EGR 2261 Unit 12
Classes and Data Abstraction
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Read Malik, Chapter 10.
Homework #12 and Lab #12 due next week.
Quiz next week.

• Recall that a struct is a collection of a fixed number of components (called members), accessed by name.
– The members may be of different types.
• struct s are very useful because they let us design complex data structures that represent real-world objects.
– Recall example (on next slide) of houseType .
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• In our examples so far, struct s have contained data members but not function
members.
– In other words, the struct s in our examples have contained variables, but not functions.
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• In the C language, struct s could only contain data members.
• C programmers came to see this as a limitation, and realized that it would be nice to be able to define struct s that contain function members as well as data members.
• This realization, along with other ideas about how to improve struct s, led to the development of C++.
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• The main new idea in C++ is the class, which is a generalization of the struct .
• Hand-in-hand with the introduction of classes was a whole new way of thinking about a program—as a collection of interacting objects. This new way of thinking is called
object-oriented design (OOD) or object-
oriented programming (OOP).
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• A class definition looks a lot like a struct definition.
• Syntax:
• Just as with struct definitions, a class definition defines a new data type; but no memory is allocated until we declare a variable of this type.
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• Each class member in classMemberList can be a variable (data member) or a function (function member).
– Data members are like the members we’ve used in struct s.
– For a function member, usually just the function prototype is listed is listed in the class definition.
The function definition is given elsewhere in the program.
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Defining a Class: Example
Seven function members
Three data members
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Note the new keywords public and private , which are called member access specifiers.

• Each class member falls into one of the following categories, as designated by the access specifiers:
– public
• The member can be accessed by any code outside the class.
– private (This is the default.)
• The member cannot be accessed by any code outside the class .
– protected
• "In between" public and private . We'll discuss in a later chapter .
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Member Access Specifiers:
Example
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Since hr is private, it can be accessed by code within the function members, but no other code can access it.
On the other hand, since setTime() is public, any code can call it.

Member Access Specifiers:
Example (cont'd.)
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In this example, all of the function members are public and all of the data members are private. This is fairly common, but it's not always the case.

• As seen above, usually only the prototypes of a class's function members are included in the class definition itself.
• Your program must also contain the code for these functions. This code is called the function's implementation.
• Note the syntax on the next slide, which uses the scope resