instruction where only one instruction
is generally expected. For example, suppose that we wanted to intialize more than one variable in
our loop:
for ( n=0, i=100 ; n!=i ; n++, i-- )
{
// whatever here...
}
This loop will execute 50 times if neither n nor i are modified within the loop:
n starts with 0 and i with 100, the condition is (n!=i) (that n be not equal to i). Beacuse n is
increased by one and i decreased by one, the loop's condition will become false after the 50th
loop, when both n and i will be equal to 50.
3.4.3 Bifurcation of control and jumps
The break instruction.
Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to
end an infinite loop, or to force it to end before its natural end. For example, we are going to stop
the count down before it naturally finishes (an engine failure maybe):
// break loop example
#include <iostream.h>
int main ()
{
int n;
for (n=10; n>0; n--) {
cout << n << ", ";
if (n==3)
{
cout << "countdown aborted!";
break;
}
}
return 0;
}
10, 9, 8, 7, 6, 5, 4, 3, countdown
aborted!
The continue instruction.
48
The continue instruction causes the program to skip the rest of the loop in the present iteration as
if the end of the statement block would have been reached, causing it to jump to the following
iteration. For example, we are going to skip the number 5 in our countdown:
// break loop example
#include <iostream.h>
int main ()
{
for (int n=10; n>0; n--) {
if (n==5) continue;
cout << n << ", ";
}
cout << "FIRE!";
return 0;
}
10, 9, 8, 7, 6, 4, 3, 2, 1, FIRE!
The goto instruction.
It allows making an absolute jump to another point in the program. You should use this feature
carefully since its execution ignores any type of nesting limitation.
The destination point is identified by a label, which is then used as an argument for the goto
instruction. A label is made of a valid identifier followed by a colon (:).
This instruction does not have a concrete utility in structured or object oriented programming
aside from those that low-level programming fans may find for it. For example, here is our
countdown loop using goto:
// goto loop example
#include <iostream.h>
int main ()
{
int n=10;
loop:
cout << n << ", ";
n--;
if (n>0) goto loop;
cout << "FIRE!";
return 0;
}
10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!
The exit function.
exit is a function defined in cstdlib (stdlib.h) library.
The purpose of exit is to terminate the running program with an specific exit code. Its prototype is:
void exit (int exit code);
The exit code is used by some operating systems and may be used by calling programs. By
convention, an exit code of 0 means that the program finished normally and any other value
means an error happened.
49
4.4.4 The selective Structure: switch.
The syntax of the switch instruction is a bit peculiar. Its objective is to check several possible
constant values for an expression, something similar to what we did at the beginning of this
section with the linking of several if and else if sentences. Its form is the following:
switch (expression) {
case constant1:
block of instructions 1
break;
case constant2:
block of instructions 2
break;
.
.
.
default:
default block of instructions
}
It works in the following way: switch evaluates expression and checks if it is equivalent to
constant1, if it is, it executes block of instructions 1 until it finds the break keyword, then the
program will jump to the end of the switch selective structure.
If expression was not equal to constant1 it will check if expression is equivalent to constant2. If it
is, it will execute block of instructions 2 until it finds the break keyword.
Finally, if the value of expression has not matched any of the previously specified constants (you
may specify as many case sentences as values you want to check), the program will execute the
instructions included in the default: section, if this one exists, since it is optional.
Both of the following code fragments are equivalent:
if-else equivalent
switch example
Switch (x) {
case 1:
cout << "x is 1";
break;
case 2:
cout << "x is 2";
break;
default:
cout << "value of x unknown";
}
if (x == 1) {
cout << "x is 1";
}
else if (x == 2) {
cout << "x is 2";
}
else {
cout << "value of x unknown";
}
I have commented before that the syntax of the switch instruction is a bit peculiar. Notice the
inclusion of the break instructions at the end of each block. This is necessary because if, for
example, we did not include it after block of instructions 1 the program would not jump to the end
of the switch selective block (}) and it would continue executing the rest of the blocks of
instructions until the first appearance of the break instruction or the end of the switch selective
block. This makes it unnecessary to include curly brackets { } in each of the cases, and it can also
be useful to execute the same block of instructions for different possible values for the expression
evaluated. For example:
50
Switch (x) {
case 1:
case 2:
case 3:
cout << "x is 1, 2 or 3";
break;
default:
cout << "x is not 1, 2 nor 3";
}
Notice that switch can only be used to compare an expression with different constants. Therefore
we cannot put variables (case (n*2):) or ranges (case (1..3):) because they are not valid constants.
51
UNIT 4
ARRAY AND POINTER
Contents
4.1
Arrays.
4.1.1 Initializing arrays
4.1.2 Access to the values of an Array
4.1.3 Multidimensional Arrays
4.1.4 Arrays as parameters
4.2
Strings
4.2.1 1Initialization of strings
4.2.2 Assigning values to strings
4.2.3 Converting strings to other types
4.2.4 Functions to manipulate strings
4.3
Pointers.
4.4
Pointers and arrays
4.5
Dynamic Memory.
4.1. Arrays
Arrays are a series of elements (variables) of the same type placed consecutively in memory that
can be individually referenced by adding an index to a unique name.
That means that, for example, we can store 5 values of type int without having to declare 5
different variables each with a different identifier. Instead, using an array we can store 5 different
values of the same type, int for example, with a unique identifier.
For example, an array to contain 5 integer values of type int called billy could be represented this
way:
where each blank panel represents an element of the array, that in this case are integer values of
type int. These are numbered from 0 to 4 since in arrays the first index is always 0,
independently of its length .
Like any other variable, an array must be declared before it is used. A typical declaration for an
array in C++ is:
type name [elements];
where type is a valid object type (int, float...), name is a valid variable identifier and the elements
field, that is enclosed within brackets [], specifies how many of these elements the array contains.
Therefore, to declare billy as shown above it is as simple as the following sentence:
int billy [5];
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NOTE: The elements field within brackets [] when declaring an array must be a constant value,
since arrays are blocks of static memory of a given size and the compiler must be able to
determine exactly how much memory it must assign to the array before any instruction is
considered.
4.1.1 Initializing arrays.
When declaring an array of local scope (within a function), if we do not specify otherwise, it will
not be initialized, so its content is undetermined until we store some values in it.
If we declare a global array (outside any function) its content will be initialized with all its
elements filled with zeros. Thus, if in the global scope we declare:
int billy [5];
every element of billy will be set initialy to 0:
But additionally, when we declare an Array, we have the possibility to assign initial values to each
one of its elements using curly brackets { }. For example:
int billy [5] = { 16, 2, 77, 40, 12071 };
this declaration would have created an array like the following one:
The number of elements in the array that we initialized within curly brackets { } must match the
length in elements that we declared for the array enclosed within square brackets [ ]. For example,
in the example of the billy array we have declared that it had 5 elements and in the list of initial
values within curly brackets { } we have set 5 different values, one for each element.
Because this can be considered useless repetition, C++ includes the possibility of leaving the
brackets empty [ ] and the size of the Array will be defined by the number of values included
between curly brackets { }:
int billy [] = { 16, 2, 77, 40, 12071 };
4.1.2 Access to the values of an Array
In any point of the program in which the array is visible we can access individually anyone of its
values for reading or modifying as if it was a normal variable. The format is the following:
name[index]
53
Following the previous examples in which billy had 5 elements and each of those elements was of
type int, the name which we can use to refer to each element is the following:
For example, to store the value 75 in the third element of billy a suitable sentence would be:
billy[2] = 75;
and, for example, to pass the value of the third element of billy to the variable a, we could write:
a = billy[2];
Therefore, for all purposes, the expression billy[2] is like any other variable of type int.
Notice that the third element of billy is specified billy[2], since first is billy[0], the second is
billy[1], and therefore, third is billy[2]. By this same reason, its last element is billy[4]. Since if
we wrote billy[5], we would be acceding to the sixth element of billy and therefore exceeding the
size of the array.
In C++ it is perfectly valid to exceed the valid range of indices for an Array, which can create
problems since they do not cause compilation errors but they can cause unexpected results or
serious errors during execution. The reason why this is allowed will be seen farther ahead when
we begin to use pointers.
At this point it is important to be able to clearly distinguish between the two uses that brackets [ ]
have related to arrays. They perform two differt tasks: one is to set the size of arrays when
declaring them; and second is to specify indices for a concrete array element when referring to it.
We must simply take care not to confuse these two possible uses of brackets [ ] with arrays:
int billy[5];
// declaration of a new Array (begins
with a type name)
// access to an element of the Array.
billy[2] = 75;
Other valid operations with arrays:
billy[0] = a;
billy[a] = 75;
b = billy [a+2];
billy[billy[a]] = billy[2] + 5;
// arrays example
#include <iostream.h>
12206
int billy [] = {16, 2, 77, 40, 12071};
int n, result=0;
int main ()
{
for ( n=0 ; n<5 ; n++ )
{
result += billy[n];
}
54
cout << result;
return 0;
}
4.1.3 Multidimensional Arrays
Multidimensional arrays can be described as arrays of arrays. For example, a bidimensional array
can be imagined as a bidimensional table of a uniform concrete data type.
jimmy represents a bidimensional array of 3 per 5 values of type int. The way to declare this array
would be:
int jimmy [3][5];
and, for example, the way to reference the second element vertically and fourth horizontally in an
expression would be:
jimmy[1][3]
(remember that array indices always begin by 0).
Multidimensional arrays are not limited to two indices (two dimensions). They can contain as
many indices as needed, although it is rare to have to represent more than 3 dimensions. Just
consider the amount of memory that an array with many indices may need. For example:
char century [100][365][24][60][60];
assigns a char for each second contained in a century, that is more than 3 billion chars! This
would consume about 3000 megabytes of RAM memory if we could declare it.
Multidimensional arrays are nothing more than an abstraction, since we can obtain the same
results with a simple array just by putting a factor between its indices:
int jimmy [3][5]; is equivalent to
int jimmy [15]; (3 * 5 = 15)
55
With the only difference that the compiler remembers for us the depth of each imaginary
dimension. Serve as example these two pieces of code, with exactly the same result, one using
bidimensional arrays and the other using only simple arrays:
// multidimensional array
#include <iostream.h>
// pseudo-multidimensional array
#include <iostream.h>
#define WIDTH 5
#define HEIGHT 3
#define WIDTH 5
#define HEIGHT 3
int jimmy [HEIGHT][WIDTH];
int n,m;
int jimmy [HEIGHT * WIDTH];
int n,m;
int main ()
{
for (n=0;n<HEIGHT;n++)
for (m=0;m<WIDTH;m++)
{
jimmy[n][m]=(n+1)*(m+1);
}
return 0;
}
int main ()
{
for (n=0;n<HEIGHT;n++)
for (m=0;m<WIDTH;m++)
{
jimmy[n * WIDTH + m]=(n+1)*(m+1);
}
return 0;
}
none of the programs above produce any output on the screen, but both assign values to the
memory block called jimmy in the following way:
We have used defined constants (#define) to simplify possible future modifications of the
program, for example, in case that we decided to enlarge the array to a height of 4 instead of 3 it
could be done by changing the line:
#define HEIGHT 3
to
#define HEIGHT 4
with no need to make any other modifications to the program.
4.1.4 Arrays as parameters
At some moment we may need to pass an array to a function as a parameter. In C++ is not
possible to pass by value a complete block of memory as a parameter to a function, even if it is
ordered as an array, but it is allowed to pass its address. This has almost the same practical effect
and it is a much faster and more efficient operation.
56
In order to admit arrays as parameters the only thing that we must do when declaring the
function is to specify in the argument the base type for the array, an identifier and a pair of void
brackets []. For example, the following function:
void procedure (int arg[])
admits a parameter of type "Array of int" called arg. In order to pass to this function an array
declared as:
int myarray [40];
it would be enough to write a call like this:
procedure (myarray);
Here you have a complete example:
// arrays as parameters
#include <iostream.h>
5 10 15
2 4 6 8 10
void printarray (int arg[], int length) {
for (int n=0; n<length; n++)
cout << arg[n] << " ";
cout << "\n";
}
int main ()
{
int firstarray[] = {5, 10, 15};
int secondarray[] = {2, 4, 6, 8, 10};
printarray (firstarray,3);
printarray (secondarray,5);
return 0;
}
As you can see, the first argument (int arg[]) admits any array of type int, wathever its length is.
For that reason we have included a second parameter that tells the function the length of each
array that we pass to it as the first parameter. This allows the for loop that prints out the array to
know the range to check in the passed array.
In a function declaration is also possible to include multidimensional arrays. The format for a
tridimensional array is:
base_type[][depth][depth]
for example, a function with a multidimensional array as argument could be:
void procedure (int myarray[][3][4])
notice that the first brackets [] are void and the following ones are not. This must always be thus
because the compiler must be able to determine within the function which is the depth of each
additional dimension.
57
Arrays, both simple or multidimensional, passed as function parameters are a quite common
source of errors for less experienced programmers.
4.2
Strings
In all programs seen until now, we have used only numerical variables, used to express numbers
exclusively. But in addition to numerical variables there also exist strings of characters, that allow
us to represent successions of characters, like words, sentences, names, texts, et cetera. Until
now we have only used them as constants, but we have never considered variables able to contain
them.
In C++ there is no specific elemental variable type to store strings of characters. In order to fulfill
this feature we can use arrays of type char, which are successions of char elements. Remember
that this data type (char) is the one used to store a single character, for that reason arrays of
them are generally used to make strings of single characters.
For example, the following array (or string of characters):
char jenny [20];
can store a string up to 20 characters long. You may imagine it thus:
This maximum size of 20 characters is not required to always be fully used. For example, jenny
could store at some moment in a program either the string of characters "Hello" or the string
"Merry christmas". Therefore, since the array of characters can store shorter strings than its total
length, a convention has been reached to end the valid content of a string with a null character,
whose constant can be written 0 or '\0'.
We could represent jenny (an array of 20 elements of type char) storing the strings of characters
"Hello" and "Merry Christmas" in the following way:
Notice how after the valid content a null character ('\0') it is included in order to indicate the end
of the string. The panels in gray color represent indeterminate values.
4.2.1 Initialization of strings
Because strings of characters are ordinary arrays they fulfill all their same rules. For example, if
we want to initialize a string of characters with predetermined values we can do it just like any
other array:
char mystring[] = { 'H', 'e', 'l', 'l', 'o', '\0' };
In this case we would have declared a string of characters (array) of 6 elements of type char
initialized with the characters that compose Hello plus a null character '\0'.
58
Nevertheless, strings of characters have an additional way to initialize their values: using constant
strings.
In the expressions we have used in examples in previous chapters constants that represented
entire strings of characters have already appeared several times. These are specified enclosed
between double quotes ("), for example:
"the result is: "
is a constant string that we have probably used on some occasion.
Unlike single quotes (') which specify single character constants, double quotes (") are constants
that specify a succession of characters. Strings enclosed between double quotes always have a
null character ('\0') automatically appended at the end.
Therefore we could initialize the string mystring with values by either of these two ways:
char mystring [] = { 'H', 'e', 'l', 'l', 'o', '\0' };
char mystring [] = "Hello";
In both cases the array or string of characters mystring is declared with a size of 6 characters
(elements of type char): the 5 characters that compose Hello plus a final null character ('\0')
which specifies the end of the string and that, in the second case, when using double quotes (") it
is automatically appended.
Before going further, notice that the assignation of multiple constants like double-quoted
constants (") to arrays are only valid when initializing the array, that is, at the moment when
declared. Expressions within the code like:
mystring = "Hello";
mystring[] = "Hello";
are not valid for arrays, like neither would be:
mystring = { 'H', 'e', 'l', 'l', 'o', '\0' };
So remember: We can "assign" a multiple constant to an Array only at the moment of initializing
it. The reason will be more comprehensible when you know a bit more about pointers, since then
it will be clarified that an array is simply a constant pointer pointing to an allocated block of
memory. And because of this constantnes, the array itself can not be assigned any value, but we
can assing values to each of the elements of the array.
The moment of initializing an Array it is a special case, since it is not an assignation, although the
same equal sign (=) is used. Anyway, always have the rule previously underlined present.
4.2.2 Assigning values to strings
Since the lvalue of an assignation can only be an element of an array and not the entire array, it
would be valid to assign a string of characters to an array of char using a method like this:
mystring[0]
mystring[1]
mystring[2]
mystring[3]
=
=
=
=
'H';
'e';
'l';
'l';
59
mystring[4] = 'o';
mystring[5] = '\0';
But as you may think, this does not seem to be a very practical method. Generally for assigning
values to an array, and more specifically to a string of characters, a series of functions like strcpy
are used. strcpy (string copy) is defined in the cstring (string.h) library and can be called the
following way:
strcpy (string1, string2);
This does copy the content of string2 into string1. string2 can be either an array, a pointer, or a
constant string, so the following line would be a valid way to assign the constant string "Hello" to
mystring:
strcpy (mystring, "Hello");
For example:
// setting value to string
#include <iostream.h>
#include <string.h>
J. Soulie
int main ()
{
char szMyName [20];
strcpy (szMyName,"J. Soulie");
cout << szMyName;
return 0;
}
Notice that we needed to include <string.h> header in order to be able to use function strcpy.
Although we can always write a simple function like the following setstring with the same
operation as cstring's strcpy:
J. Soulie
// setting value to string
#include <iostream.h>
void setstring (char szOut [], char szIn [])
{
int n=0;
do {
szOut[n] = szIn[n];
} while (szIn[n++] != '\0');
}
int main ()
{
char szMyName [20];
setstring (szMyName,"J. Soulie");
cout << szMyName;
return 0;
}
60
Another frequently used method to assign values to an array is by directly using the input stream
(cin). In this case the value of the string is assigned by the user during program execution.
When cin is used with strings of characters it is usually used with its getline method, that can be
called following this prototype:
cin.getline ( char buffer[], int length, char delimiter = ' \n');
where buffer is the address of where to store the input (like an array, for example), length is the
maximum length of the buffer (the size of the array) and delimiter is the character used to
determine the end of the user input, which by default - if we do not include that parameter - will
be the newline character ('\n').
The following example repeats whatever you type on your keyboard. It is quite simple but serves
as an example of how you can use cin.getline with strings:
// cin with strings
#include <iostream.h>
int main ()
{
char mybuffer [100];
cout << "What's your name? ";
cin.getline (mybuffer,100);
cout << "Hello " << mybuffer << ".\n";
cout << "Which is your favourite team? ";
cin.getline (mybuffer,100);
cout << "I like " << mybuffer << " too.\n";
return 0;
}
What's your name? Juan
Hello Juan.
Which is your favourite team? Inter Milan
I like Inter Milan too.
Notice how in both calls to cin.getline we used the same string identifier (mybuffer). What the
program does in the second call is simply step on the previous content of buffer with the new one
that is introduced.
If you remember the section about communication through the console, you will remember that
we used the extraction operator (>>) to receive data directly from the standard input. This method
can also be used instead of cin.getline with strings of characters. For example, in our program,
when we requested an input from the user we could have written:
cin >> mybuffer;
This would work, but this method has the following limitations that cin.getline has not:
•
It can only receive single words (no complete sentences) since this method uses as a
delimiter any occurrence of a blank character, including spaces, tabulators, newlines and
carriage returns.
•
It is not allowed to specify a size for the buffer. That makes your program unstable in case
the user input is longer than the array that will host it.
For these reasons it is recommended that whenever you require strings of characters coming from
cin you use cin.getline instead of cin >>.
61
4.2.3 Converting strings to other types
Due to that a string may contain representations of other data types like numbers, it might be
useful to translate that content to a variable of a numeric type. For example, a string may contain
"1977", but this is a sequence of 5 chars not so easily convertable to a single integer data type.
The cstdlib (stdlib.h) library provides three useful functions for this purpose:
•
atoi: converts string to int type.
•
atol: converts string to long type.
•
atof: converts string to float type.
All of these functions admit one parameter and return a value of the requested type (int, long or
float). These functions combined with getline method of cin are a more reliable way to get the
user input when requesting a number than the classic cin>> method:
Enter price: 2.75
Enter quantity: 21
Total price: 57.75
// cin and ato* functions
#include <iostream.h>
#include <stdlib.h>
int main ()
{
char mybuffer [100];
float price;
int quantity;
cout << "Enter price: ";
cin.getline (mybuffer,100);
price = atof (mybuffer);
cout << "Enter quantity: ";
cin.getline (mybuffer,100);
quantity = atoi (mybuffer);
cout << "Total price: " << price*quantity;
return 0;
}
4.2.4
Functions to manipulate strings
The cstring library (string.h) defines many functions to perform manipulation operations with Clike strings (like already explained strcpy). Here you have a brief look at the most usual:
strcat: char* strcat (char* dest, const char* src);
Appends src string at the end of dest string. Returns dest.
strcmp: int strcmp (const char* string1, const char* string2);
Compares strings string1 and string2. Returns 0 is both strings are equal.
strcpy: char* strcpy (char* dest, const char* src);
Copies the content of src to dest. Returns dest.
strlen: size_t strlen (const char* string);
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Returns the length of string.
NOTE: char* is the same as char[]
4.3 Pointers
We have already seen how variables are memory cells that we can access by an identifier. But
these variables are stored in concrete places of the computer memory. For our programs, the
computer memory is only a succession of 1 byte cells (the minimum size for a datum), each one
with a unique address.
A good simile for the computer memory can be a street in a city. On a street all houses are
consecutively numbered with an unique identifier so if we talk about 27th of Sesame Street we
will be able to find that place without trouble, since there must be only one house with that
number and, in addition, we know that the house will be between houses 26 and 28.
In the same way in which houses in a street are numbered, the operating system organizes the
memory with unique and consecutive numbers, so if we talk about location 1776 in the memory,
we know that there is only one location with that address and also that is between addresses
1775 and 1777.
Address (dereference) operator (&).
At the moment in which we declare a variable it must be stored in a concrete location in this
succession of cells (the memory). We generally do not decide where the variable is to be placed fortunately that is something automatically done by the compiler and the operating system at
runtime, but once the operating system has assigned an address there are some cases in which
we may be interested in knowing where the variable is stored.
This can be done by preceding the variable identifier by an ampersand sign (&), which literally
means "address of". For example:
ted = &andy;
would assign to variable ted the address of variable andy, since when preceding the name of the
variable andy with the ampersand (&) character we are no longer talking about the content of the
variable, but about its address in memory.
We are going to suppose that andy has been placed in the memory address 1776 and that we
write the following:
andy = 25;
fred = andy;
ted = &andy;
the result is shown in the following diagram:
63
We have assigned to fred the content of variable andy as we have done in many other occasions
in previous sections of this tutorial, but to ted we have assigned the address in memory where the
operating system stores the value of andy, that we have imagined was 1776 (it can be any
address, I have just invented this one). The reason is that in the allocation of ted we have
preceded andy with an ampersand (&) character.
The variable that stores the address of another variable (like ted in the previous example) is what
we call a pointer. In C++ pointers have certain virtues and they are used very often. Farther
ahead we will see how this type of variable is declared.
Reference operator (*)
Using a pointer we can directly access the value stored in the variable pointed by it just by
preceding the pointer identifier with the reference operator asterisk (*), that can be literally
translated to "value pointed by". Therefore, following with the values of the previous example, if
we write:
beth = *ted;
(that we could read as: "beth equal to value pointed by ted") beth would take the value 25, since
ted is 1776, and the value pointed by 1776 is 25.
You must clearly differenciate that ted stores 1776, but *ted (with an asterisk * before) refers to
the value stored in the address 1776, that is 25. Notice the difference of including or not
including the reference asterisk (I have included an explanatory commentary of how each
expression could be read):
beth = ted; // beth equal to ted ( 1776 )
beth = *ted; // beth equal to value pointed by ted ( 25 )
Operator of address or dereference (&)
It is used as a variable prefix and can be translated as "address of", thus: &variable1 can be read
as "address of variable1"
Operator of reference (*)
It indicates that what has to be evaluated is the content pointed by the expression considered as
an address. It can be translated by "value pointed by".
64
* mypointer can be read as "value pointed by mypointer".
At this point, and following with the same example initiated above where:
andy = 25;
ted = &andy;
you should be able to clearly see that all the following expressions are true:
andy == 25
&andy == 1776
ted == 1776
*ted == 25
The first expression is quite clear considering that its assignation was andy=25;. The second one
uses the address (or derefence) operator (&) that returns the address of the variable andy, that we
imagined to be 1776. The third one is quite obvious since the second was true and the
assignation of ted was ted = &andy;. The fourth expression uses the reference operator (*) that,
as we have just seen, is equivalent to the value contained in the address pointed by ted, that is
25.
So, after all that, you may also infer that while the address pointed by ted remains unchanged the
following expression will also be true:
*ted == andy
Declaring variables of type pointer
Due to the ability of a pointer to directly reference the value that it point to, it becomes necessary
to specify which data type a pointer points to when declaring it. It is not the same to point to a
char as it is to point to an int or a float type.
Therefore, the declaration of pointers follows this form:
type * pointer_name;
where type is the type of data pointed, not the type of the pointer itself. For example:
int * number;
char * character;
float * greatnumber;
They are three declarations of pointers. Each one points to a different data type, but the three are
pointers and in fact the three occupy the same amount of space in memory (the size of a pointer
depends on the operating system), but the data to which they point do not occupy the same
amount of space nor are of the same type, one is int, another one is char and the other one float.
I emphasize that the asterisk (*) that we use when declaring a pointer means only that it is a
pointer, and should not be confused with the reference operator that we have seen a bit earlier
which is also written with an asterisk (*). They are simply two different tasks represented with the
same sign.
// my first pointer
#include <iostream.h>
value1==10 / value2==20
int main ()
{
int value1 = 5, value2 = 15;
65
int * mypointer;
mypointer = &value1;
*mypointer = 10;
mypointer = &value2;
*mypointer = 20;
cout << "value1==" << value1 << "/ value2=="
<< value2;
return 0;
}
Notice how the values of value1 and value2 have changed indirectly. First we have assigned to
mypointer the address of value1 using the deference ampersand sign (&). Then we have assigned
10 to the value pointed by mypointer, which is pointing to the address of value1, so we have
modified value1 indirectly.
In order that you can see that a pointer may take several different values during the same
program we have repeated the process with value2 and the same pointer.
Here is an example a bit more complicated:
value1==10 / value2==20
// more pointers
#include <iostream.h>
int main ()
{
int value1 = 5, value2 = 15;
int *p1, *p2;
p1 = &value1;
p2 = &value2;
*p1 = 10;
*p2 = *p1;
p1 = p2;
*p1 = 20;
// p1 = address of value1
// p2 = address of value2
// value pointed by p1 = 10
// value pointed by p2 = value pointed by p1
// p1 = p2 (value of pointer copied)
// value pointed by p1 = 20
cout << "value1==" << value1 << "/ value2==" << value2;
return 0;
}
I have included as comments on each line how the code can be read: ampersand (&) as "address
of" and asterisk (*) as "value pointed by". Notice that there are expressions with pointers p1 and
p2 with and without the asterisk. The meaning of using or not using a reference asterisk is very
different: An asterisk (*) followed by the pointer refers to the place pointed by the pointer, whereas
a pointer without an asterisk (*) refers to the value of the pointer itself, that is, the address of
where it is pointing.
Another thing that can call your attention is the line:
int *p1, *p2;
66
That declares the two pointers of the previous example putting an asterisk (*) for each pointer.
The reason is that the type for all the declarations of the same line is int (and not int*). The
explanation is because of the level of precedence of the reference operator asterisk (*) that is the
same as the declaration of types, therefore, because they are associative operators from the right,
the asterisks are evaluated first than the type. We have talked about this in, although it is enough
that you know clearly that -unless you include parenthesis- you will have to put an asterisk (*)
before each pointer that you declare.
4.4 Pointers and arrays
The concept of array is very much bound to the one of pointer. In fact, the identifier of an array is
equivalent to the address of its first element, like a pointer is equivalent to the address of the first
element that it points to, so in fact they are the same thing. For example, supposing these two
declarations:
int numbers [20];
int * p;
the following allocation would be valid:
p = numbers;
At this point p and numbers are equivalent and they have the same properties, the only difference
is that we could assign another value to the pointer p whereas numbers will always point to the
first of the 20 integer numbers of type int with which it was defined. So, unlike p, that is an
ordinary variable pointer, numbers is a constant pointer (indeed an array name is a constant
pointer). Therefore, although the previous expression was valid, the following allocation is not:
numbers = p;
because numbers is an array (constant pointer), and no values can be assigned to constant
identifiers.
Due to the character of variables all the expressions that include pointers in the following
example are perfectly valid:
// more pointers
#include <iostream.h>
10, 20, 30, 40, 50,
int main ()
{
int numbers[5];
int * p;
p = numbers; *p = 10;
p++; *p = 20;
p = &numbers[2]; *p = 30;
p = numbers + 3; *p = 40;
p = numbers; *(p+4) = 50;
for (int n=0; n<5; n++)
cout << numbers[n] << ", ";
67
return 0;
}
In chapter "Arrays" we used bracket signs [] several times in order to specify the index of the
element of the Array to which we wanted to refer. Well, the bracket signs operator [] are known as
offset operators and they are equivalent to adding the number within brackets to the address of a
pointer. For example, both following expressions:
a[5] = 0;
// a [offset of 5] = 0
*(a+5) = 0;
// pointed by (a+5) = 0
are equivalent and valid either if a is a pointer or if it is an array.
Pointer initialization
When declaring pointers we may want to explicitly specify to which variable we want them to
point,
int number;
int *tommy = &number;
this is equivalent to:
int number;
int *tommy;
tommy = &number;
When a pointer assignation takes place we are always assigning the address where it points to,
never the value pointed. You must consider that at the moment of declaring a pointer, the asterisk
(*) indicates only that it is a pointer, it in no case indicates the reference operator (*). Remember,
they are two different operators, although they are written with the same sign. Thus, we must
take care not to confuse the previous with:
int number;
int *tommy;
*tommy = &number;
That anyway would not have much sense in this case.
As in the case of arrays, the compiler allows the special case that we want to initialize the content
at which the pointer points with constants at the same moment as declaring the variable pointer:
char * terry = "hello";
In this case static storage is reserved for containing "hello" and a pointer to the first char of this
memory block (that corresponds to 'h') is assigned to terry. If we imagine that "hello" is stored at
addresses 1702 and following, the previous declaration could be outlined thus:
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It is important to indicate that terry contains the value 1702 and not 'h' nor "hello", although
1702 points to these characters.
The pointer terry points to a string of characters and can be used exactly as if it was an Array
(remember that an array is just a constant pointer). For example, if our temper changed and we
wanted to replace the 'o' by a '!' sign in the content pointed by terry, we could do it by any of the
following two ways:
terry[4] = '!';
*(terry+4) = '!';
Remember that to write terry[4] is just the same as to write *(terry+4), although the most usual
expression is the first one. With either of those two expressions something like this would happen:
Arithmetic of pointers
To conduct arithmetical operations on pointers is a little different than to conduct them on other
integer data types. To begin with, only addition and subtraction operations are allowed to be
conducted, the others make no sense in the world of pointers. But both addition and subtraction
have a different behavior with pointers according to the size of the data type to which they point.
When we saw the different data types that exist, we saw that some occupy more or less space
than others in the memory. For example, in the case of integer numbers, char occupies 1 byte,
short occupies 2 bytes and long occupies 4.
Let's suppose that we have 3 pointers:
char *mychar;
short *myshort;
long *mylong;
And that we know that they point to memory locations 1000, 2000 and 3000 respectively.
So if we write:
mychar++;
myshort++;
mylong++;
mychar, as you may expect, would contain the value 1001. Nevertheless, myshort would contain
the value 2002, and mylong would contain 3004. The reason is that when adding 1 to a pointer
69
we are making it to point to the following element of the same type with which it has been defined,
and therefore the size in bytes of the type pointed is added to the pointer.
This is applicable both when adding and subtracting any number to a pointer. It would happen
exactly the same if we write:
mychar = mychar + 1;
myshort = myshort + 1;
mylong = mylong + 1;
It is important to warn you that both increase (++) and decrease (--) operators have a greater
priority than the reference operator asterisk (*), therefore the following expressions may lead to
confussion:
*p++;
*p++ = *q++;
The first one is equivalent to *(p++) and what it does is to increase p (the address where it points
to - not the value that contains).
In the second, because both increase operators (++) are after the expressions to be evaluated and
not before, first the value of *q is assigned to *p and then both q and p are increased by one. It is
equivalent to:
*p = *q;
p++;
q++;
Like always, I recommend you use parenthesis () in order to avoid unexpected results.
Pointers to pointers
C++ allows the use of pointers that point to pointers, that these, in its turn, point to data. In order
to do that we only need to add an asterisk (*) for each level of reference:
char a;
char * b;
char ** c;
a = 'z';
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b = &a;
c = &b;
this, supposing the randomly chosen memory locations of 7230, 8092 and 10502, could be
described thus:
(inside the cells there is the content of the variable; under the cells its location)
The new thing in this example is variable c, which we can talk about in three different ways, each
one of them would correspond to a different value:
c is a variable of type (char **) with a value of 8092
*c is a variable of type (char*) with a value of 7230
**c is a variable of type (char) with a value of'z'
void pointers
The type of pointer void is a special type of pointer. void pointers can point to any data type, from
an integer value or a float to a string of characters. Its sole limitation is that the pointed data
cannot be referenced directly (we can not use reference asterisk * operator on them), since its
length is always undetermined, and for that reason we will always have to resort to type casting or
assignations to turn our void pointer to a pointer of a concrete data type to which we can refer.
One of its utilities may be for passing generic parameters to a function:
6, 10, 13
// integer increaser
#include <iostream.h>
void increase (void* data, int type)
{
switch (type)
{
case sizeof(char) : (*((char*)data))++; break;
case sizeof(short): (*((short*)data))++; break;
case sizeof(long) : (*((long*)data))++; break;
}
}
int main ()
{
char a = 5;
short b = 9;
long c = 12;
increase (&a,sizeof(a));
increase (&b,sizeof(b));
increase (&c,sizeof(c));
cout << (int) a << ", " << b << ", " << c;
return 0;
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}
sizeof is an operator integrated in the C++ language that returns a constant value with the size in
bytes of its parameter, so, for example, sizeof(char) is 1, because char type is 1 byte long.
Pointers to functions
C++ allows operations with pointers to functions. The greatest use of this is for passing a function
as a parameter to another function, since these cannot be passed dereferenced. In order to
declare a pointer to a function we must declare it like the prototype of the function except the
name of the function is enclosed between parenthesis () and a pointer asterisk (*) is inserted
before the name. It might not be a very handsome syntax, but that is how it is done in C++:
8
// pointer to functions
#include <iostream.h>
int addition (int a, int b)
{ return (a+b); }
int subtraction (int a, int b)
{ return (a-b); }
int (*minus)(int,int) = subtraction;
int operation (int x, int y, int (*functocall)(int,int))
{
int g;
g = (*functocall)(x,y);
return (g);
}
int main ()
{
int m,n;
m = operation (7, 5, addition);
n = operation (20, m, minus);
cout <<n;
return 0;
}
In the example, minus is a global pointer to a function that has two parameters of type int, it is
immediately assigned to point to the function subtraction, all in a single line:
int (* minus)(int,int) = subtraction;
4.5. Dynamic Memory
Until now, in our programs, we have only had as much memory as we have requested in
declarations of variables, arrays and other objects that we included, having the size of all of them
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fixed before the execution of the program. But, what if we need a variable amount of memory that
can only be determined during the program execution (runtime), for example, in case that we need
an user input to determine the necessary amount of space?
The answer is dynamic memory, for which C++ integrates the operators new and delete.
Operators new and delete are exclusive of C++. Farther ahead in this section are shown the C
equivalents for these operators.
Operators new and new[ ]
In order to request dynamic memory, the operator new exists. new is followed by a data type and
optionally the number of elements required within brackets []. It returns a pointer to the
beginning of the new block of assigned memory. Its form is:
pointer = new type
or
pointer = new type [elements]
The first expression is used to assign memory to contain one single element of type. The second
one is used to assign a block (an array) of elements of type.
For example:
int * bobby;
bobby = new int [5];
In this case, the operating system has assigned space for 5 elements of type int in a heap and it
has returned a pointer to its beginning that has been assigned to bobby. Therefore, now, bobby
points to a valid block of memory with space for 5 int elements.
You could ask what is the difference between declaring a normal array and assigning memory to a
pointer as we have just done. The most important one is that the size of an array must be a
constant value, which limits its size to what we decide at the moment of designing the program
before its execution, whereas the dynamic memory allocation allows assigning memory during the
execution of the program using any variable, constant or combination of both as size.
The dynamic memory is generally managed by the operating system, and in multitask interfaces it
can be shared between several applications, so there is a possibility that the memory exhausts. If
this happens and the operating system cannot assign the memory that we request with the
operator new, a null pointer will be returned. For that reason it is recommended to always check
to see if the returned pointer is null after a call to new.
int * bobby;
bobby = new int [5];
if (bobby == NULL) {
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// error assigning memory. Take measures.
};
Operator delete.
Since the necessity of dynamic memory is usually limited to concrete moments within a program,
once it is no longer needed it should be freed so that it becomes available for future requests of
dynamic memory. The operator delete exists for this purpose, whose form is:
delete pointer;
or
delete [] pointer;
The first expression should be used to delete memory alloccated for a single element, and the
second one for memory allocated for multiple elements (arrays). In most compilers both
expressions are equivalent and can be used without distinction, although indeed they are two
different operators and so must be considered for operator overloading.
// rememb-o-matic
#include <iostream.h>
#include <stdlib.h>
int main ()
{
char input [100];
int i,n;
long * l;
cout << "How many numbers do you want to
type in? ";
cin.getline (input,100); i=atoi (input);
l= new long[i];
if (l == NULL) exit (1);
for (n=0; n<i; n++)
{
cout << "Enter number: ";
cin.getline (input,100); l[n]=atol (input);
}
cout << "You have entered: ";
for (n=0; n<i; n++)
cout << l[n] << ", ";
delete[] l;
return 0;
}
How many numbers do you want to type in?
5
Enter number : 75
Enter number : 436
Enter number : 1067
Enter number : 8
Enter number : 32
You have entered: 75, 436, 1067, 8, 32,
This simple example that memorizes numbers does not have a limited amount of numbers that
can be introduced, thanks to us requesting to the system to provide as much space as is
necessary to store all the numbers that the user wishes to introduce.
NULL is a constant value defined in manyfold C++ libraries specially designed to indicate null
pointers. In case that this constant is not defined you can do it yourself by defining it to 0:
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#define NULL 0
It is indifferent to put 0 or NULL when checking pointers, but the use of NULL with pointers is
widely extended and it is recommended for greater legibility. The reason is that a pointer is rarely
compared or set directly to a numerical literal constant except precisely number 0, and this way
this action is symbolically masked.
Dynamic memory in ANSI-C
Operators new and delete are exclusive of C++ and they are not available in C language. In C
language, in order to assign dynamic memory we have to resort to the library stdlib.h. We are
going to see them, since they are also valid in C++ and they are used in some existing programs.
The function malloc
It is the generic function to assign dynamic memory to pointers. Its prototype is:
void * malloc (size_t nbytes);
where nbytes is the number of bytes that we want to be assigned to the pointer. The function
returns a pointer of type void*, which is the reason why we have to type cast the value to the type
of the destination pointer, for example:
char * ronny;
ronny = (char *) malloc (10);
This assigns to ronny a pointer to an usable block of 10 bytes. When we want to assign a block of
data of a different type other than char (different from 1 byte) we must multiply the number of
elements desired by the size of each element. Luckyly we have at our disposition the operator
sizeof, that returns the size of the type of a concrete datum.
int * bobby;
bobby = (int *) malloc (5 * sizeof(int));
This piece of code assigns to bobby a pointer to a block of 5 integers of type int, this size can be
equal to 2, 4 or more bytes according to the system where the program is compiled.
The function calloc.
calloc is very similar to malloc in its operation, its main difference is in its prototype:
void * calloc (size_t nelements, size_t size);
Since it admits 2 parameters instead of one. These two parameters are multiplied to obtain the
total size of the memory block to be assigned. Usually the first parameter (nelements) is the
number of elements and the second one (size) serves to specify the size of each element. For
example, we could define bobby with calloc thus:
int * bobby;
bobby = (int *) calloc (5, sizeof(int));
Another difference between malloc and calloc is that calloc initializates all its elements to 0.
The function realloc.
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It changes the size of a block of memory already assigned to a pointer.
void * realloc (void * pointer, size_t size);
pointer parameter receives a pointer to an already assigned memory block or a null pointer, and
size specifies the new size that the memory block shall have. The function assigns size bytes of
memory to the pointer. The function may need to change the location of the memory block so that
the new size can fit, in that case the present content of the block is copied to the new one to
guarantee that the existing data is not lost. The new pointer is returned by the function. If it has
not been posible to assign the memory block with the new size it returns a null pointer but the
pointer specified as parameter and its content remains unchanged.
The function free.
It releases a block of dynamic memory previously assigned using malloc, calloc or realloc.
void free (void * pointer);
This function must only be used to release memory assigned with functions malloc, calloc and
realloc.
76
UNIT 5
STRUCTURES AND UNION
Contents
5.1
5.2
5.2.1
5.2.2
5.2.3
Structures.
5.1.1 Poniters to structures
5.1.2 Nesting structures
User defined data types.
Typedef
Union
Enum
5.1 Structures
A data structure is a set of diverse types of data that may have different lengths grouped together
under a unique declaration. Its form is the following:
struct model_name {
type1 element1;
type2 element2;
type3 element3;
.
.
} object_name;
where model_name is a name for the model of the structure type and the optional parameter
object_name is a valid identifier (or identifiers) for structure object instantiations. Within curly
brackets { } they are the types and their sub-identifiers corresponding to the elements that
compose the structure.
If the structure definition includes the parameter model_name (optional), that parameter becomes
a valid type name equivalent to the structure. For example:
struct products {
char name [30];
float price;
};
products apple;
products orange, melon;
We have first defined the structure model products with two fields: name and price, each of a
different type. We have then used the name of the structure type (products) to declare three
objects of that type: apple, orange and melon.
Once declared, products has become a new valid type name like the fundamental ones int, char or
short and we are able to declare objects (variables) of that type.
77
The optional field object_name that can go at the end of the structure declaration serves to directly
declare objects of the structure type. For example, we can also declare the structure objects
apple, orange and melon this way:
struct products {
char name [30];
float price;
} apple, orange, melon;
Moreover, in cases like the last one in which we took advantage of the declaration of the structure
model to declare objects of it, the parameter model_name (in this case products) becomes
optional. Although if model_name is not included it will not be possible to declare more objects of
this same model later.
It is important to clearly differentiate between what is a structure model, and what is a structure
object. Using the terms we used with variables, the model is the type, and the object is the
variable. We can instantiate many objects (variables) from a single model (type).
Once we have declared our three objects of a determined structure model (apple, orange and
melon) we can operate with the fields that form them. To do that we have to use a point (.)
inserted between the object name and the field name. For example, we could operate with any of
these elements as if they were standard variables of their respective types:
apple.name
apple.price
orange.name
orange.price
melon.name
melon.price
each one being of its corresponding data type: apple.name, orange.name and melon.name are of
type char[30], and apple.price, orange.price and melon.price are of type float.
We are going to leave apples, oranges and melons and go with an example about movies:
// example about structures
#include <iostream.h>
#include <string.h>
#include <stdlib.h>
struct movies_t {
char title [50];
int year;
} mine, yours;
Enter title: Alien
Enter year: 1979
My favourite movie is:
2001 A Space Odyssey (1968)
And yours:
Alien (1979)
void printmovie (movies_t movie);
int main ()
{
char buffer [50];
strcpy (mine.title, "2001 A Space Odyssey");
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mine.year = 1968;
cout << "Enter title: ";
cin.getline (yours.title,50);
cout << "Enter year: ";
cin.getline (buffer,50);
yours.year = atoi (buffer);
cout << "My favourite movie is:\n ";
printmovie (mine);
cout << "And yours:\n ";
printmovie (yours);
return 0;
}
void printmovie (movies_t movie)
{
cout << movie.title;
cout << " (" << movie.year << ")\n";
}
The example shows how we can use the elements of a structure and the structure itself as normal
variables. For example, yours.year is a valid variable of type int, and mine.title is a valid array of
50 chars.
Notice that mine and yours are also treated as valid variables of type movies_t when being
passed to the function printmovie(). Therefore, one of the most important advantages of
structures is that we can refer either to their elements individually or to the entire structure as a
block.
Structures are a feature used very often to build data bases, specially if we consider the possibility
of building arrays of them.
// array of structures
#include <iostream.h>
#include <stdlib.h>
#define N_MOVIES 5
struct movies_t {
char title [50];
int year;
} films [N_MOVIES];
void printmovie (movies_t movie);
int main ()
{
char buffer [50];
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
title: Alien
year: 1979
title: Blade Runner
year: 1982
title: Matrix
year: 1999
title: Rear Window
year: 1954
title: Taxi Driver
year: 1975
You have entered these movies:
Alien (1979)
Blade Runner (1982)
Matrix (1999)
Rear Window (1954)
79
int n;
for (n=0; n<N_MOVIES; n++)
{
cout << "Enter title: ";
cin.getline (films[n].title,50);
cout << "Enter year: ";
cin.getline (buffer,50);
films[n].year = atoi (buffer);
}
cout << "\nYou have entered these
movies:\n";
for (n=0; n<N_MOVIES; n++)
printmovie (films[n]);
return 0;
}
Taxi Driver (1975)
void printmovie (movies_t movie)
{
cout << movie.title;
cout << " (" << movie.year << ")\n";
}
5.1.1 Pointers to structures
Like any other type, structures can be pointed by pointers. The rules are the same as for any
fundamental data type: The pointer must be declared as a pointer to the structure:
struct movies_t {
char title [50];
int year;
};
movies_t amovie;
movies_t * pmovie;
Here amovie is an object of struct type movies_t and pmovie is a pointer to point to objects of
struct type movies_t. So, the following, as with fundamental types, would also be valid:
pmovie = &amovie;
Ok, we will now go with another example, that will serve to introduce a new operator:
// pointers to structures
#include <iostream.h>
#include <stdlib.h>
struct movies_t {
Enter title: Matrix
Enter year: 1999
You have entered:
Matrix (1999)
80
char title [50];
int year;
};
int main ()
{
char buffer[50];
movies_t amovie;
movies_t * pmovie;
pmovie = & amovie;
cout << "Enter title: ";
cin.getline (pmovie->title,50);
cout << "Enter year: ";
cin.getline (buffer,50);
pmovie->year = atoi (buffer);
cout << "\nYou have entered:\n";
cout << pmovie->title;
cout << " (" << pmovie->year << ")\n";
return 0;
}
The previous code includes an important introduction: operator ->. This is a reference operator
that is used exclusively with pointers to structures and pointers to classes. It allows us not to
have to use parenthesis on each reference to a structure member. In the example we used:
pmovie->title
that could be translated to:
(*pmovie).title
both expressions pmovie->title and (*pmovie).title are valid and mean that we are evaluating
the element title of the structure pointed by pmovie. You must distinguish it clearly from:
*pmovie.title
that is equivalent to
*(pmovie.title)
and that would serve to evaluate the value pointed by element title of structure movies, that in
this case (where title is not a pointer) it would not make much sense. The following panel
summarizes possible combinations of pointers and structures:
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Expression
Description
pmovie.title
Element title of structure pmovie
Equivalent
pmovie->title
Element title of structure pointed by pmovie
(*pmovie).title
*pmovie.title
Value pointed by element title of structure pmovie
*(pmovie.title)
5.1.2 Nesting structures
Structures can also be nested so that a valid element of a structure can also be another structure.
struct movies_t {
char title [50];
int year;
}
struct friends_t {
char name [50];
char email [50];
movies_t favourite_movie;
} charlie, maria;
friends_t * pfriends = &charlie;
Therefore, after the previous declaration we could use the following expressions:
charlie.name
maria.favourite_movie.title
charlie.favourite_movie.year
pfriends->favourite_movie.year
(where, by the way, the last two expressions are equivalent).
The concept of structures that has been discussed in this section is the same as used in C
language, nevertheless, in C++, the structure concept has been extended up to the same
functionality of a class with the peculiarity that all of its elements are considered public.
5.2 User Defined Data Types
We have already seen a data type that is defined by the user (programmer): the structures. But in
addition to these there are other kinds of user defined data types:
5.2.1 Typedef
C++ allows us to define our own types based on other existing data types. In order to do that we
shall use keyword typedef, whose form is:
typedef
existing_type new_type_name ;
where existing_type is a C++ fundamental or any other defined type and new_type_name is the
name that the new type we are going to define will receive. For example:
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typedef
typedef
typedef
typedef
char C;
unsigned int WORD;
char * string_t;
char field [50];
In this case we have defined four new data types: C, WORD, string_t and field as char, unsigned
int, char* and char[50] respectively, that we could perfectly use later as valid types:
C achar, anotherchar, *ptchar1;
WORD myword;
string_t ptchar2;
field name;
Typedef can be useful to define a type that is repeatedly used within a program and it is possible
that we will need to change it in a later version, or if a type you want to use has too long a name
and you want it to be shorter.
5.2.2 Unions
Unions allow a portion of memory to be accessed as different data types, since all of them are in
fact the same location in memory. Its declaration and use is similar to the one of structures but
its functionality is totally different:
union model_name {
type1 element1;
type2 element2;
type3 element3;
.
.
} object_name;
All the elements of the union declaration occupy the same space of memory. Its size is the one of
the greatest element of the declaration. For example:
union mytypes_t {
char c;
int i;
float f;
} mytypes;
defines three elements:
mytypes.c
mytypes.f
mytypes.i
each one of a different data type. Since all of them are referring to a same location in memory, the
modification of one of the elements will afect the value of all of them.
83
One of the uses a union may have is to unite an elementary type with an array or structures of
smaller elements. For example,
union mix_t{
long l;
struct {
short hi;
short lo;
} s;
char c[4];
} mix;
defines three names that allow us to access the same group of 4 bytes: mix.l, mix.s and mix.c
and which we can use according to how we want to access it, as long, short or char respectively. I
have mixed types, arrays and structures in the union so that you can see the different ways that
we can access the data:
Anonymous unions
In C++ we have the option that unions be anonymous. If we include a union in a structure
without any object name (the one that goes after the curly brackets { }) the union will be
anonymous and we will be able to access the elements directly by its name. For example, look at
the difference between these two declarations:
anonymous union
Union
struct {
char title[50];
char author[50];
union {
float dollars;
int yens;
} price;
} book;
struct {
char title[50];
char author[50];
union {
float dollars;
int yens;
};
} book;
The only difference between the two pieces of code is that in the first one we gave a name to the
union (price) and in the second we did not. The difference is when accessing members dollars
and yens of an object. In the first case it would be:
book.price.dollars
book.price.yens
whereas in the second it would be:
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book.dollars
book.yens
Once again I remind you that because it is a union, the fields dollars and yens occupy the same
space in the memory so they cannot be used to store two different values. That means that you
can include a price in dollars or yens, but not both.
Enumerations (enum)
Enumerations serve to create data types to contain something different that is not limited to
either numerical or character constants nor to the constants true and false. Its form is the
following:
enum model_name {
value1,
value2,
value3,
.
.
} object_name;
For example, we could create a new type of variable called color to store colors with the following
declaration:
enum colors_t {black, blue, green, cyan, red, purple, yellow, white};
Notice that we do not include any fundamental data type in the declaration. To say it another
way, we have created a new data type without it being based on any existing one: the type color_t,
whose possible values are the colors that we have enclosed within curly brackets {}. For example,
once declared the colors_t enumeration in the following expressions will be valid:
colors_t mycolor;
mycolor = blue;
if (mycolor == green) mycolor = red;
In fact our enumerated data type is compiled as an integer and its possible values are any type of
integer constant specified. If it is not specified, the integer value equivalent to the first possible
value is 0 and the following ones follow a +1 progression. Thus, in our data type colors_t that we
defined before, black would be equivalent to 0, blue would be equivalent to 1, green to 2 and so
on.
If we explicitly specify an integer value for some of the possible values of our enumerated type (for
example the first one) the following values will be the increases of this, for example:
enum months_t { january=1, february, march, april,
may, june, july, august,
september, october, november, december} y2k;
85
In this case, variable y2k of the enumerated type months_t can contain any of the 12 possible
values that go from january to december and that are equivalent to values between 1 and 12, not
between 0 and 11 since we have made January equal to 1.
86
UNIT 6
FUNCTIONS
Contents
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
Functions
Default values in arguments
Void Functions
Call by value and reference
Passing Reference to Functions.
Returning References from Functions
Inline function
Recursive function
Prototyping function
6.1.Functions
Using functions we can structure our programs in a more modular way, accessing all the
potential that structured programming in C++ can offer us.
A function is a block of instructions that is executed when it is called from some other point of the
program. The following is its format:
type name ( argument1, argument2, ...) statement
where:
— type is the type of data returned by the function.
— name is the name by which it will be possible to call the function.
— arguments (as many as wanted can be specified). Each argument consists of a type of data
followed by its identifier, like in a variable declaration (for example, int x) and which acts within
the function like any other variable. They allow passing parameters to the function when it is
called. The different parameters are separated by commas.
— statement is the function's body. It can be a single instruction or a block of instructions. In
the latter case it must be delimited by curly brackets {}.
Here you have the first function example:
// function example
#include <iostream.h>
The result is 8
int addition (int a, int b)
{
int r;
r=a+b;
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return (r);
}
int main ()
{
int z;
z = addition (5,3);
cout << "The result is " << z;
return 0;
}
In order to examine this code, first of all remember something said at the beginning of this
tutorial: a C++ program always begins its execution with the main function. So we will begin
there.
We can see how the main function begins by declaring the variable z of type int. Right after that
we see a call to addition function. If we pay attention we will be able to see the similarity between
the structure of the call to the function and the declaration of the function itself in the code lines
above:
The parameters have a clear correspondence. Within the main function we called to addition
passing two values: 5 and 3 that correspond to the int a and int b parameters declared for the
function addition.
At the moment at which the function is called from main, control is lost by main and passed to
function addition. The value of both parameters passed in the call (5 and 3) are copied to the
local variables int a and int b within the function.
Function addition declares a new variable (int r;), and by means of the expression r=a+b;, it
assigns to r the result of a plus b. Because the passed parameters for a and b are 5 and 3
respectively, the result is 8.
The following line of code:
return (r);
Finalizes function addition, and returns the control back to the function that called it (main)
following the program from the same point at which it was interrupted by the call to addition. But
additionally, return was called with the content of variable r (return (r);), which at that moment
was 8, so this value is said to be returned by the function.
The value returned by a function is the value given to the function when it is evaluated. Therefore,
z will store the value returned by addition (5, 3), that is 8. To explain it another way, you can
imagine that the call to a function (addition (5,3)) is literally replaced by the value it returns (8).
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The following line of code in main is:
cout << "The result is " << z;
That, as you may already suppose, produces the printing of the result on the screen.
Scope of variables [re]
You must consider that the scope
of variables declared within a
function or any other block of
instructions is only their own
function or their own block of
instructions and cannot be used
outside of them. For example, in
the previous example it had been
impossible to use the variables a, b
or r directly in function main since
they were local variables to
function addition. Also, it had
been impossible to use the variable
z directly within function addition,
since this was a local variable to the function main.
Therefore, the scope of local variables is limited to the same nesting level in which they are
declared. Nevertheless you can also declare global variables that are visible from any point of the
code, inside and outside any function. In order to declare global variables you must do it outside
any function or block of instructions, that means, directly in the body of the program.
And here is another example about functions:
The
The
The
The
// function example
#include <iostream.h>
int subtraction (int a, int b)
{
int r;
r=a-b;
return (r);
}
int main ()
{
int x=5, y=3, z;
z = subtraction (7,2);
cout << "The first result is " << z << '\n';
cout << "The second result is " << subtraction (7,2) <<
'\n';
cout << "The third result is " << subtraction (x,y) <<
'\n';
z= 4 + subtraction (x,y);
cout << "The fourth result is " << z << '\n';
return 0;
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first result is 5
second result is 5
third result is 2
fourth result is 6
}
In this case we have created the function subtraction. The only thing that this function does is to
subtract both passed parameters and to return the result.
Nevertheless, if we examine the function main we will see that we have made several calls to
function subtraction. We have used some different calling methods so that you see other ways or
moments when a function can be called.
In order to understand well these examples you must consider once again that a call to a function
could be perfectly replaced by its return value. For example the first case (that you should already
know beacause it is the same pattern that we have used in previous examples):
z = subtraction (7,2);
cout << "The first result is " << z;
If we replace the function call by its result (that is 5), we would have:
z = 5;
cout << "The first result is " << z;
As well as
cout << "The second result is " << subtraction (7,2);
has the same result as the previous call, but in this case we made the call to subtraction directly
as a parameter for cout. Simply imagine that we had written:
cout << "The second result is " << 5;
since 5 is the result of subtraction (7,2).
In the case of
cout << "The third result is " << subtraction (x,y);
The only new thing that we introduced is that the parameters of subtraction are variables instead
of constants. That is perfectly valid. In this case the values passed to the function subtraction are
the values of x and y, that are 5 and 3 respectively, giving 2 as result.
The fourth case is more of the same. Simply note that instead of:
z = 4 + subtraction (x,y);
we could have put:
z = subtraction (x,y) + 4;
with exactly the same result. Notice that the semicolon sign (;) goes at the end of the whole
expression. It does not necessarily have to go right after the function call. The explanation might
be once again that you imagine that a function can be replaced by its result:
z = 4 + 2;
z = 2 + 4;
6.2. Default values in arguments
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When declaring a function we can specify a default value for each parameter. This value will be
used if that parameter is left blank when calling to the function. To do that we simply have to
assign a value to the arguments in the function declaration. If a value for that parameter is not
passed when the function is called, the default value is used, but if a value is specified this
default value is stepped on and the passed value is used. For example:
// default values in functions
#include <iostream.h>
6
5
int divide (int a, int b=2)
{
int r;
r=a/b;
return (r);
}
int main ()
{
cout << divide (12);
cout << endl;
cout << divide (20,4);
return 0;
}
As we can see in the body of the program there are two calls to the function divide. In the first
one:
divide (12)
we have only specified one argument, but the function divide allows up to two. So the function
divide has assumed that the second parameter is 2 since that is what we have specified to
happen if this parameter is lacking (notice the function declaration, which finishes with int b=2).
Therefore the result of this function call is 6 (12/2).
In the second call:
divide (20,4)
there are two parameters, so the default assignation (int b=2) is stepped on by the passed
parameter, that is 4, making the result equal to 5 (20/4).
6.3. Void function
If you remember the syntax of a function declaration:
type name ( argument1, argument2 ...) statement
you will see that it is obligatory that this declaration begins with a type, that is the type of the
data that will be returned by the function with the return instruction. But what if we want to
return no value?
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Imagine that we want to make a function just to show a message on the screen. We do not need it
to return any value, moreover, we do not need it to receive any parameters. For these cases, the
void type was devised in the C language. Take a look at:
// void function example
#include <iostream.h>
I'm a function!
void dummyfunction (void)
{
cout << "I'm a function!";
}
int main ()
{
dummyfunction ();
return 0;
}
Although in C++ it is not necessary to specify void, its use is considered suitable to signify that it
is a function without parameters or arguments and not something else.
What you must always be aware of is that the format for calling a function includes specifing its
name and enclosing the arguments between parenthesis. The non-existence of arguments does
not exempt us from the obligation to use parenthesis. For that reason the call to
dummyfunction is
dummyfunction ();
This clearly indicates that it is a call to a function and not the name of a variable or anything else.
6.4 Call by value and reference.
Until now, in all the functions we have seen, the parameters passed to the functions have been
passed by value. This means that when calling a function with parameters, what we have passed
to the function were values but never the specified variables themselves. For example, suppose
that we called our first function addition using the following code:
int x=5, y=3, z;
z = addition ( x , y );
What we did in this case was to call function addition passing the values of x and y, that means
5 and 3 respectively, not the variables themselves.
This way, when function addition is being called the value of its variables a and b become 5 and
3 respectively, but any modification of a or b within the function addition will not affect the
values of x and y outside it, because variables x and y were not passed themselves to the
function, only their values.
92
But there might be some cases where you need to manipulate from inside a function the value of
an external variable. For that purpose we have to use arguments passed by reference, as in the
function duplicate of the following example:
// passing parameters by reference
#include <iostream.h>
x=2, y=6, z=14
void duplicate (int& a, int& b, int& c)
{
a*=2;
b*=2;
c*=2;
}
int main ()
{
int x=1, y=3, z=7;
duplicate (x, y, z);
cout << "x=" << x << ", y=" << y << ", z=" << z;
return 0;
}
The first thing that should call your attention is that in the declaration of duplicate the type of
each argument was followed by an ampersand sign (&), that serves to specify that the variable has
to be passed by reference instead of by value, as usual.
When passing a variable by reference we are passing the variable itself and any modification that
we do to that parameter within the function will have effect in the passed variable outside it.
To express it another way, we have associated a, b and c with the parameters used when calling
the function (x, y and z) and any change that we do on a within the function will affect the value
of x outside. Any change that we do on b will affect y, and the same with c and z.
That is why our program's output, that shows the values stored in x, y and z after the call to
duplicate, shows the values of the three variables of main doubled.
If when declaring the following function:
void duplicate (int& a, int& b, int& c)
we had declared it thus:
void duplicate (int a, int b, int c)
That is, without the ampersand (&) signs, we would have not passed the variables by reference,
but their values, and therefore, the output on screen for our program would have been the values
of x, y and z without having been modified.
93
This type of declaration "by reference" using the ampersand (&) sign is exclusive of C++.
Passing by reference is an effective way to allow a function to return more than one single value.
For example, here is a function that returns the previous and next numbers of the first parameter
passed.
// more than one returning value
#include <iostream.h>
Previous=99, Next=101
void prevnext (int x, int& prev, int& next)
{
prev = x-1;
next = x+1;
}
int main ()
{
int x=100, y, z;
prevnext (x, y, z);
cout << "Previous=" << y << ", Next=" << z;
return 0;
}
6.5. Passing Reference to Functions.
Reference variables are particularly useful when passing to functions. The changes made in the
called functions are reflected back to the calling function . The program uses the classic problem
in programming, swapping the values of two variables.
e.g.
void val_swap(int x, int y)
// Call by Value
{
int t;
t = x;
x = y;
y = t;
}
void add_swap(int *x, int *y) // Call by Address
{
int t;
t = *x;
*x = *y;
*y = t;
}
void val_swap(int &x, int &y) // Call by Reference
{
94
int t;
t = x;
x = y;
y = t;
}
void main()
{
int n1 = 25, n2 = 50;
cout << “Before call by value : “;
cout << “ n1 = “ << n1 << “ n2 = “ << n2 << endl;
val_swap( n1, n2 );
cout << “ After call by value : “;
cout << “ n1 = “ << n1 << “ n2 = “ << n2 << endl;
cout << “Before call by address : “;
cout << “ n1 = “ << n1 << “ n2 = “ << n2 << endl;
val_swap( &n1, &n2 );
cout << “ After call by address : “;
cout << “ n1 = “ << n1 << “ n2 = “ << n2 << endl;
cout << “Before call by reference: “;
cout << “ n1 = “ << n1 << “ n2 = “ << n2 << endl;
val_swap( n1, n2 );
cout << “ After call by value : “ ;
cout << “ n1 = “ << n1 << “ n2 = “ << n2 << endl;
}
Output :
Before call by value : n1 = 25 n2 = 50
After call by value
: n1 = 25 n2 = 50 // x = 50, y = 25
Before call by address : n1 = 25 n2 = 50
After call by address : n1 = 50 n2 = 25 //x = 50, y = 25
Before call by reference: n1 = 50 n2 = 25
After call by reference : n1 = 25 n2 = 50 //x = 25, y = 50
You can see that the only difference in writing the functions in call by value and call by reference
is while receiving the parameters where as in pass by address the function body has some
changes, i.e. they use (*) indirection operator to manipulate the variables.
6.6. Returning References from Functions
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Just as in passing the parameters by reference, returning a reference also doesn’t return back a
copy of the variable , instead an alias is returned.
e.g.
int &func(int &num)
{
:
:
return(num);
}
void main()
{
int n1,n2;
:
:
n1 = fn( n2);
}
Notice that the function header contains an ampersand (&) before the function name. This is how
a function is made to return reference variable. In this case, it takes a reference to an integer as
its argument and returns a reference to an integer. This facility can be very useful for returning
objects and even structure variables.
6.7. Inline functions
The inline directive can be included before a function declaration to specify that the function must
be compiled as code at the same point where it is called. This is equivalent to declaring a macro.
Its advantage is only appreciated in very short functions, in which the resulting code from
compiling the program may be faster if the overhead of calling a function (stacking of arguments)
is avoided.
The format for its declaration is:
inline type name ( arguments ... ) { instructions ... }
and the call is just like the call to any other function. It is not necessary to include the inline
keyword before each call, only in the declaration.
6.8. Recursive function
Recursivity is the property that functions have to be called by themselves. It is useful for tasks
such as some sorting methods or to calculate the factorial of a number. For example, to obtain the
factorial of a number (n) its mathematical formula is:
n! = n * (n-1) * (n-2) * (n-3) ... * 1
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more concretely, 5! (factorial of 5) would be:
5! = 5 * 4 * 3 * 2 * 1 = 120
and a recursive function to do that could be this:
// factorial calculator
#include <iostream.h>
Type a number: 9
!9 = 362880
long factorial (long a)
{
if (a > 1)
return (a * factorial (a-1));
else
return (1);
}
int main ()
{
long l;
cout << "Type a number: ";
cin >> l;
cout << "!" << l << " = " << factorial (l);
return 0;
}
Notice how in function factorial we included a call to itself, but only if the argument is greater
than 1, since otherwise the function would perform an infinite recursive loop in which once it
arrived at 0 it would continue multiplying by all the negative numbers (probably provoking a
stack overflow error on runtime).
This function has a limitation because of the data type used in its design (long) for more
simplicity. In a standard system, the type long would not allow storing factorials greater than 12!.
6.9 Prototyping functions
Until now, we have defined the all of the functions before the first appearance of calls to them,
that generally was in main, leaving the function main for the end. If you try to repeat some of the
examples of functions described so far, but placing the function main before any other function
that is called from within it, you will most likely obtain an error. The reason is that to be able to
call a function it must have been declared previously (it must be known), like we have done in all
our examples.
But there is an alternative way to avoid writing all the code of all functions before they can be
used in main or in another function. It is by prototyping functions. This consists in making a
previous shorter, but quite significant, declaration of the complete definition so that the compiler
can know the arguments and the return type needed.
Its form is:
type name ( argument_type1, argument_type2, ...);
97
It is identical to the header of a function definition, except:
•
It does not include a statement for the function. That means that it does not include the
body with all the instructions that are usually enclose within curly brackets { }.
•
It ends with a semicolon sign (;).
In the argument enumeration it is enough to put the type of each argument. The inclusion
of a name for each argument as in the definition of a standard function is optional,
although recommended.
For example:
•
// prototyping
#include <iostream.h>
void odd (int a);
void even (int a);
int main ()
{
int i;
do {
cout << "Type a number: (0 to exit)";
cin >> i;
odd (i);
} while (i!=0);
return 0;
}
Type a number (0
Number is odd.
Type a number (0
Number is even.
Type a number (0
Number is even.
Type a number (0
Number is even.
to exit): 9
to exit): 6
to exit): 1030
to exit): 0
void odd (int a)
{
if ((a%2)!=0) cout << "Number is odd.\n";
else even (a);
}
void even (int a)
{
if ((a%2)==0) cout << "Number is even.\n";
else odd (a);
}
This example is indeed not an example of effectiveness, I am sure that at this point you can
already make a program with the same result using only half of the code lines. But this example
ilustrates how protyping works. Moreover, in this concrete case the prototyping of -at least- one of
the two functions is necessary.
The first things that we see are the prototypes of functions odd and even:
void odd (int a);
void even (int a);
that allows these functions to be used before they are completely defined, for example, in main,
which now is located in a more logical place: the beginning of the program's code.
98
Nevertheless, the specific reason why this program needs at least one of the functions prototyped
is because in odd there is a call to even and in even there is a call to odd. If none of the two
functions had been previously declared, an error would have happened, since either odd would
not be visible from even (because it has not still been declared), or even would not be visible from
odd.
Many programmers recommend that all functions be prototyped. It is also my recommendation,
mainly in case that there are many functions or in case that they are very long. Having the
prototype of all the functions in the same place can spare us some time when determining how to
call it or even ease the creation of a header file.
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UNIT7
CLASSES AND OBJECTS
Contents
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
Introduction to class.
Class Definition
Classes and Objects
Access specifiers – Private, Public and Protected.
Member functions of the class.
Passing and returning objects.
Pointers to objects.
Array of objects.
The special ‘this’ pointer
self test
7.1 Introduction to Classes
Object-oriented programming (OOP) is a conceptual approach to design programs. It can be
implemented in many languages, whether they directly support OOP concepts or not. The C
language also can be used to implement many of the object-oriented principles. However, C++
supports the object-oriented features directly. All these features like Data abstraction, Data
encapsulation, Information hiding etc have one thing in common – the vehicle that is used to
implement them. The vehicle is “ class.”
Class is a user defined data type just like structures, but with a difference. It also has three
sections namely private, public and protected. Using these, access to member variables of a class
can be strictly controlled.
7.2. Class Definition
The following is the general format of defining a class template:
class class_name {
permission_label_1:
member1;
permission_label_2:
member2;
...
} object_name;
example:-
100
class tag_name
{
public :
type member_variable_name;
:
type member_function_name();
:
private:
// Must
// Optional
type member_variable_name;
:
type member_function_name();
:
};
The keyword class is used to define a class template. The private and public sections of a class are
given by the keywords ‘private’ and ‘public’ respectively. They determine the accessibility of the
members. All the variables declared in the class, whether in the private or the public section, are
the members of the class. Since the class scope is private by default, you can also omit the
keyword private. In such cases you must declare the variables before public, as writing public
overrides the private scope of the class.
e.g.
class tag_name
{
type member_variable_name; // private
:
type member_function_name(); // private
:
public :
// Must
type member_variable_name;
:
type member_function_name();
:
};
The variables and functions from the public section are accessible to any function of the program.
However, a program can access the private members of a class only by using the public member
functions of the class. This insulation of data members from direct access in a program is called
information hiding.
e.g.
101
class player
{
public :
void getstats(void);
void showstats(void);
int no_player;
private :
char name[40];
int age;
int runs;
int tests;
float average;
float calcaverage(void);
};
The above example models a cricket player. The variables in the private section – name, age, runs,
highest, tests, and average – can be accessed only by member functions of the class calcaverage(),
getstats() and showstats(). The functions in the public section - getstats() and showstats() can be
called from the program directly , but function calcaverage() can be called only from the member
functions of the class – getstats() and showstats().
With information hiding one need not know how actually the data is represented or functions
implemented. The program need not know about the changes in the private data and functions.
The interface(public) functions take care of this. The OOP methodology is to hide the
implementation specific details, thus reducing the complexities involved.
7.3. Classes and Objects
As seen earlier, a class is a vehicle to implement the OOP features in the C++ language. Once a
class is declared, an object of that type can be defined. An object is said to be a specific instance of
a class just like Maruti car is an instance of a vehicle or pigeon is the instance of a bird. Once a
class has been defined several objects of that type can be declared. For instance, an object of the
class defined above can be declared with the following statement:
player Sachin, Dravid, Mohan ;
[Or]
class player Sachin , Dravid, Mohan ;
where Sachin and Dravid are two objects of the class player. Both the objects have their own set
of member variables. Once the object is declared, its public members can be accessed using the
dot operator with the name of the object. We can also use the variable no_player in the public
section with a dot operator in functions other than the functions declared in the public section of
the class.
102
e.g.
Sachin.getstats();
Dravid.showstats();
Mohan.no_player = 10;
7.4. Access specifis- Private Public and Protected members
Class members can either be declared in public’,’protected’ or in the ‘private’ sections of the class.
But as one of the features of OOP is to prevent data from unrestricted access, the data members
of the class are normally declared in the private section. The member functions that form the
interface between the object and the program are declared in public section ( otherwise these
functions can not be called from the program ). The member functions which may have been
broken down further or those, which do not form a part of the interface, are declared in the
private section of the class. By default all the members of the class are private. The third access
specifier ‘protected’ that is not used in the above example, pertains to the member functions of
some new class that will be inherited from the base class. As far as non-member functions are
concerned, private and protected are one and the same.
Summary of Access specifiers
•
private members of a class are accessible only from other members of their same class or
from their "friend" classes.
•
protected members are accessible from members of their same class and friend classes,
and also from members of their derived classes.
•
Finally, public members are accessible from anywhere the class is visible.
7.5. Member Functions of a Class
A member function of the class is same as an ordinary function. Its declaration in a class
template must define its return value as well as the list of its arguments. You can declare or
define the function in the class specifier itself, in which case it is just like a normal function. But
since the functions within the class specifier is considered inline by the compiler we should not
define large functions and functions with control structures, iterative statements etc should not
be written inside the class specifier. However, the definition of a member function differs from that
of an ordinary function if written outside the class specifier. The header of a member function
uses the scope operator (::) to specify the class to which it belongs. The syntax is:
return_type class_name :: function_name (parameter list)
{
:
}
e.g.
void player :: getstats (void)
{
:
}
103
void player :: showstats (void)
{
:
:
}
This notation indicates that the functions getstats () and showstats() belong to the class
player.
COMPLETE EXAMPLE OF CLASS: Find the area of the rectangle
// class example
#include <iostream.h>
class CRectangle {
int x, y;
public:
void set_values (int,int);
int area (void) {return (x*y);}
};
void CRectangle::set_values (int a, int b) {
x = a;
y = b;
}
int main () {
CRectangle rect, rectb;
rect.set_values (3,4);
rectb.set_values (5,6);
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl;
}
------------------------------------------------------------OUTPUT
rect area: 12
rectb area: 30
7.6. Passing and Returning Objects
Objects can be passed to a function and returned back just like normal variables. When an object
is passed by content , the compiler creates another object as a formal variable in the called
function and copies all the data members from the actual variable to it. Objects can also be
passed by address, which will be discussed later.
104
e.g.
class check
{
public :
check add(check);
void get()
{
cin >> a;
}
void put()
{
cout << a;
}
private :
int a;
};
void main()
{
check c1,c2,c3;
c1.get();
c2.get();
c3 = c1.add(c2);
c3.put();
}
check check :: add ( check c2)
{
check temp;
temp.a = a + c2.a;
return ( temp);
}
The above example creates three objects of class check. It adds the member variable of two
classes, the invoking class c1 and the object that is passed to the function , c2 and returns the
result to another object c3.
You can also notice that in the class add() the variable of the object c1 is just referred as ‘a’ where
as the member of the object passed .i.e. c2 is referred as ‘c2.a’ . This is because the member
function will be pointed by the pointer named this in the compiler where as what we pass should
be accessed by the extraction operator ‘.’. we may pass more than one object and also normal
variable. we can return an object or a normal variable from the function. We have made use of a
temporary object in the function add() in order to facilitate return of the object.
105
7.7. Pointers to Objects
Passing and returning of objects is, however, not very efficient since it involves passing and
returning a copy of the data members. This problem can be eliminated using pointers. Like other
variables, objects of class can also have pointers. Declaring a pointer to an object of a particular
class is same as declaring a pointer to a variable of any other data type. A pointer variable
containing the address of an object is said to be pointing to that object. Pointers to objects can be
used to make a call by address or for dynamic memory allocation. Just like structure pointer, a
pointer to an object also uses the arrow operator to access its members. Like pointers to other
data types, a pointer to an object also has only one word of memory. It has to be made to point to
an already existing object or allocated memory using the keyword ‘new’.
e.g.
string str;
string *sp;
// Object
// Pointer to an object
sp = &str;
// Assigns address of an existing object
sp = new string
// Allocates memory with new.
It is perfectly valid to create pointers pointing to objects, in order to do that we must simply
consider that once declared, the class becomes a valid type, so use the class name as the type for
the pointer. For example:
CRectangle * prect;
is a pointer to an object of class CRectangle.
As it happens with data structures, to refer directly to a member of an object pointed by a pointer
you should use operator ->. Here is an example with some possible combinations:
a area: 2
*b area: 12
*c area: 2
d[0] area: 30
d[1] area: 56
// pointer to objects example
#include <iostream.h>
class CRectangle {
int width, height;
public:
void set_values (int, int);
int area (void) {return (width * height);}
};
void CRectangle::set_values (int a, int b) {
width = a;
height = b;
}
int main () {
CRectangle a, *b, *c;
CRectangle * d = new CRectangle[2];
106
b= new CRectangle;
c= &a;
a.set_values (1,2);
b->set_values (3,4);
d->set_values (5,6);
d[1].set_values (7,8);
cout << "a area: " << a.area() << endl;
cout << "*b area: " << b->area() << endl;
cout << "*c area: " << c->area() << endl;
cout << "d[0] area: " << d[0].area() << endl;
cout << "d[1] area: " << d[1].area() << endl;
return 0;
}
Next you have a summary on how can you read some pointer and class operators (*, &, ., ->, [ ])
that appear in the previous example:
*x
can be read: pointed by x
&x
can be read: address of x
x.y can be read: member y of object x
(*x).y can be read: member y of object pointed by x
x->y can be read: member y of object pointed by x (equivalent to the previous one)
x[0] can be read: first object pointed by x
x[1] can be read: second object pointed by x
x[n] can be read: (n+1)th object pointed by x
7.8. Array of Objects
As seen earlier, a class is a template, which can contain data items as well as member functions
to operate on the data items. Several objects of the class can also be declared and used. Also, an
array of objects can be declared and used just like an array of any other data type. An example
will demonstrate the use of array of objects.
e.g.
class student
{
public :
void getdetails();
void printdetails();
private :
int rollno;
char name[25];
int marks[6];
float percent;
};
void student :: getdetails()
107
{
int ctr,total;
cout << ”enter rollno”;
cin >> rollno ;
cout << ”enter name”;
cin >> name;
cout << ” enter 6 marks “ ;
for( ctr = 1 ;ctr <= 6 ; ctr++ )
{
cin >> marks[ctr];
total = total + marks[ctr];
}
percent = total / 6;
}
void student :: printdetails ()
{
cout << rollno << name << percent ;
}
void main()
{
student records[50];
int x=0;
cout << “ How many students “;
cin >> x;
for ( int i =1; i<= x; i++)
{
records[i].getdeatils();
}
for ( int i =1; i<= x; i++)
{
records[i].printdeatils();
}
}
As can be seen above, an array of objects is declared just like any other array. Members of the
class are accessed, using the array name qualified by a subscript.
The statement,
108
records[y].printdetails();
Invokes the member funs printdetails() for the object given by the subscript y. For different values
of subscript, it invokes the same member function, but for different objects.
7.9 The Special Pointer ‘this’
When several instances of a class come into existence, it naturally follows that each instance has
its own copy of member variables. If this were not the case, then for obvious reasons it would be
impossible to create more than one instance of the class. On the other hand, even though the
class member functions are encapsulated with the data members inside the class definition, it
would be very inefficient in terms of memory usage to replicate all these member functions and
store the code for them within each instance. Consequently, only one copy of each member
function per class is stored in memory, and must be shared by all of the instances of the class.
But this poses a big problem for the compiler: How can any given member function of a
class knows which instance it is supposed to be working on ? In other words, up to now in a class
member function you have simply been referring to the members directly without regard to the
fact that when the instantiations occur each data member will have a different memory address.
In other words, all the compiler knows is the offset of each data member from the start of the
class.
The solution to this dilemma is that, in point of fact, each member function does have
access to a pointer variable that points to the instance being manipulated. Fortunately this
pointer is supplied to each member function automatically when the function is called, so that
this burden is not placed upon the programmer.
This pointer variable has a special name ‘this’ (reserved word). Even though the this pointer is
implicitly declared, you always have access to it and may use the variable name anywhere you
seem appropriate.
e.g.
class try_this
{
public :
void print();
try_this add(int);
private :
int ivar;
};
109
void print()
{
cout << ivar;
cout << this -> ivar ;
}
The function print refers to the member variable ivar directly. Also, an explicit reference is made
using the this pointer. This special pointer is generally used to return the object, which invoked
the member function. For example,
void main()
{
try_this t1,t2;
t2 = t1.add(3);
t2.print();
}
try_this try_this :: add(int v)
{
ivar = ivar + v;
return ( *this);
}
In the above example if ivar for t1 is 10 and value in
v is 2, then the function add() adds them and ivar for t1 becomes 12 . We want to store this in
another object t2, which can be done by returning the object t1 using *this to t2. The result of
t2.print() now will be 12.
Dereferencing the Pointer this
Sometimes a member function needs to make a copy of the invoking instance so that it can
modify the copy without affecting the original instance. This can be done as follows :
try_this temp(*this);
try_this temp = *this ;
In OOP emphasis is on how the program represents data. It is a design concept with less
emphasis on operational aspects of the program. The primary concepts of OOP is implemented
using class and objects. A class contains data members as well as function members. The access
specifiers control the access of data members. Only the public members of the class can access
the data members declared in private section. Once class has been defined, many objects of that
class can be declared. Data members of different objects of the same class occupy different
memory area but function members of different objects of the same class share the same set of
functions. This is possible because of the internal pointer ‘*this’ which keeps track of which
function is invoked by which object.
110
7.10 Self test
Exercises:
1.
Define a class to model a banking system. The function members should allow initializing
the data members, a query to facilitate for account and a facility to deposit and withdraw
from the account. WAP to implement the same.
2.
Create a class called Time that has separate int member data for hours, minutes and
seconds. Write functions for accepting time and displaying time.
111
UNIT 8
CONSTRUCTOR AND DESTRUCTOR
Contents
8.1
8.2
8.3
8.4
Constructors
8.1.1
Syntax rules for writing constructor functions
8.1.2
Different ways of calling contructor
Overloading Constructors
Destructor
Self test
Since C++ supports the concept of user-defined classes and the subsequent initiations of
these classes, it is important that initialization of these instantiations be performed so that the
state of any object does not reflect “ garbage”. One of the principles of C++ is that objects know
how to initialize and cleanup after themselves. This automatic initialization and clean up is
accomplished by two member functions – the constructor and the destructor.
8.1 Constructors
By definition, a constructor function of some class is a member function that automatically gets
executed whenever an instance of the class to which the constructor belongs comes into
existence. The execution of such a function guarantees that the instance variables of the class will
be initialized properly.
A constructor function is unique from all other functions in a class because it is not called using
some instance of the class, but is invoked whenever we create an object of that class.
A constructor may be overloaded to accommodate many different forms of initialization for
instances of the class. i.e. for a single class many constructors can be written with different
argument lists .
8.1.1 Syntax rules for writing constructor functions
•
Its name must be same as that of the class to which it belongs.
•
It is declared with no return type (not even void). However, it will implicitly return a
temporary copy of the instance itself that is being created.
•
It cannot be declared static (a function which does not belong to a particular instance),
const( in which you can not make changes).
•
It should have public or protected access within the class. Only in very rare circumstances
the programmers declare it in private section.
e.g.
112
We are going to implement CRectangle including a constructor:
rect area: 12
rectb area: 30
// classes example
#include <iostream.h>
class CRectangle {
int width, height;
public:
CRectangle (int,int);
int area (void) {return (width*height);}
};
CRectangle::CRectangle (int a, int b) {
width = a;
height = b;
}
int main () {
CRectangle rect (3,4);
CRectangle rectb (5,6);
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl;
}
. Notice the way in which the parameters are passed to the constructor at the moment at which
the instances of the class are created:
CRectangle rect (3,4);
CRectangle rectb (5,6);
8.1.2 Different ways of calling contructor
There are basically three ways of calling constructor.
The first way to call the constructor is explicitly as :
CRectangle rect = CRectangle (3,4);
This statement creates an object with the name bigbox and initializes the data members with the
parameters passed to the constructor function. The above object can also be created with an
implicit call to the constructor :
CRectangle rect (3,4);
Both the statements given above are equivalent. Yet, another way of creating and initializing an
object is by direct assignment of the data item to the object name. But, this approach works if
there is only one data item in the class.
This is obvious because we cannot assign more than one value at a time to a variable.
113
e.g.
class counter
{
public :
counter ( int c) // constructor.
{
count = c;
};
private :
int count;
};
we can now create an object as,
counter cnt = 0;
In the above example , object cnt is initialized by a value zero at the time of declaration.
This value is actually assigned to its data member count. This is the third way to initialize an
object’s data member. Thus, all the following statements to initialize the objects of the class
counter are equivalent:
counter c1(20);
counter c1 = counter(30);
counter c1= 10;
8.2 Overloading Constructors
Like any other function, a constructor can also be overloaded with several functions that have the
same name but different types or numbers of parameters. Remember that the compiler will
execute the one that matches at the moment at which a function with that name is called. In this
case, at the moment at which a class object is declared.
In fact, in the cases where we declare a class and we do not specify any constructor the compiler
automatically assumes two overloaded constructors ("default constructor" and "copy constructor").
For example, for the class:
class CExample {
public:
int a,b,c;
void multiply (int n, int m) { a=n; b=m; c=a*b; };
};
with no constructors, the compiler automatically assumes that it has the following constructor
member functions:
114
•
Empty constructor
It is a constructor with no parameters defined as nop (empty block of instructions). It does
nothing.
CExample::CExample () { };
Copy constructor
It is a constructor with only one parameter of its same type that assigns to every non static
class member variable of the object a copy of the passed object.
CExample::CExample (const CExample& rv)
{
a=rv.a; b=rv.b; c=rv.c;
}
It is important to realize that both default constructors: the empty construction and the copy
constructor exist only if no other constructor is explicitly declared. In case that any constructor
with any number of parameters is declared, none of these two default constructors will exist. So if
you want them to be there, you must define your own ones.
•
Of course, you can also overload the class constructor providing different constructors for when
you pass parameters between parenthesis and when you do not (empty):
rect area: 12
rectb area: 25
// overloading class constructors
#include <iostream.h>
class CRectangle {
int width, height;
public:
CRectangle ();
CRectangle (int,int);
int area (void) {return (width*height);}
};
CRectangle::CRectangle () {
width = 5;
height = 5;
}
CRectangle::CRectangle (int a, int b) {
width = a;
height = b;
}
int main () {
CRectangle rect (3,4);
CRectangle rectb;
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl;
}
In this case rectb was declared without parameters, so it has been initialized with the constructor
that has no parameters, which declares both width and height with a value of 5.
115
Notice that if we declare a new object and we do not want to pass parameters to it we do not
include parentheses ():
CRectangle rectb; // right
CRectangle rectb(); // wrong!
8.3 Destructors
The Destructor fulfills the opposite functionality. It is automatically called when an object is
released from the memory, either because its scope of existence has finished (for example, if it was
defined as a local object within a function and the function ends) or because it is an object
dynamically assigned and it is released using operator delete.
A destructor function gets executed whenever an instance of the class to which it belongs goes out
of existence. The primary usage of a destructor function is to release memory space that the
instance currently has reserved.
Syntax rules for writing a destructor function
•
Its name is the same as that of the class to which it belongs, except that the first character
of the name is the symbol tilde ( ~ ).
•
It is declared with no return type ( not even void ) since it cannot ever return a value.
•
It cannot be static, const or volatile.
•
It takes no input arguments , and therefore cannot be overloaded.
•
It should have public access in class declaration.
Generally the destructor cannot be called explicitly (directly) from the program. The compiler
generates a class to destructor when the object expires. Class destructor is normally used to clean
up the mess from an object. Class destructors become extremely necessary when class
constructor use the new operator, otherwise it can be given as an empty function. However, the
destructor function may be called explicitly allowing you to release the memory not required and
allocate this memory to new resources, in Borland C++ version 3.1.
Eg
class employee
{
public :
employee()
{
}
~employee();
{
}
};
// example on constructors and destructors
116
#include <iostream.h>
class CRectangle {
int *width, *height;
public:
CRectangle (int,int);
~CRectangle ();
int area (void) {return (*width * *height);}
};
CRectangle::CRectangle (int a, int b) {
width = new int;
height = new int;
*width = a;
*height = b;
}
CRectangle::~CRectangle () {
delete width;
delete height;
}
int main () {
CRectangle rect (3,4), rectb (5,6);
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl;
return 0;
}
8.4
Self test
1.
Create a class called Time that has a separate data members for day, month and year. A
constructor should be used to initialize these members. Then write a function to add these
dates and store the result in a third object and display it.
2.
WAP to add co-ordinates of the plane. The class contains x and y co-ordinates. Create three
objects. Use a constructor to pass one pair of co-ordinates and a function to accept the
second pair. Add these variables of two objects and store the result in the third.
117
UNIT 9
FUNCTION OVERLOADING
Contents
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
Function overloading
Precautions to be taken while overloading functions.
Static Class Members
Static Member Functions
Friend Functions
Friend for Overloading Operators
Granting friendship to another class
Granting friendship to a member function of another class
9.1 Function Overloading
Function overloading is a form of polymorphism. Function overloading facilitates defining one
function having many forms. In other words it facilitates defining several functions with the same
name, thus overloading the function names. Like in operator overloading, even here, the compiler
uses context to determine which definition of an overloaded function is to be invoked.
Function overloading is used to define a set of functions that essentially, do the same thing, but
use different argument lists. The argument list of the function is also called as the function’s
signature. You can overload the function only if their signatures are different.
The differences can be 1. In number of arguments,
2. Data types of the arguments,
3. Order of arguments, if the number and
data types of the arguments are same.
e.g.
int add( int, int );
int add( int, int, int );
float add( float, float );
float add( int, float, int );
float add(int,int,float);
The compiler cannot distinguish if the signature is same and only return type is different. Hence,
it is a must, that their signature is different. The following functions therefore raise a compilation
error.
e.g.
118
float add( int, float, int );
int add( int, float, int );
Consider the following example
// overloaded function
#include <iostream.h>
2
2.5
int divide (int a, int b)
{
return (a/b);
}
float divide (float a, float b)
{
return (a/b);
}
int main ()
{
int x=5,y=2;
float n=5.0,m=2.0;
cout << divide (x,y);
cout << "\n";
cout << divide (n,m);
cout << "\n";
return 0;
}
In this case we have defined two functions with the same name, but one of them accepts two
arguments of type int and the other accepts them of type float. The compiler knows which one to
call in each case by examining the types when the function is called. If it is called with two ints as
arguments it calls to the function that has two int arguments in the prototype and if it is called
with two floats it will call to the one which has two floats in its prototype.
For simplicity I have included the same code within both functions, but this is not compulsory.
You can make two functions with the same name but with completely different behaviors.
9.2 Precautions to be taken while overloading function
Function overloading is a boon to designers, since different names for similar functions need not
be thought of, which often is a cumbersome process given that many times people run out of
names. But, this facility should not be overused, lest it becomes an overhead in terms of
readability and maintenance. Only those functions, which basically do the same task, on different
sets of data, should be overloaded.
119
We have already seen the powerful features of OOP that make C++ such a strong language. In this
session we will continue exploring some other features, which make this language more powerful.
9.3 Static Class Members
As we already know all the objects of the class have different data members but invoke the same
member functions. However, there is an exception to this rule. If the data member is declared with
the keyword static, then only one such data item is created for the entire class, no matter how
many objects it has. Static data members are useful, if all objects of a class must share a common
data item. Whereas, the visibility of this data item remains same, the duration of this variable is
for entire lifetime of the program.
For example, such a variable can be particularly useful if an object requires to know how many
objects of its kind exist.
class counter
{
public :
counter ();
int getcount();
private:
static int count;
};
counter::counter ()
{
count++;
}
int counter::getcount()
{
return ( count );
}
int counter :: count = 0;// INITIALIZATION OF STATIC MEMBER.
void main()
{
counter c1,c2;
cout << “ Count = “ << c1.getcount() << endl;
cout << “ Count = “ << c2.getcount() << endl;
counter c3;
cout << “ Count = “ << c3.getcount() << endl;
counter c4,c5;
120
cout << “ Count = “ << c4.getcount() << endl;
cout << “ Count = “ << c5.getcount() << endl;
}
Output:
Count
Count
Count
Count
Count
=
=
=
=
=
2
2
3
5
5
// not 1 because 2 objects are already created.
In the above example, the class counter demonstrates the use of static data members. It contains
just one data member count. Notice the initialization of this static class member. For some
compilers it is mandatory. Even though the data member is in the private section it can be
accessed in this case as a global variable directly, but has to be preceded by the name of the class
and the scope resolution operator. This example contains a constructor to increment this variable.
Similarly, there can be a destructor to decrement it.
You can use these to generate register numbers for student objects from a student class.
Whenever an object is created, he will be automatically assigned a register number if the register
number is a static variable and a constructor is used to write an equation to generate separate
register numbers in some order. You could initialize the variable to give the first register number
and then use this in the constructor for further operations.
9.4 Static Member Functions:All the objects of the class share static data members of a class. The example above demonstrates
how to keep track of all the objects of a class which are4 in existence. However, this function uses
existing objects to invoke a member function getcount(), which returns the value of the static data
member. What if the programme does not want to use objects to invoke this function and still the
programme would like to know how many objects have been created? If there is no object how the
member function is invoked? Further, as can be seen from the previous output, the number of
objects (count) remains same at a given instance no matter which object is used to invoke the
member function. In fact, the use of existing objects, like in the above example, is not an effective
way to access the value of the static data member. A specific object should not be used to refer to
this member, since it does not belong to that object; it belongs to the entire class. C++ gives a
facility to define static function members, for the same. That is, to invoke such a function, an
object is not required. It can be invoked with the name of the class. The programme given below
illustrates its use.
class counter
{
public :
121
counter ();
static int getcount();
private:
static int count;
};
counter::counter ()
{
count++;
}
int counter::getcount()
{
}
int counter :: count = 0;
// INITIALIZATION OF
STATIC MEMBER.
void main()
{
counter c1,c2;
cout << “ Count = “ << counter :: getcount() << endl;
cout << “ Count = “ << counter :: getcount() << endl;
counter c3;
cout << “ Count = “ << counter :: getcount() << endl;
counter c4,c5;
cout << “ Count = “ << counter :: getcount() << endl;
cout << “ Count = “ << counter :: getcount() << endl;
}
Output:
Count
Count
Count
Count
Count
=
=
=
=
=
2
2
3
5
5
122
9.5 Friend Functions
One of the main features of OOP is information hiding. A class encapsulates data and methods to
operate on that data in a single unit . The data from the class can be accessed only through
member functions of the class. This restricted access not only hides the implementation details of
the methods and the data structure, it also saves the data from any possible misuse, accidental or
otherwise. However, the concept of data encapsulation sometimes takes information hiding too
far. There are situations where a rigid and controlled access leads to inconvenience and
hardships.
For instance, consider a case where a function is required to operate on object of two different
classes. This function cannot be made a member of both the classes. What can be done is that a
function can be defined outside both the classes and made to operate on both. That is, a function
not belonging to either, but able to access both. Such a function is called as a friend function. In
other words, a friend function is a nonmember function, which has access to a class’s private
members. It is like allowing access to one’s personal belongings to a friend.
Using a friend function is quite simple. The following example defines a friend function to access
members of two classes.
class Bclass;
// Forward Declaration
class Aclass
{
public :
Aclass(int v)
{
Avar = v;
}
friend int addup(Aclass &ac, Bclass &bc);
private :
int Avar;
};
class Bclass
{
public :
Bclass(int v)
{
Bvar = v;
}
friend int addup(Aclass &ac, Bclass &bc);
private :
int Bvar;
};
123
int addup(Aclass &ac, Bclass &bc)
{
return( ac.Avar + bc.Bvar);
}
void main()
{
Aclass aobj;
Bclass bobj;
int total;
total = addup(aobj,bobj);
}
The program defines two classes- Aclass and Bclass. It also has constructors for these classes. A
friend function, addup(), is declared in the definition of both the classes, although it does not
belong to either. This friend function returns the sum of the two private members of the classes.
Notice, that this function can directly access the private members of the classes. To access the
private members, the name of the member has to be prefixed with the name of the object , along
with the dot operator.
The first line in the program is called forward declaration. This is required to inform the compiler
that the definition for class Bclass comes after class Aclass and therefore it will not show any
error on encountering the object of Bclass in the declaration of the friend function. Since it does
not belong to both the classes , while defining it outside we do not give any scope resolution and
we do not invoke it with the help of any object. Also , the keyword friend is just written before the
declaration and not used while defining.
Sometimes friend functions can be avoided using inheritance and they are preferred. Excessive
use of friend over many classes suggests a poorly designed program structure. But sometimes
they are unavoidable.
9.6 Friend for Overloading Operators
Some times friend functions cannot be avoided. For instance with the operator overloading.
Consider the following class that contains data members to simulate a matrix. Several operations
can be performed on the matrices. One of them is to multiply the given matrix by a number(
constant literal). There are two ways in which we can do this. The two ways are :
Matrix * num;
[or]
num * Matrix;
124
In the first case we can overload * to perform the operation and an object invokes this as the
statement gets converted to :
Mobj.operator*(num);
Where Mobj is an object of Matrix and num is a normal integer variable. What happens to the
second one ? It gets converted by the compiler as :
num.operator*(Mobj);
Let us see this program in detail.
class Matrix
{
public:
:
:
Matrix &operator*(int num);
friend Matrix &operator*(int n, Matrix &m);
private:
int mat[20][20];
int rows, cols;
}
Matrix Matrix::operator*(int num)
{
Matrix temp;
temp.rows=rows;
temp.cols=cols;
for(int i=1; i<=rows; i++)
for(int j=1; j<=cols; j++)
temp.mat[i][j]=mat[i][j]*num;
return (temp);
}
Matrix operator*(int n, Matrix &m)
{
Matrix temp;
temp= m*n;
return temp;
}
void main()
{
Matrix M1, M2, M3;
int num;
:
:
// accept matrix one and num
125
M2=M1*num; // calls member operator function.
M3=num*M1; // calls friend function.
}
Here when the compiler comes across the multiplication of an object by a number it invokes the
operator member function. When is encountered before multiplication symbol as in the second
call in the program, the compiler calls friend function . in friend function we have just reversed
the arguments, which causes the member function to be invoked . Intelligent use of friend
functions makes the code more readable.
9.7 Granting friendship to another class:To grant friendship to another class, write the keyword followed by the class name. The keyword
class is optional. Note that this declaration also implies a forward declaration of the class to which
the friendship is being granted. The implication of this declaration is that all off the member
functions of the friend class are friend functions of the class that bestows the friendship.
e.g.
class Aclass
{
public :
:
:
:
friend class Bclass; // Friend declaration.
private :
int Avar;
};
class Bclass
{
public :
:
:
void fn1(Aclass ac)
{
Bvar = ac. Avar; // Avar can be accessed.
}
private :
int Bvar;
};
void main()
{
Aclass aobj;
126
Bclass bobj;
Bobj,fn1(aobj);
}
The program declares class Bclass to be a friend of Aclass. It means that all member functions of
Bclass have been granted direct access to all member functions of Aclass.
9.8 Granting friendship to a member function of another class
If you want class A to grant friendship to one or more individual member functions of class B,
then you must code the classes and their member functions in this manner:
•
Forward declare class A;
•
Define class B and declare (not define) the member functions:
•
Define class A in which you declare the friendship for the member functions of class B. Of
course, you must qualify the names of these functions using the class name B and the
scope resolution operator.
•
Define the member functions of class B;
e.g.
class Aclass
class Bclass
{
public :
:
:
void fn1();
// Can’t define here
void fn3()
{
:
:
}
private :
int Bvar;
};
class Aclass
{
public :
:
127
:
void fn1()
{
:
}
friend classB:: fn1();
friend classB:: fn2();
private :
int Avar;
};
void classB:: fn1()
{
Bvar = Avar;
}
void classB:: fn2()
{
Bvar = variable +25;
}
128
UNIT 10
OPERATOR OVERLOADING
Contents
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.9
Introduction to Operator Overloading.
Operator Overloading Fundamentals.
Implementing the operator functions.
Rules for overloading the operators.
Pointer oddities (assignment) and Operator Overloading.
Overloading the Extraction and Insertion Operators
Conversion functions.
10.7.1 Conversion from basic to user-defined variable.
10.7.2 Conversion from User-Defined to Basic data type
10.7.3 Conversion Between Objects of Different Classes
10.7.4 Conversion function in the Destination Class
Table for Type Conversions
Self Test
10.1 Introduction to Operator Overloading
All computer languages have built in types like integers, real numbers, characters and so on.
Some languages allow us to create our own data types like dates, complex numbers, co-ordinates
of a point. Operations like addition, comparisons can be done only on basic data types and not on
derived (user-defined) data types. If we want to operate on them we must write functions like
compare (), add ().
e.g.
if (compare (v1, v2) = = 0)
:
:
Where v1 and v2 are variables of the new data type and compare () is a function that will contain
actual comparison instructions for comparing their member variables. However, the concept of
Operator Overloading, in C++, allows a statement like
if (v1 = = v2)
:
:
Where the operation of comparing them is defined in a member function and associated with
comparison operator(==).
129
The ability to create new data types, on which direct operations can be performed is called as
extensibility and the ability to associate an existing operator with a member function and use it
with the objects of its class, as its operands, is called as Operator Overloading.
Operator Overloading is one form of Polymorphism ,an important feature of object-oriented
programming .Polymorphism means one thing having many forms, i.e. here an operator can be
overloaded to perform different operations on different data types on different contexts. Operator
Overloading is also called operational polymorphism. Another form of polymorphism is function
overloading.
10.2 Operator Overloading Fundamentals
The C language uses the concept of Operator Overloading Discreetly. The asterisk (*) is used as
multiplication operator as well as indirection (pointer) operator. The ampersand (&) is used as
address operator and also as the bitwise logical ‘AND’ operator. The compiler decides what
operation is to be performed by the context in which the operator is used.
Thus, the C language has been using Operator Overloading internally. Now, C++ has made this
facility public. C++ can overload existing operators with some other operations. If the operator is
not used in the context as defined by the language, then the overloaded operation, if defined will
be carried out.
For example, in the statement
x = y + z;
If x, y and z are integer variables, then the compiler knows the operation to be performed. But, if
they are objects of some class, then the compiler will carry out the instructions, which will be
written for that operator in the class.
10.3 Implementing Operator Functions
The general format of the Operator function is:
return_type operator op ( argument list );
Where op is the symbol for the operator being overloaded. Op
has to be a valid C++ operator, a new symbol cannot be used.
e.g.
Let us consider an example where we overload
unary arithmetic operator ‘++’.
class Counter
{
public :
Counter();
130
void operator++(void);
private :
int Count;
};
Counter::Counter()
{
Count = 0;
}
void Counter::operator++(void)
{
++ Count ;
}
void main()
{
Counter c1;
c1++;
++c1;
// increments Count to 1
// increments Count to 2
}
In main() the increment operator is applied to a specific object. The function itself does not take
any arguments. It increments the data member Count. Similarly, to decrement the Counter object
can also be coded in the class definition as:
void operator--(void)
{
-- Count ;
}
and invoked with the statement
--c1; or c1--;
In the above example , the compiler checks if the operator is overloaded and if an operator
function is found in the class description of the object, then the statement to increment gets
converted, by the compiler, to the following:
c1.operator++();
This is just like a normal function call qualified by the object’s name. It has some special
characters ( ++) in it. Once this conversion takes place, the compiler treats it just like any other
member function from the class. Hence, it can be seen that such a facility is not a very big
overhead on the compiler.
131
However, the operator function in the above example has a potential glitch. On overloading , it
does not work exactly like it does for the basic data types. With the increment and decrement
operators overloaded, the operator function is executed first, regardless of whether the operator is
postfix or prefix.
If we want to assign values to another object in main() we have to return values to the calling
function.
Counter Counter :: operator++(void)
{
Counter temp;
temp.Count = ++ Count ;
return ( temp );
}
void main()
{
Counter c1,c2;
c1 = c2 ++;
//increments to 1, then assigns.
}
In this example , the operator function creates a new object temp of the class Counter, assigns
the incremented value of Count to the data member of temp and returns the new object. This
object is returned to main(). We can do this in another way by creating a nameless temporary
object and return it.
class Counter
{
public :
Counter(); // CONSTRUCTOR WITHOUT ARGUMENTS
Counter( int c); // CONSTRUCTOR WITH 1 ARGUMENT
Counter operator++(void);
private :
int Count;
};
Counter::Counter() // CONSTRUCTOR WITHOUT ARGUMENTS
{
Count = 0;
}
Counter::Counter( int c) // CONSTRUCTOR WITH 1 ARGUMENT
{
Count = c;
132
}
Counter Counter::operator++(void)
{
++ Count ;
return Counter(Count);
}
One change we can see is a constructor with one argument. No new temporary object is explicitly
created. However return statement creates an unnamed temporary object of the class Counter
initializes it with the value in Count and returns the newly created object. Hence one argument
constructor is required.
Yet another way of returning an object from the member function is by using the this pointer. This
special pointer points to the object, which invokes the function. The constructor with one
argument is not required in this approach.
Counter Counter :: operator++(void)
{
++ Count ;
return ( * this);
}
Consider a class COMPLEX for Complex numbers. It will have a real and an imaginary member
variable. Here we can see binary operator overloaded and also how to return values from the
functions.
class COMPLEX
{
public:
COMPLEX operator+(COMPLEX);
private:
int real, imaginary;
};
Suppose that C1, C2 and C3 are objects of this class. Symbolically addition can be carried out as
C3 = C1 + C2;
The actual instructions of the operator are written in a special member function.
e.g.
COMPLEX COMPLEX :: operator+( COMPLEX C2)
{
COMPLEX temp;
temp.real
= real + C2.real;
133
temp.imaginary = imaginary + C2. imaginary;
return (temp);
}
The above example shows how Operator Overloading is implemented. It overloads “+” operator to
perform addition on objects of COMPLEX class. Here we have overloaded a binary operator(+).
10.4 Rules for overloading the operators
This summarizes the most important points you need to know in order to do operator function
overloading.
•
The only operators you may overload are the ones from the C++ list and not all of those are
available. You cannot arbitrarily choose a new symbol (such as @) and attempt to “overload it.
Start by declaring a function in the normal function fashion, but for the function name use
the expression:
Operator op
Where op is the operator to be overloaded. You may leave one or more spaces before op.
•
•
The pre-defined precedence rules cannot be changed. i.e. you cannot, for example,
make binary ‘+’ have higher precedence than binary ‘*’. In addition, you cannot change
the associativity of the operators.
The unary operators that you may overload are:
->
indirect member operator
!
not
&
address
*
dereference
+
plus
minus
++
prefix increment
++
postfix increment (possible in AT & T
version 2.1)
-postfix decrement
-prefix decrement (possible in AT & T
version 2.1)
~
one’s complement
•
•
•
The binary operators that you may overload are:
(), [], new, delete, *, / , %, + , - , <<,>>,
<, <=, >, >=, ==,! =, &, ^, |, &&, ||, =, *=, /=, %=, +=, -, =, <<=, >>=, &=,! =, ^=,
','(Comma).
The operators that can not be overloaded are:
.
.*
134
direct member
direct pointer to member
::
?:
scope resolution
ternary
•
No default arguments are allowed in overloaded operator functions.
•
As with the predefined operators, an overloaded operator may be unary or binary. If it
is normally unary, then it cannot be defined to be binary and vice versa. However, if an
operator can be both unary and binary, then it can be overloaded either way or both.
•
The operator function for a class may be either a non-static member or global friend
function. A non-static member function automatically has one argument implicitly
defined, namely the address of the invoking instance (as specified by the pointer
variable this). Since a friend function has no this pointer, it needs to have all its
arguments explicitly defined).
At least one of the arguments to the overloaded function explicit or implicit must be an
instance of the class to which the operator belongs.
Here you have a table with a summary on how the different operator functions must be declared
(replace @ by the operator in each case):
•
Expression
Operator (@)
@a
+ - * & ! ~ ++ --
A::operator@()
operator@(A)
a@
++ --
A::operator@(int)
operator@(A, int)
a@b
+-*/%^&|<
> == != <= >= <<
>> && || ,
A::operator@(B)
operator@(A, B)
a@b
= += -= *= /= %=
^= &= |= <<= >>=
[]
A::operator@(B)
-
a(b, c...)
()
A::operator()(B, C...)
-
a->b
->
A::operator->()
-
Function member
Global function
* where a is an object of class A, b is an object of class B and c is an object of class C.
You can see in this panel that there are two ways to overload some class operators: as member
function and as global function. Its use is indistinct, nevertheless I remind you that functions that
are not members of a class cannot access the private or protected members of the class unless the
global function is friend of the class (friend is explained later).
Consider an example, which depicts overloading of += (Compound assignment), <, >, ==
(Equality),!=, + (Concatenation) using String class.
class String
{
public :
String ();
String ( char str [] );
135
void putstr();
String operator + (String);
String operator += (String s2);
int operator < (String s2);
int operator > (String s2);
int operator == (String s2);
int operator != (String s2);
private :
char s[100];
};
String::String ()
// CONSTRUCTOR WITH
{
// NO ARGUMENTS
s[0] = 0;
};
String:: String( char str [] ) // CONSTRUCTOR WITH
{
// ONE ARGUMENT
strcpy(s,str)
};
void String:: putstr()// FUNCTION TO PRINT STRING
{
cout << s ;
};
String String :: operator+(String s2)
{
String temp;
strcpy(temp.s,s);
strcat(temp.s,s2.s);
return (temp);
}
String String :: operator+=(String s2)
{
strcat(s,s2.s);
return (*this);
}
136
int String::operator < (String s2)
{
return (strcmp (s, s2.s ) < 0);
}
int String::operator > (String s2)
{
return (strcmp (s, s2.s ) > 0);
}
int String::operator == (String s2)
{
return (strcmp (s, s2.s ) == 0);
}
int String::operator != (String s2)
{
return (strcmp (s, s2.s ) != 0);
}
void main()
{
String s1 = “welcome “;
String s2 = “ to the world of c++”;
String s3;
cout << endl << “s1 = “;
s1.putstr();
cout << endl << “s2 = “;
s2.putstr();
s3 = s1 + s2;
cout << endl << “ s3 = “;
s3.putstr();
String s4;
cout <<endl<<” *********************”;
s4 = s1 + = s2;
cout << endl << “ s4 = “;
137
s4.putstr();
String s5 = “ Azzzz “;
String s6 = “ Apple “;
if( s5 < s6 )
{
s5.putstr();
cout << ” < ”;
s6.putstr();
}
else if( s5 > s6 )
{
s5.putstr();
cout << ” > ”;
s6.putstr();
}
else
{
if( s5 == s6 )
s5.putstr();
cout << ” = ”;
s6.putstr();
}
else
{
if( s5 != s6 )
s5.putstr();
cout << ” < ”;
s6.putstr();
}
}
Output:
S1 = welcome
S2 = to the world of C++
S3 = welcome to the world of c++
**************************
S4 = welcome to the world of c++
// vectors: overloading operators example
#include <iostream.h>
class CVector {
public:
int x,y;
CVector () {};
CVector (int,int);
CVector operator + (CVector);
};
138
CVector::CVector (int a, int b) {
x = a;
y = b;
}
CVector CVector::operator+ (CVector param) {
CVector temp;
temp.x = x + param.x;
temp.y = y + param.y;
return (temp);
}
int main () {
CVector a (3,1);
CVector b (1,2);
CVector c;
c = a + b;
cout << c.x << "," << c.y;
return 0;
}
10.5 Pointer Oddities and Operator Overloading
Consider an example, where the data members contain pointers and have been allocated memory
using the operator new. In this case using an assignment operator to assign one object to another
will result in the pointer variable being copied rather than the contents at the address. The
following example explains this problem.
class String
{
public :
String (char *s = “”)
// CONSTRUCTOR
{
size = strlen(s);
cptr = new char [size + 1];
strcpy(cptr,s);
};
~String()
{
delete cptr;
139
}
void putstr()
//
FUNCTION TO PRINT STRING
{
cout << cptr ;
};
private :
char *cptr;
int size;
};
void main()
{
String s1(“hello students “);
String s2;
s2 = s1; // Assignment
s1.putstr();
s2.putstr();
}
The constructor function allocates a string and copies the contents of its formal variable in it. The
assignment operator in main() assigns the object s1 to s2. the data members’ cptr and size , of
object s1, gets assigned to s2. The output of the program is :
hello students
hello students
Null pointer assignment
Why does the program give a ‘null pointer assignment’ message ? After the program is over, the
destructor function is called automatically, which releases the memory allocated by new to the
data member cptr. But, after the assignment, the data member cptr of both the objects point to
the same location in the memory. Thus, the delete operator called for the first object releases the
memory and for the second call attempts to release the same memory location again, resulting in
the error message.
The solution to this problem is to define an operator function for the assignment operator . It can
be done as follows:
String operator = (String s2)
{
delete cptr;
size = strlen ( s2.cptr );
cptr = new char [size + 1];
strcpy(cptr, s2.cptr);
return (*this);
140
}
On including this member function in the class definition of the above example , the program
outputs the following :
hello students
hello students
Hence, the operator function for assignment of an object to another of the same class removes the
quirks associated with pointers as data members.
10.6 Overloading the Extraction and Insertion Operators
We’ll finish this session by showing how to overload the extraction and insertion operators. Here,
you can accept the input and output the results of the user-defined variables or objects just like
normal variables. Consider a class Complex that consists of two system defined variables, both
float to denote the real and complex part of a complex number. In general cases to accept these
member variables, we need to write some function say getval() and invoke it using the object of the
class say Comobj.getval() and similar method would be required to display them. Using operator
overloading we can accept or display the user-defined object just like a normal variable . i.e. by
overloading the extraction operator >> we can accept the complex object as,
cin >> comobj;
Similarly, by overloading the insertion operator we can display the member variables of the object
as,
cout << comobj;
just as if it were a basic data type. Consider the example :
class Complex
{
public:
friend istream& operator >> (istream &is, Complex &c2)
friend ostream& operator << (ostream &os, Complex &c2)
private:
float real, imaginary;
};
istream& operator >> (istream &is, Complex &c2)
{
cout << “ enter real and imaginary “ << endl;
141
is >> c2.real >> c2.imaginary;
return (is);
}
istream& operator << (ostream &os, Complex &c2)
{
os << “ the complex number is “ <<endl;
os << c2.real << “+i”<< c2.imaginary;
return (os);
}
void main()
{
Complex c1,c2;
cin >> c1;
cout << c1;
cin >> c2;
cout << c2;
}
The operator functions have to be declared friends , since they have to access the user class and
the objects of istream and ostream classes that are system defined. Since these operator functions
are friend functions, the two objects – cin and cout are passed as arguments, along with the
objects of the user-class. They return the istream and ostream objects so that the operator can be
chained. That is the above two input statements can also be written as,
cin >> c1 >> c2;
cout << c1 << c2;
10.7 Conversion functions
Conversion functions are member functions used for the following purposes:
1.
2.
3.
Conversion of object to basic data type.
Conversion of basic data type to object.
Conversion of objects of different classes.
Conversions of one basic data type to another are automatically done by the compiler using its
own built-in routines (implicit) and it can be forced using built-in conversion routines (explicit).
However, since the compiler does not know anything about user-defined types (classes), the
program has to define conversion functions.
e.g.
int i = 2, j =19;
142
float f = 3.5;
i = f; // i gets the value 3 , implicit conversion
f = (float) j; // f gets 19.00, explicit conversion
10.7.1 Conversion from Basic to User-Defined variable
Consider the following example.
class Distance
{
public :
Distance(void) // Constructor with no
{
// argument
feet = 0;
inches = 0.0;
};
Distance(float metres)
{
float f;
// Constructor with
f = 3.28 * metres;
// one argument
feet = int(f);
// also used for
inches = 12 * ( f – feet);// conversion
void display(void)
{
cout << “ Feet = “ << feet <<”,”;
cout << “ Inches = “ << inches << endl;
};
private :
int feet;
float inches;
};
void main (void)
{
Distance d1 = 1.25; // Uses 2nd constructor
Distance d2;
// Uses 1st constructor
float m;
d2 = 2.0 ;
// Uses 2nd constructor
cout << “ 1.25 metres is : “ << d1.showdist() ;
143
};
cout << “ 2.0 metres is :“ << d2.showdist();
}
Output :
1.25 metres is :FEET = 4 , INCHES = 1.199999
2.0 metres is :FEET = 6 , INCHES = 6.719999
The above program converts distance in metres ( basic data type) into feet and inches ( members
of an object of class Distance ).
The declaration of first object d1 uses the second constructor and conversion takes place.
However, when the statement encountered is
d2 = 2.0;
The compiler first checks for an operator function for the assignment operator. If the assignment
operator is not overloaded, then it uses the constructor to do the conversion.
10.7.2. Conversion from User-Defined to Basic data type
The following program uses the program in the previous section to convert the Distance into
metres(float).
class Distance
{
public :
Distance(void) // Constructor with no
{
// argument
feet = 0;
inches = 0.0;
};
Distance(float metres)
{
float f;
// Constructor with
f = 3.28 * metres; // one argument
feet = int(f);
// Also used for
= 12 * ( f – feet); //conversion
};
operator float(void)
// Conversion function
{
// from Distance to float
float f;
f = inches / 12;
f = f + float (feet);
return ( f/ 3.28 );
};
144
inches
void display(void)
{
cout << “ Feet = “ << feet <<”,”;
cout << “ Inches = “ << inches << endl;
};
private :
int feet;
float inches;
};
void main (void)
{
Distance d1 = 1.25; // Uses 2nd constructor
Distance d2;
// Uses 1st constructor
float m;
d2 = 2.0 ;
// Uses 2nd constructor
cout << “ 1.25 metres is :“ << d1.showdist ();
cout << “ 2.0 metres is :“ << d2.showdist ();
cout << “ CONVERTED BACK TO METRES “;
m = float ( d1 ); // Calls function explicitly.
cout << “ d1 = “ << m;
m = d2;
// Calls function explicitly.
cout << “ d2 = “ << m;
}
Output:
1.25 metres is :FEET = 4 ,INCHES = 1.199999
2.0 metres is :FEET = 6 ,INCHES = 6.719999
CONVERTED BACK TO METRES
d1 = 1.25
d2 = 2.00
Actually, this conversion function is nothing but overloading the typecast operator float(). The
conversion is achieved explicitly and implicitly.
145
m = float (d1);
is forced where as , in the second assignment statement
m = d2;
first the compiler checks for an operator function for assignment ( = ) operator and if not found it
uses the conversion function.
The conversion function must not define a return type nor should it have any arguments.
10.7.3 Conversion Between Objects of Different Classes
Since the compiler does not know anything about the user-defined type, the conversion
instructions are to be specified in a function. The function can be a member function of the
source class or a member function of the destination class. We will consider both the cases.
Consider a class DistFeet which stores distance in terms of feet and inches and has a constructor
to receive these. The second class DistMetres store distance in metres and has a constructor to
receive the member.
Conversion function in the Source Class
class DistFeet
{
public :
DistFeet(void)
{
// Constructor with no
// argument
feet = 0;
inches = 0.0;
};
DistFeet(int ft,float in)
{
feet = ft;
inches = in
};
void ShowFeet(void)
{
cout << “ Feet = “ << feet << “,”;
cout << “ Inches = “ << inches << endl;
};
private :
int feet;
float inches;
};
146
class DistMetres
{
public:
DistMetres(void)
{
metres = 0 ; // constructor 1.
}
DistMetres(float m)
{
metres = m ; // constructor 2.
}
void ShowMetres(void)
{
cout << “ Metres = “ << metres << endl;
};
operator DistFeet(void) // conversion
{
// function
float ffeet, inches;
int ifeet;
ffeet = 3.28 * metres;
ifeet = int (ffeet);
inches = 12 * (ffeet – ifeet);
return(DistFeet(inches,ifeet);
};
private:
float metres;
};
void main (void)
{
DistMetres dm1 = 1.0;
DistFeet df1;
df1 = dm1 ; // OR df1 = DistFeet(dm1);
// Uses conversion function
dm1.ShowMetres();
df1.ShowFeet();
}
147
In the above example, DistMetres contains a conversion function to convert the distance from
DistMetres ( source class), to DistFeet ( destination class). The statement to convert one object to
another
df1 = dm1;
calls the conversion function implicitly. It could also have been called explicitly as
df1 = DistFeet(dm1);
10.7.4 Conversion function in the Destination Class
class DistMetres
{
public:
DistMetres(void)
{
metres = 0 ; // Constructor 1.
}
DistMetres(float m)
{
metres = m ; // constructor 2.
}
void ShowMetres(void)
{
cout << “ Metres = “ << metres << endl;
};
float GetMetres(void)
{
return(metres);
}
private:
float metres;
};
class DistFeet
{
public :
DistFeet(void) // Constructor1 with no
{
// argument
feet = 0;
inches = 0.0;
148
};
DistFeet(int ft,float in)
{
feet = ft;
inches = in;
};
void ShowFeet(void)
{
cout << “ Feet = “ << feet << endl;
cout << “ Inches = “ << inches << endl;
};
DistFeet( DistMetres dm) // Constructor 3
{
float ffeet;
ffeet = 3.28 * dm.GetMetres();
feet = int (ffeet);
inches = 12 * (ffeet – ifeet);
};
private :
int feet;
float inches;
};
void main (void)
{
DistMetres dm1 = 1.0;
DistFeet df1;
df1 = dm1 ;
// Uses 2nd constructor
// class DistMetres
// Uses 1st constructor
// class DistFeet
// OR df1 = DistFeet(dm1);
// Uses 3rd conversion function
dm1.ShowMetres();
df1.ShowFeet();
}
This program works same as previous function. Here constructor is written in the destination
class. Also, we can see a new function GetMetres() . The function returns the data member
metres of the invoking object. The function is required because the constructor is defined in the
DistFeet class and since metres is a private member of the DistMetres class, it cannot be
accessed directly in the constructor function in the DistFeet class.
149
Since you can use any of the above methods, it is strictly a matter of choice which method you
choose to implement.
10.8 Table for Type Conversions
Operation Function
in Destination Class
Operation Function
in Source
Class
Basic to class
Constructor
Not Allowed
Class to Basic
Not Allowed
Conversion Function
Class to Class
Constructor
Conversion Function
This session covered yet another concept of OOP – Polymorphism. It means one thing
having many forms. It is very powerful, yet, simple concept which gives the C++ language a facility
to redefine itself into a new language. There are two types of Polymorphism- operational and
functional. This session covered operational polymorphism also called as operator overloading. We
will see function overloading in future.
10.9 Self Test
1.
WAP to add 2 complex number using OOT(Operator Overloading techniques).
2.
WAP to add 2 times using OOT and display the resultant time in watch format.
3. WAP to add, subtract and multiply 2 matrices using OOT.
Sort an array of objects. Each object has a string as a member variable. Overload >= or <=
operators to compare the two strings.
{ make use of constructors and destructors whenever possible }
4.
WAP to create a class called DATE . Accept 2 valid dates in the form of dd/mm/yyyy.
Implement the following by overloading the operators – and + . Display the result after
every operation.
no_of_dasy = d1 – d2, where d1 and d2 are DATE objects;
d1 > = d2 ; and no_of_days is an integer.
b) d1 = d1 + no_of_days - where d1 is a DATE object and
no_of_days is an integer.
a)
150
5.
Modify the matrix program (program 3) slightly. Overload == operator to compare 2
matrices to be added or subtracted. i.e., whether the column of first and the row of
second matrix are same or not.
if(m1==m2)
{
m3=m1+m2;
m4=m1-m2;
}
else
display error;
6. WAP to concatenate 2 strings by using a copy constructor.
------------ --------------- ---------------- --------------
151
UNIT 11
INHERITANCE
Contents
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
Reusability.
Inheritance concept- single inheritance.
11.2.1
Private derivation
11.2.2
Public derivation
11.2.3
The Protected Access
11.2.4
Summary of derivation
11.2.5
Table of derivation and access specifiers
Using the derived class
Constructor and destructor in derived class.
Object initialization and conversion.
Nested classes (Container classes).
Multilevel inheritance.
Multiple inheritance.
Hybrid Inheritance.
Virtual base class.
Object-oriented programming as seen in the preceding sessions emphasizes the data,
rather than emphasizing algorithms. The previous sessions covered OOP features like
extensibility, data encapsulation, information hiding, functional polymorphism and operational
polymorphism. OOP, however, has more jargon associated to it, like reusability, inheritance. This
session covers reusability and inheritance.
11.1 Reusability
Reusability means reusing code written earlier ,may be from some previous project or from the
library. Reusing old code not only saves development time, but also saves the testing and
debugging time. It is better to use existing code, which has been time-tested rather than reinvent
it. Moreover, writing new code may introduce bugs in the program. Code, written and tested
earlier, may relieve a programmer of the nitty-gritty. Details about the hardware, user-interface,
files and so on. It leaves the programmer more time to concentrate on the overall logistics of the
program.
What is inheritance?
Class, the vehicle, which is used to implement object-oriented concepts in C++, has given a
new dimension to this idea of reusability. Many vendors now offer libraries of classes. A class
library consists of data and methods to operate on that data, encapsulated in a class . The source
152
code of these libraries need not be available to modify them. The new dimension of OOP uses a
method called inheritance to modify a class to suit one’s needs. Inheritance means deriving new
classes from the old ones. The old class is called the base class or parent class or super class and
the class which is derived from this base class is called as derived class or child class or sub class.
Deriving a new class from an existing one , allows redefining a member function of the base class
and also adding new members to the derived class . and this is possible without having the source
of the course definition also. In other words, a derived class not only inherits all properties of the
base class but also has some refinements of its own. The base class remains unchanged in the
process. In other words, the derived class “is a “ type of base class, but with more details added.
For this reason, the relationship between a derived class and its base class is called an “is-a”
relationship.
Class A
Class B
Single Inheritance
Here class A is a base class and the class B is the derived class.
How to define a derived class ?
A singly inherited derived class id defined by writing :
•
The keyword class.
•
The name of the derived class .
•
A single colon (:).
•
The type of derivation ( private , protected, or public ).
•
The name of the base, or parent class.
•
The remainder of the class definition.
e.g.
class A
{
public :
int public_A;
void public_function_A();
private :
int pri_A;
void private_function_A();
153
protected :
int protected_A;
void protected_function_A();
};
class B : private A
{
public :
int public_B;
void public_function_B();
private :
int pri_B;
void private_function_B();
};
class C : public A
{
public :
int public_C;
void public_function_C();
private :
int pri_C;
void private_function_C();
};
class D : protected A
{
public :
int public_D;
void public_function_D();
private :
int pri_D;
void private_function_D();
};
A derived class always contains all of the member members from its base class . you cannot
“subtract” anything from a base class. However, accessing the inherited variables is a different
matter. It is also important to understand the privileges that the derived class has insofar as
access to members of its base class are concerned. In other words, just because you happen to
derive a class does not mean that you are automatically granted complete and unlimited access
privileges to the members of the base class. to understand this you must look at the different
types of derivation and the effect of each one.
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11.2.1. Private derivation
If no specific derivation is listed, then a private derivation is assumed. If a new class is derived
privately from its parent class , then :
•
The private members inherited from its base class are inaccessible to new member
functions in the derived class . this means that the creator of the base class has absolute
control over the accessibility of these members , and there is no way that you can override
this.
•
The public members inherited from the base class have private access privilege. In other
words, they are treated as though they were declared as new private members of the
derived class, so that new member functions can access them. However, if another private
derivation occurs from this derived class, then these members are inaccessible to new
member functions.
e.g.
class base
{
private :
int number;
};
class derived : private base
{
public :
void f()
{
++number;
// Private base member not
accessible
}
};
The compiler error message is
‘ base :: number ‘ is not accessible in the function derived :: f();
e.g.
class base
{
public :
int number;
};
class derived : private base
{
155
public :
void f()
{
++number;
// Access to number O.K.
}
};
class derived2 : private derived
{
public :
void g()
{
++number;
// Access to number is
prohibited.
}
};
The compiler error message is
‘ base :: number ‘ is not accessible in the function derived2 :: g();
Since public members of a base class are inherited as private in the derived class, the function
derived :: f() has no problem accessing it . however, when another class is derived from the class
derived , this new class inherits number but cannot access it. Of course, if derived1::g() were to
call upon derived::f(), there is no problem since derived::f() is public and inherited into derived2 as
private.
i.e. In derived2 we can write,
void g()
{
f();
}
or there is another way. Writing access declaration does this.
class base
{
public :
int number;
};
class derived : private base
{
public : base :: number ;
void f()
{
156
++number;
// Access to number O.K.
}
};
class derived2 : private derived
{
public :
void g()
{
++number;
}
};
// Access to number O.K
As you have just seen private derivations are very restrictive in terms of accessibility of the
base class members . therefore, this type of derivation is rarely used.
11.2.2 Public derivation
Public derivations are much more common than private derivations. In this situation :
•
The private members inherited from the base class are inaccessible to new members
functions in the derived class.
•
The public members inherited from the base class may be accessed by new members
functions in the derived class and by instances of the derived class .
e.g.
class base
{
private :
int number;
};
class derived : public base
{
public :
void f()
{
++number;
// Private base member not
accessible
}
};
157
The compiller error message is
‘ base :: number ‘ is not accessible in the function derived::f();
Here, only if the number is public then you can access it.
Note : However example 2 and 3 in the above section works here if you derive them as “public”.
11.2.3 The Protected Access
In the preceding example, declaring the data member number as private is much too
restrictive because clearly new members function in the derived class need to gain access to it and
in order to perform some useful work.
To solve this dilemma, the C++ syntax provides another class access specification called
protected . here is how protected works :
•
In a private derivation the protected members inherited from the base class have private
access privileges. Therefore, new member functions and friend of the derived class may
access them.
•
In a public derivation the protected members inherited from the base class retain their
protected status. They may be accessed by new members function and friends of the
derived class .
In both situations the new members functions and friends of the derived class have unrestricted
access to protected members . however, as the instances of the derived class are concerned,
protected and private are one and the same, so that direct access id always denied. Thus, you can
see that the new category of protected provides a middle ground between public and private by
granting access to new function and friends of the derived class while still blocking out access to
non-derived class members and
friend functions .
class base
{
protected :
int number;
};
class derived : public base
{
public :
void f()
{
++number;
}
};
// base member access O.K.
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Protected derivation
In addition to doing private and public derivations, you may also do a protected derivation.
In this situation :
•
The private members inherited from the base class are inaccessible to new member
functions in the derived class.
( this is exactly same as if a private or public derivation
has occurred.)
•
The protected members inherited from the base class have protected access privilege.
•
The public members inherited from the base class have protected have protected access.
Thus , the only difference between a public and a protected derivation is how the public
members of the parent class are inherited. It is unlikely that you will ever have occasion to do this
type of derivation.
Summary of access privileges
1. If the designer of the base class wants no one, not even a derived class to access a member
, then that member should be made private .
2. If the designer wants any derived class function to have access to it, then that member
must be protected.
3. if the designer wants to let everyone , including the instances, have access to that member
, then that member should be made public .
11.2.4 Summary of derivations
1. Regardless of the type of derivation, private members are inherited by the derived class ,
but cannot be accessed by the new member function of the derived class , and certainly
not by the instances of the derived class .
2. In a private derivation, the derived class inherits public and protected members as private
. a new members function can access these members, but instances of the derived class
may not. Also any members of subsequently derived classes may not gain access to these
members because of the first rule.
3. In public derivation, the derived class inherits public members as public , and protected as
protected . a new member function of the derived class may access the public and
protected members of the base class ,but instances of the derived class may access only
the public members.
4. In a protected derivation, the derived class inherits public and protected members as
protected .a new members function of the derived class may access the public and
protected members of the base class, both instances of the derived class may access only
the public members .
11.2.5 Table of Derivation and access specifiers
159
Derivation Type
Base Class Member
Private
Public
Protected
Access in Derived Class
Private
(inaccessible )
Public
Private
Protected
Private
Private
(inaccessible )
Public
Public
Protected
Protected
Private
(inaccessible )
Public
Protected
Protected
Protected
We can summarize the different access types according to whom can access them in the following
way:
Access
public
protected
private
members of the same class
yes
yes
yes
members of derived classes
yes
yes
no
Not-members
yes
no
no
where "not-members" represent any reference from outside the class, such as from main(), from
another class or from any function, either global or local.
11.3. Using the Derived Class
An instance of a derived class has complete access to the public members of the base class .
assuming that the same name does not exist within the scope of the derived class , the members
from the base class will automatically be used. Because there is no ambiguity involved in this
situation, you do not need to use scope resolution operator to refer to this base class member.
class base
{
public :
base(int n = 0)
{
number = n;
}
int get_number();
protected :
int number;
};
160
int base :: get_number()
{
return number;
}
class derived : public base
{
};
void main()
{
derived d;
// First checks class derived , then class base
cout << d.get_number();
// Goes directly to class base
cout<< d.base ::get_number();
}
Output:
0
0.
11.4 Constructor and destructor in derived class.
What is inherited from the base class?
In principle every member of a base class is inherited by a derived class except:
•
Constructor and destructor
•
operator=() member
•
friends
Although the constructor and destructor of the base class are not inherited, the default
constructor (i.e. constructor with no parameters) and the destructor of the base class are always
called when a new object of a derived class is created or destroyed.
If the base class has no default constructor or you want that an overloaded constructor is called
when a new derived object is created, you can specify it in each constructor definition of the
derived class:
derived_class_name (parameters) : base_class_name (parameters) {}
For example:
mother: no parameters
daughter: int parameter
// constructors and derivated classes
#include <iostream.h>
161
class mother {
public:
mother ()
{ cout << "mother: no parameters\n"; }
mother (int a)
{ cout << "mother: int parameter\n"; }
};
mother: int parameter
son: int parameter
class daughter : public mother {
public:
daughter (int a)
{ cout << "daughter: int parameter\n\n"; }
};
class son : public mother {
public:
son (int a) : mother (a)
{ cout << "son: int parameter\n\n"; }
};
int main () {
daughter cynthia (1);
son daniel(1);
return 0;
}
Observe the difference between which mother's constructor is called when a new daughter object
is created and which when it is a son object. The difference is because the constructor declaration
of daughter and son:
daughter (int a)
// nothing specified: call default constructor
son (int a) : mother (a) // constructor specified: call this one
11.5 Object Initialization and conversion
Initialization
An object of a derived class can be initialized to an object of a base class . If both the classes have
same data members , then no specific constructor needs to be defined in the derived class . It
uses the constructor of the base class . An object of a base class can be assigned to the object of
the derived class , if the derived class doesn’t contain any additional data members . However, if it
does , then the assignment operator will have to be overloaded for the same.
Conversions
162
Just like initialization , conversions are also done automatically when an object of a derived class
is assigned to an object of the base class . However, the compiler resorts to a member-wise
assignment in the absence of an overloaded function for the assignment operator .
11.6 Nested Classes
Many a times, it becomes necessary to have a class contain properties of two other classes. One
way is to define a class within another – that is a class with member classes also called nested
classes. This has nothing to do with inheritance. Another way is multiple inheritance , which will
be discussed later.
e.g.
class Aclass
{
public :
Aclass(int pv)
{
private_variable_A = pv;
}
private :
int private_variable_A;
};
class Bclass
{
public :
Bclass(int bpv, int apv): Aobj(apv)
{
private_variable_B = bpv;
}
private :
int private_variable_B;
Aclass Aobj; // Declaring an object here.
};
As can be seen , the class Bclass contains an object Aobj in its private section as one of its
members. Also, it contains a constructor function with the same name , to which the two
variables passed are bpv and apv. The variable bpv is used to initialize the private variable of
Bclass.
However, the constructor contains something after the colon. The part after the colon in the
definition of a constructor is called as initialization section and the actual body of the constructor
is called as assignment section . Initialization section initializes the base class members , whereas
assignment section contains statements to initialize the derived class members .
163
As seen earlier , in case of the derived classes, the name of the base class constructor is written
after colon in the initialization section. This base class constructor is called before the constructor
in the derived class. However, this example does not contain a derived class. This is the example
of the nested class . In this case, the name of the object of the member class ‘Aclass’ is written
after the colon. It tells the compiler to initialize the Aobj data member of Bclass with the value in
apv. It is exactly like declaring an object of Aclass with the statement :
Aclass Aobj(apv);
The only change is that, it is written after the colon in the initialization section of the Bclass
constructor. Its assignment section contains code to initialize its own members. The same
constructor function can also be written as :
Bclass (int bovine apv):Aobj(apv),private_variable_B = bpv;
11.7 Multilevel Inheritance
In multilevel inheritance there is a parent class , from whom we derive another class . now from
this derived class we can derive another class and so on.
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Class A
Class B
Class C
Multilevel Inheritance
class Aclass
{
:
:
}
class Bclass : public Aclass
{
:
:
}
class Cclass : public Bclass
{
:
:
}
11.8 Multiple Inheritance
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Multiple inheritance , as the name suggests , is deriving a class from more than
one class . The derived class inherits all the properties of all its base classes. Consider
the following example :
Class A
Class B
Class C
Multiple Inheritance
class Aclass
{
:
:
};
class Bclass
{
:
:
};
class Cclass : public Aclass , public Bclass
{
private :
:
:
public :
Cclass(...) : Aclass (...), Bclass(...)
{
};
};
The class Cclass in the above example is derived from two classes – Aclass and Bclass –
therefore the first line of its class definition contains the name of two classes, both
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publicly inherited. Like with normal inheritance , constructors have to be defined for
initializing the data members of all classes. The constructor in Cclass
calls
constructors for base classes. The
constructor calls are separated by commas.
11. 9 Multiple inheritance with a common base (Hybrid Inheritance)
Inheritance is an important and powerful feature of OOP. Only the imagination of the
person concerned is the limit. There are many combinations in which inheritance can
be put to use. For instance, inheriting a class from two different classes, which in turn
have been derived from the same base class .
e.g.
base
Class A
Class B
derived
Hybrid Inheritance
class base
{
:
:
};
class Aclass : public base
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{
:
:
};
class Bclass : public base
{
:
:
};
class derived : public Aclass, public Bclass
{
:
:
};
Aclass and Bclass are two classes derived from the same base class . The class derived
has a common ancestor – class base. This is multiple inheritance with a common base
class . However, this subtlety of class inheritance is not all that simple. One potential
problem here is that both, Aclass and Bclass, are derived from base and therefore both
of them, contains a copy of the data members base class. The class derived is derived
from these two classes. That means it contains two copies of base class members – one
from Aclass and the other from Bclass. This gives rise to ambiguity between the base
data members. Another problem is that declaring an object of class derived will invoke
the base class constructor twice. The solution to this problem is provided by virtual
base classes.
11. 10 Virtual Base Classes
This ambiguity can be resolved if the class derived contains only one copy of the class
base. This can be done by making the base class a virtual class. This keyword makes
the two classes share a single copy of their base class . It can be done as follows :
class base
{
:
:
};
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class Aclass : virtual public base
{
:
:
};
class Bclass : virtual public base
{
:
:
};
class derived : public Aclass, public Bclass
{
:
:
};
This will resolve the ambiguity involved.
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UNIT 12
ABSTRACT AND VIRTUAL FUNCTION
Contents
12.1
Abstract class.
12.2
Virtual function.
12.3
Pure virtual function
12.4
Self test
12.1. Abstract Classes
Abstract classes are the classes, which are written just to act as base classes. Consider
the following classes.
class base
{
:
:
};
class Aclass : public base
{
:
:
};
class Bclass : public base
{
:
:
};
class Cclass : public base
{
:
:
};
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void main()
{
Aclass objA;
Bclass objB;
Cclass objC;
:
:
}
There are three classes – Aclass, Bclass, Cclass – each of which is derived from the
class base. The main () function declares three objects of each of these three classes.
However, it does not declare any object of the class base. This class is a general class
whose sole purpose is to serve as a base class for the other three. Classes used only for
the purpose of deriving other classes from them are called as abstract classes. They
simply serve as base class , and no objects for such classes are created.
12.2 Virtual Functions
The keyword virtual was earlier used to resolve ambiguity for a class derived from two
classes, both having a common ancestor. These classes are called virtual base classes.
This time it helps in implementing the idea of polymorphism with class inheritance .
The function of the base class can be declared with the keyword virtual. The program
with this change and its output is given below.
class Shape
{
public :
virtual void print()
{
cout << “ I am a Shape “ << endl;
}
};
class Triangle : public Shape
{
public :
void print()
{
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cout << “ I am a Triangle “ << endl;
}
};
class Circle : public Shape
{
public :
void print()
{
cout << “ I am a Circle “ << endl;
}
};
void main()
{
Shape S;
Triangle T;
Circle C;
S.print();
T.print();
C.print();
Shape *ptr;
ptr = &S;
ptr -> print();
ptr = &T;
ptr -> print();
ptr = &C;
ptr -> print();
}
The output of the program is given below:
I am a Shape
I am a Triangle
I am a Circle
I am a Shape
I am a Triangle
I am a Circle
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Now, the output of the derived classes are invoked correctly. When declared with the
keyword virtual , the compiler selects the function to be invoked, based upon the
contents of the pointer and not the type of the pointer. This facility can be very
effectively used when many such classes are derived from one base class . Member
functions of each of these can be ,then, invoked using a pointer to the base class .
12.3 Pure Virtual Functions
As discussed earlier, an abstract class is one, which is used just for deriving some other
classes. No object of this class is declared and used in the program. Similarly, there are
pure virtual functions which themselves won’t be used. Consider the above example
with some changes.
class Shape
{
public :
virtual void print() = 0; // Pure virtual
function
};
class Triangle : public Shape
{
public :
void print()
{
cout << “ I am a Triangle “ << endl;
}
};
class Circle : public Shape
{
public :
void print()
{
cout << “ I am a Circle “ << endl;
}
};
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void main()
{
Shape S;
Triangle T;
Circle C;
Shape *ptr;
ptr = &T;
ptr -> print();
ptr = &C;
ptr -> print();
}
The output of the program is given below:
I am a Triangle
I am a Circle
It can be seen from the above example that , the print() function from the base class is
not invoked at all . even though the function is not necessary, it cannot be avoided,
because , the pointer of the class Shape must point to its members.
Object oriented programming has altered the program design process. Exciting OOP
concepts like polymorphism have given a big boost to all this. Inheritance has further
enhanced the language. This session has covered some of the finer aspects of
inheritance. The next session will resolve some finer aspects of
the language.
EXAMPLE:20
10
0
// virtual members
#include <iostream.h>
class CPolygon {
protected:
int width, height;
public:
void set_values (int a, int b)
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{ width=a; height=b; }
virtual int area (void)
{ return (0); }
};
class CRectangle: public CPolygon {
public:
int area (void)
{ return (width * height); }
};
class CTriangle: public CPolygon {
public:
int area (void)
{ return (width * height / 2); }
};
int main () {
CRectangle rect;
CTriangle trgl;
CPolygon poly;
CPolygon * ppoly1 = &rect;
CPolygon * ppoly2 = &trgl;
CPolygon * ppoly3 = &poly;
ppoly1->set_values (4,5);
ppoly2->set_values (4,5);
ppoly3->set_values (4,5);
cout << ppoly1->area() << endl;
cout << ppoly2->area() << endl;
cout << ppoly3->area() << endl;
return 0;
}
Now the three classes (CPolygon, CRectangle and CTriangle) have the same members:
width, height, set_values() and area(). area() has been defined as virtual because it is
later redefined in derived classes. You can verify if you want that if you remove this
word (virtual) from the code and then you execute the program the result will be 0 for
the three polygons instead of 20,10,0. That is because instead of calling the
corresponding area() function for each object (CRectangle::area(), CTriangle::area()
and CPolygon::area(), respectively), CPolygon::area() will be called for all of them since
the calls are via a pointer to CPolygon.
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Therefore what the word virtual does is to allow that a member of a derived class with
the same name as one in the base class be suitably called when a pointer to it is used,
as in the above example.
Note that in spite of its virtuality we have also been able to declare an object of type
CPolygon and to call its area() function, that always returns 0 as the result.
12.4 Self test
Create a class drugs containing encapsulated data for medicine name, whether solid or
liquid, price and purpose of use. From this class derive two classes, Ayurvedic and
Allopathic. The class Ayurvedic should additionally store data on the herbs used,
association to be used (whether honey or water). The class Allopathic should
additionally include data on the chemicals used and the weight in milligrams. The
classes should contain constructors and destructors. They should contain functions to
accept data and display the data. The main() should test the derived classes.
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UNIT 13
TEMPLATES AND EXCEPTION HANDLING
Contents
13.1
Templates.
13.1.1
Function template
13.1.2
Class templates
13.1.3
Template specialization
13.1.4
Parameter values for templates
13.1.5
Templates and multiple -file project
13.2
Exception handling
13.2.1
Exception not caught
13.2.2
Standard exception
13.3
Advanced class type-casting.
13.3.1
reinterpret cast
13.3.2
static cast
13.3.3
dynamic cast
13.3.4
const_cast
13.3.5
typeid
13.4
Preprocessor directives.
13.1 Templates
13.1.1 Function templates
Templates allow to create generic functions that admit any data type as parameters and
return a value without having to overload the function with all the possible data types.
Until certain point they fulfill the functionality of a macro. Its prototype is any of the
two following ones:
template <class identifier> function_declaration;
template <typename identifier> function_declaration;
the only difference between both prototypes is the use of keyword class or typename,
its use is indistinct since both expressions have exactly the same meaning and behave
exactly the same way.
For example, to create a template function that returns the greater one of two objects
we could use:
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template <class GenericType>
GenericType GetMax (GenericType a, GenericType b) {
return (a>b?a:b);
}
As the first line specifies, we have created a template for a generic data type that we
have called GenericType. Therefore in the function that follows, GenericType becomes
a valid data type and it is used as the type for its two parameters a and b and as the
return type for the function GetMax.
GenericType still does not represent any concrete data type; when the function
GetMax will be called we will be able to call it with any valid data type. This data type
will serve as a pattern and will replace GenericType in the function. The way to call a
template class with a type pattern is the following:
function <pattern> (parameters);
Thus, for example, to call GetMax and to compare two integer values of type int we can
write:
int x,y;
GetMax <int> (x,y);
so GetMax will be called as if each appearance of GenericType was replaced by an int
expression.
Here is the complete example:
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// function template
#include <iostream.h>
template <class T>
T GetMax (T a, T b) {
T result;
result = (a>b)? a : b;
return (result);
}
int main () {
int i=5, j=6, k;
long l=10, m=5, n;
k=GetMax<int>(i,j);
n=GetMax<long>(l,m);
cout << k << endl;
cout << n << endl;
return 0;
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}
(In this case we have called the generic type T instead of GenericType because it is
shorter and in addition is one of the most usual identifiers used for templates, although
it is valid to use any valid identifier).
In the example above we used the same function GetMax() with arguments of type int
and long having written a single implementation of the function. That is to say, we have
written a function template and called it with two different patterns.
As you can see, within our GetMax() template function the type T can be used to
declare new objects:
T result;
result is an object of type T, like a and b, that is to say, of the type that we enclose
between angle-brackets <> when calling our template function.
In this concrete case where the generic T type is used as a parameter for function
GetMax the compiler can find out automatically which data type is passed to it without
having to specify it with patterns <int> or <long>. So we could have written:
int i,j;
GetMax (i,j);
since both i and j are of type int the compiler would assume automatically that the
wished function is for type int. This implicit method is more usual and would produce
the same result:
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// function template II
#include <iostream.h>
template <class T>
T GetMax (T a, T b) {
return (a>b?a:b);
}
int main () {
int i=5, j=6, k;
long l=10, m=5, n;
k=GetMax(i,j);
n=GetMax(l,m);
cout << k << endl;
cout << n << endl;
return 0;
}
Notice how in this case, within function main() we called our template function
GetMax() without explicitly specifying the type between angle-brackets <>. The compiler
automatically determines what type is needed on each call.
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Because our template function includes only one data type (class T) and both
arguments it admits are both of that same type, we cannot call our template function
with two objects of different types as parameters:
int i;
long l;
k = GetMax (i,l);
This would be incorrect, since our function waits for two arguments of the same type (or
class).
We can also make template-functions that admit more than one generic class or data
type. For example:
template <class T, class U>
T GetMin (T a, U b) {
return (a<b?a:b);}
In this case, our template function GetMin() admits two parameters of different types
and returns an object of the same type as the first parameter (T) that is passed. For
example, after that declaration we could call the function by writing:
int i,j;
long l;
i = GetMin<int,long> (j,l);
or simply
i = GetMin (j,l);
even though j and l are of different types.
13.1.2 Class templates
We also have the possibility to write class templates, so that a class can have members
based on generic types that do not need to be defined at the moment of creating the
class or whose members use these generic types. For example:
template <class T>
class pair {
T values [2];
public:
pair (T first, T second)
{
values[0]=first; values[1]=second;
}
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};
The class that we have just defined serves to store two elements of any valid type. For
example, if we wanted to declare an object of this class to store two integer values of
type int with the values 115 and 36 we would write:
pair<int> myobject (115, 36);
this same class would also serve to create an object to store any other type:
pair<float> myfloats (3.0, 2.18);
The only member function has been defined inline within the class declaration. If we
define a function member outside the declaration we must always precede the definition
with the prefix template <... >.
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// class templates
#include <iostream.h>
template <class T>
class pair {
T value1, value2;
public:
pair (T first, T second)
{value1=first; value2=second;}
T getmax ();
};
template <class T>
T pair<T>::getmax ()
{
T retval;
retval = value1>value2? value1 : value2;
return retval;
}
int main () {
pair <int> myobject (100, 75);
cout << myobject.getmax();
return 0;
}
notice how the definition of member function getmax begins:
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template <class T>
T pair<T>::getmax ()
All Ts that appear are necessary because whenever you declare member functions you
have to follow a format similar to this (the second T makes reference to the type
returned by the function, so this may vary).
13.1.3 Template Specialization
A template specialization allows a template to make specific implementations when the
pattern is of a determined type. For example, suppose that our class template pair
included a function to return the result of the module operation between the objects
contained in it, but we only want it to work when the contained type is int. For the rest
of the types we want this function to return 0. This can be done the following way:
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// Template specialization
#include <iostream.h>
template <class T>
class pair {
T value1, value2;
public:
pair (T first, T second)
{value1=first; value2=second;}
T module () {return 0;}
};
template <>
class pair <int> {
int value1, value2;
public:
pair (int first, int second)
{value1=first; value2=second;}
int module ();
};
template <>
int pair<int>::module() {
return value1%value2;
}
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int main () {
pair <int> myints (100,75);
pair <float> myfloats (100.0,75.0);
cout << myints.module() << '\n';
cout << myfloats.module() << '\n';
return 0;
}
As you can see in the code the specialization is defined this way:
template <> class class_name <type>
The specialization is part of a template, for that reason we must begin the declaration
with template <>. And indeed because it is a specialization for a concrete type, the
generic type cannot be used in it and the first angle-brackets <> must appear empty.
After the class name we must include the type that is being specialized enclosed
between angle-brackets <>.
When we specialize a type of a template we must also define all the members equating
them to the specialization (if one pays attention, in the example above we have had to
include its own constructor, although it is identical to the one in the generic template).
The reason is that no member is "inherited" from the generic template to the specialized
one.
13.1.4 Parameter values for templates
Besides the template arguments preceded by the class or typename keywords that
represent a type, function templates and class templates can include other parameters
that are not types whenever they are also constant values, like for example values of
fundamental types. As an example see this class template that serves to store arrays:
100
3.1416
// array template
#include <iostream.h>
template <class T, int N>
class array {
T memblock [N];
public:
void setmember (int x, T value);
T getmember (int x);
};
template <class T, int N>
array<T,N>::setmember (int x, T value) {
memblock[x]=value;
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}
template <class T, int N>
T array<T,N>::getmember (int x) {
return memblock[x];
}
int main () {
array <int,5> myints;
array <float,5> myfloats;
myints.setmember (0,100);
myfloats.setmember (3,3.1416);
cout << myints.getmember(0) << '\n';
cout << myfloats.getmember(3) << '\n';
return 0;
}
It is also possible to set default values for any template parameter just as it is done with
function parameters.
Some possible template examples seen above:
template
template
template
template
template
<class T>
// The most usual: one class parameter.
<class T, class U>
// Two class parameters.
<class T, int N>
// A class and an integer.
<class T = char>
// With a default value.
<int Tfunc (int)>
// A function as parameter.
13.1.5 Templates and multiple-file projects
From the point of view of the compiler, templates are not normal functions or classes.
They are compiled on demand, meaning that the code of a template function is not
compiled until an instantiation is required. At that moment, when an instantiation is
required, the compiler generates a function specifically for that type from the template.
When projects grow it is usual to split the code of a program in different source files. In
these cases, generally the interface and implementation are separated. Taking a library
of functions as example, the interface generally consists of the prototypes of all the
functions that can be called. These are generally declared in a "header file" with .h
extension, and the implementation (the definition of these functions) is in an
independent file of c++ code.
The macro-like functionality of templates, forces a restriction for multi-file projects: the
implementation (definition) of a template class or function must be in the same file as
the declaration. That means we cannot separate the interface in a separate header file
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and we must include both interface and implementation in any file that uses the
templates.
Going back to the library of functions, if we wanted to make a library of function
templates, instead of creating a header file (.h) we should create a "template file" with
both the interface and implementation of the function templates (there is no convention
on the extension for this type of file other than there be no extension at all or to keep
the .h). The inclusion more than once of the same template file with both declarations
and definitions in a project doesn't generate linkage errors, since they are compiled on
demand and compilers that allow templates should be prepared to not generate
duplicate code in these cases.
13.2 Exception Handling
During the development of a program, there may be some cases where we do not have
the certainty that a piece of the code is going to work right, either because it accesses
resources that do not exist or because it gets out of an expected range, etc...
These types of anomalous situations are included in what we consider exceptions and
C++ has recently incorporated three new operators to help us handle these situations:
try, throw and catch.
Their form of use is the following:
try {
// code to be tried
throw exception;
}
catch (type exception)
{
// code to be executed in case of exception
}
And its operation:
- The code within the try block is executed normally. In case that an exception takes
place, this code must use the throw keyword and a parameter to throw an exception.
The type of the parameter details the exception and can be of any valid type.
- If an exception has taken place, that is to say, if it has executed a throw instruction
within the try block, the catch block is executed receiving as parameter the exception
passed by throw.
For example:
Exception: Out of range
// exceptions
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#include <iostream.h>
int main () {
char myarray[10];
try
{
for (int n=0; n<=10; n++)
{
if (n>9) throw "Out of range";
myarray[n]='z';
}
}
catch (char * str)
{
cout << "Exception: " << str << endl;
}
return 0;
}
In this example, if within the n loop, n gets to be more than 9 an exception is thrown,
since myarray[n] would in that case point to a non-trustworthy memory address. When
throw is executed, the try block finalizes right away and every object created within the
try block is destroyed. After that, the control is passed to the corresponding catch
block (that is only executed in these cases). Finally the program continues right after
the catch block, in this case: return 0;.
The syntax used by throw is similar to that of return: Only the parameter does not
need to be enclosed between parenthesis.
The catch block must go right after the try block without including any code line
between them. The parameter that catch accepts can be of any valid type. Even more,
catch can be overloaded so that it can accept different types as parameters. In that
case the catch block executed is the one that matches the type of the exception sent
(the parameter of throw):
Exception: index 10 is out
of range
// exceptions: multiple catch blocks
#include <iostream.h>
int main () {
try
{
char * mystring;
mystring = new char [10];
if (mystring == NULL) throw "Allocation failure";
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for (int n=0; n<=100; n++)
{
if (n>9) throw n;
mystring[n]='z';
}
}
catch (int i)
{
cout << "Exception: ";
cout << "index " << i << " is out of range" << endl;
}
catch (char * str)
{
cout << "Exception: " << str << endl;
}
return 0;
}
In this case there is a possibility that at least two different exceptions could happen:
1. That the required block of 10 characters cannot be assigned (something rare,
but possible): in this case an exception is thrown that will be caught by catch
(char * str).
2. That the maximum index for mystring is exceeded: in this case the exception
thrown will be caught by catch (int i), since the parameter is an integer
number.
We can also define a catch block that captures all the exceptions independently of the
type used in the call to throw. For that we have to write three points instead of the
parameter type and name accepted by catch:
try {
// code here
}
catch (...) {
cout << "Exception occurred";
}
It is also possible to nest try-catch blocks within more external try blocks. In these
cases, we have the possibility that an internal catch block forwards the exception
received to the external level, for that the expression throw; with no arguments is used.
For example:
try {
try {
// code here
}
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catch (int n) {
throw;
}
}
catch (...) {
cout << "Exception occurred";
}
13.2.1 Exceptions not caught
If an exception is not caught by any catch statement because there is no catch
statement with a matching type, the special function terminate will be called.
This function is generally defined so that it terminates the current process immediately
showing an "Abnormal termination" error message. Its format is:
void terminate();
13.2.2 Standard exceptions
Some functions of the standard C++ language library send exceptions that can be
captured if we include them within a try block. These exceptions are sent with a class
derived from std::exception as type. This class (std::exception) is defined in the C++
standard header file <exception> and serves as a pattern for the standard hierarchy of
exceptions:
Exception
bad_alloc
(thrown by new)
bad_cast
(thrown by dynamic_cast when fails with a referenced
type)
bad_exception
(thrown when an exception doesn't match any catch)
bad_typeid
(thrown by typeid)
logic_error
domain_error
invalid_argument
length_error
out_of_range
runtime_error
overflow_error
range_error
underflow_error
ios_base::failure
(thrown by ios::clear)
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Because this is a class hierarchy, if you include a catch block to capture any of the
exceptions of this hierarchy using the argument by reference (i.e. adding an ampersand
& after the type) you will also capture all the derived ones (rules of inheritance in C++).
The following example catches an exception of type bad_typeid (derived from
exception) that is generated when requesting information about the type pointed by a
null pointer:
Exception: Attempted typeid of NULL
pointer
// standard exceptions
#include <iostream.h>
#include <exception>
#include <typeinfo>
class A {virtual f() {}; };
int main () {
try {
A * a = NULL;
typeid (*a);
}
catch (std::exception& e)
{
cout << "Exception: " << e.what();
}
return 0;
}
You can use the classes of standard hierarchy of exceptions to throw your exceptions or
derive new classes from them.
13.3 Advanced Class Type-Casting
Until now, in order to type-cast a simple object to another we have used the traditional
type casting operator. For example, to cast a floating point number of type double to an
integer of type int we have used:
int i;
double d;
i = (int) d;
or also
i = int (d);
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This is quite good for basic types that have standard defined conversions, however this
operators can also be indiscriminately applied on classes and pointers to classes. So, it
is perfectly valid to write things like:
// class type-casting
#include <iostream.h>
class CDummy {
int i;
};
class CAddition {
int x,y;
public:
CAddition (int a, int b) { x=a; y=b;
}
int result() { return x+y;}
};
int main () {
CDummy d;
CAddition * padd;
padd = (CAddition*) &d;
cout << padd->result();
return 0;
}
Although the previous program in syntactically correct in C++ (in fact it will compile
with no warnings on most compilers) it is code with not much sense since we use
function result, that is a member of CAddition, without having declared an object of
that class: padd is not an object, it is only a pointer which we have assigned the
address of a non related object. When accessing its result member it will produce a
run-time error or, at best, just an unexpected result.
13.3.1 Reinterpret Cast
In order to control these types of conversions between classes, ANSI-C++ standard has
defined four new casting operators: reinterpret_cast, static_cast, dynamic_cast and
const_cast. All of them have the same format when used:
reinterpret_cast <new_type> (expression)
dynamic_cast <new_type> (expression)
static_cast <new_type> (expression)
const_cast <new_type> (expression)
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Where new_type is the destination type to which expression has to be casted. To make
an easily understandable parallelism with traditional type-casting operators these
expression mean:
(new_type) expression
new_type (expression)
but with their own special characteristics.
reinterpret_cast
reinterpret_cast casts a pointer to any other type of pointer. It also allows casting from
a pointer to an integer type and vice versa.
This operator can cast pointers between non-related classed. The operation results is a
simple binary copy of the value from one pointer to the other. The content pointed does
not pass any kind of check nor transformation between types.
In the case that the copy is performed from a pointer to an integer, the interpretation of
its content is system dependent and therefore any implementation is non portable. A
pointer casted to an integer large enough to fully contain it can be casted back to a
valid pointer.
class A {};
class B {};
A * a = new A;
B * b = reinterpret_cast<B*>(a);
reinterpret_cast treats all pointers exactly as traditional type-casting operators do.
13.3.2 static_cast
static_cast performs any casting that can be implicitly performed as well as the inverse
cast (even if this is not allowed implicitly).
Applied to pointers to classes, that is to say that it allows to cast a pointer of a derived
class to its base class (this is a valid conversion that can be implicitly performed) and it
can also perform the inverse: cast a base class to its derivated class.
In this last case the base class that is being casted is not checked to determine wether
this is a complete class of the destination type or not.
class Base {};
class Derived: public Base {};
Base * a = new Base;
Derived * b = static_cast<Derived*>(a);
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static_cast, aside from manipulating pointers to classes, can also be used to perform
conversions explicitly defined in classes, as well as to perform standard conversions
between fundamental types:
double d=3.14159265;
int i = static_cast<int>(d);
13.3.3 dynamic_cast
dynamic_cast is exclusively used with pointers and references to objects. It allows any
type-casting that can be implicitly performed as well as the inverse one when used with
polymorphic classes, however, unlike static_cast, dynamic_cast checks, in this last
case, if the operation is valid. That is to say, it checks if the casting is going to return a
valid complete object of the requested type.
Checking is performed during run-time execution. If the pointer being casted is not a
pointer to a valid complete object of the requested type, the value returned is a NULL
pointer.
class Base { virtual dummy(){}; };
class Derived : public Base { };
Base* b1 = new Derived;
Base* b2 = new Base;
Derived* d1 = dynamic_cast<Derived*>(b1); // succeeds
Derived* d2 = dynamic_cast<Derived*>(b2); // fails: returns NULL
If the type-casting is performed to a reference type and this casting is not possible an
exception of type bad_cast is thrown:
class Base { virtual dummy(){}; };
class Derived : public Base { };
Base* b1 =
Base* b2 =
Derived d1
Derived d2
new Derived;
new Base;
= dynamic_cast<Derived&*>(b1); // succeeds
= dynamic_cast<Derived&*>(b2); // fails: exception thrown
const_cast
This type of casting manipulates the const attribute of the passed object, either to be set
or removed:
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class C {};
const C * a = new C;
C * b = const_cast<C*> (a);
Neither of the other three new cast operators can modify the constness of an object.
13.3.4 Typeid
ANSI-C++ also defines a new operator called typeid that allows checking the type of an
expression:
typeid (expression)
This operator returns a refernece to a constant object of type type_info that is defined
in the standard header file <typeinfo>. This returned value can be compared with
another using operators == and != or can serve to obtain a string of characters
representing the data type or class name by using its name() method.
a and b are of different
types:
a is: class CDummy *
b is: class CDummy
// typeid, typeinfo
#include <iostream.h>
#include <typeinfo>
class CDummy { };
int main () {
CDummy* a,b;
if (typeid(a) != typeid(b))
{
cout << "a and b are of different types:\n";
cout << "a is: " << typeid(a).name() << '\n';
cout << "b is: " << typeid(b).name() << '\n';
}
return 0;
}
Preprocessor Director
Preprocessor directives are orders that we include within the code of our programs that
are not instructions for the program itself but for the preprocessor. The preprocessor is
executed automatically by the compiler when we compile a program in C++ and is in
charge of making the first verifications and digestions of the program's code.
All these directives must be specified in a single line of code and they do not have to
include an ending semicolon ;.
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#define
At the beginning of this tutorial we have already spoken about a preprocessor directive:
#define, that serves to generate what we called defined constannts or macros and whose
form is the following:
#define name value
Its function is to define a macro called name that whenever it is found in some point of
the code is replaced by value. For example:
#define MAX_WIDTH 100
char str1[MAX_WIDTH];
char str2[MAX_WIDTH];
It defines two strings to store up to 100 characters.
#define can also be used to generate macro functions:
#define getmax(a,b) a>b?a:b
int x=5, y;
y = getmax(x,2);
after the execution of this code y would contain 5.
#undef
#undef fulfills the inverse functionality of #define. It eliminates from the list of defined
constants the one that has the name passed as a parameter to #undef:
#define MAX_WIDTH 100
char str1[MAX_WIDTH];
#undef MAX_WIDTH
#define MAX_WIDTH 200
char str2[MAX_WIDTH];
#ifdef, #ifndef, #if, #endif, #else and #elif
These directives allow to discard part of the code of a program if a certain condition is
not fulfilled.
#ifdef allows that a section of a program is compiled only if the defined constant that is
specified as the parameter has been defined, independently of its value. Its operation is:
#ifdef name
// code here
#endif
For example:
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#ifdef MAX_WIDTH
char str[MAX_WIDTH];
#endif
In this case, the line char str[MAX_WIDTH]; is only considered by the compiler if the
defined constant MAX_WIDTH has been previously defined, independently of its value.
If it has not been defined, that line will not be included in the program.
#ifndef serves for the opposite: the code between the #ifndef directive and the #endif
directive is only compiled if the constant name that is specified has not been defined
previously. For example:
#ifndef MAX_WIDTH
#define MAX_WIDTH 100
#endif
char str[MAX_WIDTH];
In this case, if when arriving at this piece of code the defined constant MAX_WIDTH has
not yet been defined it would be defined with a value of 100. If it already existed it
would maintain the value that it had (because the #define statement won't be executed).
The #if, #else and #elif (elif = else if) directives serve so that the portion of code that
follows is compiled only if the specified condition is met. The condition can only serve to
evaluate constant expressions. For example:
#if MAX_WIDTH>200
#undef MAX_WIDTH
#define MAX_WIDTH 200
#elsif MAX_WIDTH<50
#undef MAX_WIDTH
#define MAX_WIDTH 50
#else
#undef MAX_WIDTH
#define MAX_WIDTH 100
#endif
char str[MAX_WIDTH];
Notice how the structure of chained directives #if, #elsif and #else finishes with #endif.
#line
When we compile a program and errors happen during the compiling process, the
compiler shows the error that happened preceded by the name of the file and the line
within the file where it has taken place.
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The #line directive allows us to control both things, the line numbers within the code
files as well as the file name that we want to appear when an error takes place. Its form
is the following one:
#line number "filename"
Where number is the new line number that will be assigned to the next code line. The
line number of successive lines will be increased one by one from this.
filename is an optional parameter that serves to replace the file name that will be shown
in case of error from this directive until another one changes it again or the end of the
file is reached. For example:
#line 1 "assigning variable"
int a?;
This code will generate an error that will be shown as error in file "assigning variable",
line 1.
#error
This directive aborts the compilation process when it is found returning the error that is
specified as the parameter:
#ifndef __cplusplus
#error A C++ compiler is required
#endif
This example aborts the compilation process if the defined constant __cplusplus is not
defined.
#include
This directive has also been used assiduously in other sections of this tutorial. When
the preprocessor finds an #include directive it replaces it by the whole content of the
specified file. There are two ways to specify a file to be included:
#include "file"
#include <file>
The only difference between both expressions is the directories in which the compiler is
going to look for the file. In the first case where the file is specified between quotes, the
file is looked for in the same directory that includes the file containing the directive. In
case that it is not there, the compiler looks for the file in the default directories where it
is configured to look for the standard header files.
If the file name is enclosed between angle-brackets <> the file is looked for directly
where the compiler is configured to look for the standard header files.
#pragma
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This directive is used to specify diverse options to the compiler. These options are
specific for the platform and the compiler you use. Consult the manual or the reference
of your compiler for more information on the possible parameters that you can define
with #pragma.
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UNIT 14
FILE INPUT OUTPUT
Contents
14.1
Input/Output with files.
14.2
Open a file
14.3
Closing a file
14.4
Methods of Input and Output Classes
14.5
Text mode files
14.6
state flags
14.7
Binary files
14.8
Buffers and Synchronization
14.1 Input Output With Files
The techniques for file input and output, i/o, in C++ are virtually identical to those
introduced in earlier lessons for writing and reading to the standard output devices, the
screen and keyboard. To perform file input and output the include file fstream must be
used.
#include <fstream>
Fstream contains class definitions for classes used in file i/o. Within a program needing
file i/o, for each output file required, an object of class ofstream is instantiated. For
each input file required, an object of class ifstream is instantiated. The ofstream object
is used exactly as the cout object for standard output is used. The ifstream object is
used exactly as the cin object for standard input is used. This is best understood by
studying an example.
C++ has support both for input and output with files through the following classes:
•
ofstream: File class for writing operations (derived from ostream)
•
ifstream : File class for reading operations (derived from istream)
•
fstream : File class for both reading and writing operations (derived from
iostream)
14.2 Open a file
The first operation generally done on an object of one of these classes is to associate it
to a real file, that is to say, to open a file. The open file is represented within the
program by a stream object (an instantiation of one of these classes) and any input or
output performed on this stream object will be applied to the physical file.
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In order to open a file with a stream object we use its member function open():
void open (const char * filename, openmode mode);
where filename is a string of characters representing the name of the file to be opened
and mode is a combination of the following flags:
ios::in
Open file for reading
ios::out
Open file for writing
ios::ate
Initial position: end of file
ios::app
Every output is appended at the end of file
ios::trunc
If the file already existed it is erased
ios::binary
Binary mode
These flags can be combined using bitwise operator OR: |. For example, if we want to
open the file "example.bin" in binary mode to add data we could do it by the following
call to function-member open:
ofstream file;
file.open ("example.bin", ios::out | ios::app | ios::binary);
All of the member functions open of classes ofstream, ifstream and fstream include a
default mode when opening files that varies from one to the other:
class
default mode to parameter
ofstream
ios::out | ios::trunc
ifstream
ios::in
fstream
ios::in | ios::out
The default value is only applied if the function is called without specifying a mode
parameter. If the function is called with any value in that parameter the default mode is
stepped on, not combined.
Since the first task that is performed on an object of classes ofstream, ifstream and
fstream is frequently to open a file, these three classes include a constructor that
directly calls the open member function and has the same parameters as this. This
way, we could also have declared the previous object and conducted the same opening
operation just by writing:
ofstream file ("example.bin", ios::out | ios::app | ios::binary);
Both forms to open a file are valid.
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You can check if a file has been correctly opened by calling the member function
is_open():
bool is_open();
that returns a bool type value indicating true in case that indeed the object has been
correctly associated with an open file or false otherwise.
14.3 Closing a file
When reading, writing or consulting operations on a file are complete we must close it
so that it becomes available again. In order to do that we shall call the member function
close(), that is in charge of flushing the buffers and closing the file. Its form is quite
simple:
void close ();
Once this member function is called, the stream object can be used to open another file,
and the file is available again to be opened by other processes.
In case that an object is destructed while still associated with an open file, the
destructor automatically calls the member function close.
14.4 Methods of Input and Output Classes
The ifstream class has several useful methods for input. These method are also in the
class cin, which is used to read from standard input. These methods are used to read
from any input stream. An input stream is a source of input such as the keyboard, a
file or a buffer.
•
Extraction Operator, >>
This overloaded operator handles all built in C++ data types. By default, any
intervening white space is disregarded. That is, blanks, tabs, new lines,
formfeeds and carriage returns are skipped over.
•
get()
This form of get extracts a single character from the input stream, that is, from
the standard input, a file or a buffer. It does not skip white space. It returns type
int.
•
get(char &ch)
This form of get also extracts a single character from the input stream, but it
stores the value in the character variable passed in as an argument.
•
get(char *buff, int buffsize, char delimiter='\n')
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This form of get reads characters into the C-style buffer passed as an argument
buffsize characters are read, the delimiter is encountered or an end of file is
encountered. The '\n' is the new line character. The delimiter is not read into
the buffer but is instead left in the input stream. It must be removed separately
but using either another get or an ignore. Because of this added step, this form
of get is a frequent source of errors and should be avoided. Fortunately, another
method shown below, getline, reads in the delimiter as well and should be used
in place of this form of get.
•
Getline
There are several useful forms of getline.
•
ignore(int count=1, int delim=traits_type::eof)
This method reads and discards "count" characters from the input stream.
•
peek()
This method returns the next character from the input stream, but does not
remove it. It is useful to look ahead at what the next character read will be.
•
putback(char &ch)
This method puts ch onto the input stream. The character in ch will then be the
next character read from the input stream.
•
unget()
This method puts the last read character back into the input stream.
•
read(char *buff, int count)
This method is used to perform an unformatted read of count bytes from the
input stream into a character buffer.
The ofstream class has several useful methods for writing to an output stream. An
output stream is standard output (usually the screen), a file or a buffer. These methods
are also in the object cout, which is used for standard output.
The simplest way to understand how to use these methods is by looking at a few
examples. Since we have seen the extraction, >>, and insertion, << in several lessons,
let's look at the other methods. Getline, which is very useful to read entire lines of text
into a string.
Suppose we need to read a file and determine the number of alphanumeric characters,
the number of blanks and the number of sentences. To determine the number of
sentences we will count the number of periods (dots). We will disregard newlines and
tabs.
Here is a program that solves the problem.
#include <iostream>
#include <fstream>
using namespace std;
int main()
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{
int blank_count = 0;
int char_count = 0;
int sentence_count = 0;
char ch;
ifstream iFile("c:/lesson12.txt");
if (! iFile)
{
cout << "Error opening input file" << endl;
return -1;
}
while (iFile.get(ch))
{
switch (ch) {
case ' ':
blank_count++;
break;
case '\n':
case '\t':
break;
case '.':
sentence_count++;
break;
default:
char_count++;
break;
}
}
cout << "There are " << blank_count << " blanks" << endl;
cout << "There are " << char_count << " characters" << endl;
cout << "There are " << sentence_count << " sentences" << endl;
return 0;
}
As a second example, let's implement a program that will copy the contents of one file to
another. The program will prompt the user for the input and output file names, and
then copy.
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#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
char ch;
string iFileName;
string oFileName;
cout << "Enter the source file name: ";
cin >> iFileName;
cout << "Enter the destination file name: ";
cin >> oFileName;
ofstream oFile(oFileName.c_str());
ifstream iFile(iFileName.c_str());
//Error checking on file opens omitted for brevity.
while (iFile.get(ch))
{
oFile.put(ch);
}
return 0;
}
14.5 Text mode files
Classes ofstream, ifstream and fstream are derived from ostream, istream and
iostream respectively. That's why fstream objects can use the members of these parent
classes to access data.
Generally, when using text files we shall use the same members of these classes that we
used in communication with the console (cin and cout). As in the following example,
where we use the overloaded insertion operator <<:
// writing on a text file
#include <fstream.h>
This is a line.
This is another line.
int main () {
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ofstream examplefile ("example.txt");
if (examplefile.is_open())
{
examplefile << "This is a line.\n";
examplefile << "This is another line.\n";
examplefile.close();
}
return 0;
}
Data input from file can also be performed in the same way that we did with cin:
// reading a text file
#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>
This is a line.
This is another line.
int main () {
char buffer[256];
ifstream examplefile ("example.txt");
if (! examplefile.is_open())
{ cout << "Error opening file"; exit (1); }
while (! examplefile.eof() )
{
examplefile.getline (buffer,100);
cout << buffer << endl;
}
return 0;
}
This last example reads a text file and prints out its content on the screen. Notice how
we have used a new member function, called eof that ifstream inherits from class ios
and that returns true in case that the end of the file has been reached.
14.6 State flags
In addition to eof(), other member functions exist to verify the state of the stream (all of
them return a bool value):
bad()
Returns true if a failure occurs in a reading or writing operation. For example in case
we try to write to a file that is not open for writing or if the device where we try to write
has no space left.
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fail()
Returns true in the same cases as bad() plus in case that a format error happens, as
trying to read an integer number and an alphabetical character is received.
eof()
Returns true if a file opened for reading has reached the end.
good()
It is the most generic: returns false in the same cases in which calling any of the
previous functions would return true.
In order to reset the state flags checked by the previous member functions you can use
member function clear(), with no parameters.
get and put stream pointers
All i/o streams objects have, at least, one stream pointer:
•
ifstream, like istream, has a pointer known as get pointer that points to the
next element to be read.
•
ofstream, like ostream, has a pointer put pointer that points to the location
where the next element has to be written.
• Finally fstream, like iostream, inherits both: get and put
These stream pointers that point to the reading or writing locations within a stream can
be read and/or manipulated using the following member functions:
tellg() and tellp()
These two member functions admit no parameters and return a value of type pos_type
(according ANSI-C++ standard) that is an integer data type representing the current
position of get stream pointer (in case of tellg) or put stream pointer (in case of tellp).
seekg() and seekp()
This pair of functions serve respectively to change the position of stream pointers get
and put. Both functions are overloaded with two different prototypes:
seekg ( pos_type position );
seekp ( pos_type position );
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Using this prototype the stream pointer is changed to an absolute position from the
beginning of the file. The type required is the same as that returned by functions tellg
and tellp.
seekg ( off_type offset, seekdir direction );
seekp ( off_type offset, seekdir direction );
Using this prototype, an offset from a concrete point determined by parameter direction
can be specified. It can be:
ios::beg
Offset specified from the beginning of the stream
ios::cur
Offset specified from the current position of the stream
pointer
ios::end
Offset specified from the end of the stream
The values of both stream pointers get and put are counted in different ways for text
files than for binary files, since in text mode files some modifications to the appearance
of some special characters can occur. For that reason it is advisable to use only the first
prototype of seekg and seekp with files opened in text mode and always use nonmodified values returned by tellg or tellp. With binary files, you can freely use all the
implementations for these functions. They should not have any unexpected behavior.
The following example uses the member functions just seen to obtain the size of a
binary file:
// obtaining file size
#include <iostream.h>
#include <fstream.h>
size of example.txt is 40 bytes.
const char * filename = "example.txt";
int main () {
long l,m;
ifstream file (filename,
ios::in|ios::binary);
l = file.tellg();
file.seekg (0, ios::end);
m = file.tellg();
file.close();
cout << "size of " << filename;
cout << " is " << (m-l) << " bytes.\n";
return 0;
}
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14.7 Binary files
In binary files inputting and outputting data with operators like << and >> and
functions like getline, does not make too much sense, although they are perfectly valid.
File streams include two member functions specially designed for input and output of
data sequentially: write and read. The first one (write) is a member function of
ostream, also inherited by ofstream. And read is member function of istream and it is
inherited by ifstream. Objects of class fstream have both. Their prototypes are:
write ( char * buffer, streamsize size );
read ( char * buffer, streamsize size );
Where buffer is the address of a memory block where the read data are stored or from
where the data to be written are taken. The size parameter is an integer value that
specifies the number of characters to be read/written from/to the buffer.
// reading binary file
#include <iostream.h>
#include <fstream.h>
the complete file is in a buffer
const char * filename = "example.txt";
int main () {
char * buffer;
long size;
ifstream file (filename,
ios::in|ios::binary|ios::ate);
size = file.tellg();
file.seekg (0, ios::beg);
buffer = new char [size];
file.read (buffer, size);
file.close();
cout << "the complete file is in a buffer";
delete[] buffer;
return 0;
}
14.8 Buffers and Synchronization
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When we operate with file streams, these are associated to a buffer of type streambuf.
This buffer is a memory block that acts as an intermediary between the stream and the
physical file. For example, with an out stream, each time the member function put
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(write a single character) is called, the character is not written directly to the physical
file with which the stream is associated. Instead of that, the character is inserted in the
buffer for that stream.
When the buffer is flushed, all data that it contains is written to the physic media (if it
is an out stream) or simply erased (if it is an in stream). This process is called
synchronization and it takes place under any of the following circumstances:
•
When the file is closed: before closing a file all buffers that have not yet been
completely written or read are synchronized.
•
When the buffer is full: Buffers have a certain size. When the buffer is full it is
automatically synchronized.
•
Explicitly with manipulators: When certain manipulators are used on streams
synchronization takes place. These manipulators are: flush and endl.
•
Explicitly with function sync(): Calling member function sync() (no
parameters) causes an immediate syncronization. This function returns an int
value equal to -1 if the stream has no associated buffer or in case of failure.
14.9 I/O Manipulators
Up till now, we have accepted the default output formatting. C++ defines a set of
manipulators which are used to modify the state of iostream objects. These control how
data is formatted. They are defined in the include file, <ios>. It is not usually necessary
to explicitly include this file because it is included indirectly via the use of other
includes such as <iostream> or <fstream>.
Let's see how some of these manipulators work in a simple program.
Manipulator
Use
boolalpha
Causes bool variables to be output as true or false.
noboolalhpa (default)
Causes bool variables to be displayed as 0 or 1.
dec (default)
Specifies that integers are displayed in base 10.
hex
Specifies that integers are displayed in hexadecimal.
oct
Specified that integers are displayed in octal.
left
Causes text to be left justified in the output field.
right
Causes text to be right justified in the output field.
internal
Causes the sign of a number to be left justified and the value
to be right justified.
noshowbase (default)
Turns off displaying a prefix indicating the base of a number.
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showbase
Turns on displaying a prefix indicating the base of a number.
noshowpoint (default)
Displays decimal point only if a fractional part exists.
showpoint
Displays decimal point always.
noshowpos (default)
No "+" prefixing a positive number.
showpos
Displays a "+" prefixing a positive number.
skipws (default)
Causes white space (blanks, tabs, newlines) to be skipped by
the input operator, >>.
noskipws
White space not skipped by the extraction operator, >>.
fixed (default)
Causes floating point numbers to be displayed in fixed
notation.
scientific
Causes floating point numbers to be displayed in scientific
notation.
nouppercase (default)
0x displayed for hexadecimal numbers, e for scientific notation
uppercase
0X displayed for hexadecimal numbers, E for scientific
notation
The manipulators in the above table modify the state of the iostream object. This means
that once used on an iostream object they will effect all subsequent input or output
done with the object. There are several other manipulators that are used to format a
particular
output but do no modify the state of the object.
Setting Output Width
setw(w) - sets output or input width to w; requires <iomanip> to be included.
width(w) - a member function of the iostream classes.
Filling White Space
setfill(ch) - fills white space in output fields with ch; requires <iomanip> to be included.
fill(ch) = a member function of the iostream classes.
Setting Precision
setprecision(n) - sets the display of floating point numbers at precision n. This does not
effect the way floating point numbers are handled during calculations in your program.
Here is a simple program illustrating the use of the i/o manipulators.
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
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int main()
{
int intValue = 15;
cout
cout
cout
cout
cout
cout
cout
cout
cout
cout
<<
<<
<<
<<
<<
<<
<<
<<
<<
<<
"Integer Number" << endl;
"Default: " << intValue << endl;
"Octal: " << oct << intValue << endl;
"Hex: " << hex << intValue << endl;
"Turning showbase on" << showbase << endl;
"Dec: " << dec << intValue << endl;
"Octal: " << oct << intValue << endl;
"Hex: " << hex << intValue << endl;
"Turning showbase off" << noshowbase << endl;
endl;
double doubleVal = 12.345678;
cout
cout
cout
cout
cout
cout
cout
cout
<<
<<
<<
<<
<<
<<
<<
<<
"Floating Point Number" << endl;
"Default: " << doubleVal << endl;
setprecision(10);
"Precision of 10: " << doubleVal << endl;
scientific << "Scientific Notation: " << doubleVal << endl;
uppercase;
"Uppercase: " << doubleVal << endl;
endl;
bool theBool = true;
cout
cout
cout
cout
<<
<<
<<
<<
"Boolean" << endl;
"Default: " << theBool << endl;
boolalpha << "BoolAlpha set: " << theBool << endl;
endl;
string myName = "John";
cout << "Strings" << endl;
cout << "Default: " << myName << endl;
cout << setw(35) << right << "With setw(35) and right: "
<< myName << endl;
cout.width(20);
cout << "With width(20): " << myName << endl;
cout << endl;
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return 0;
}
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213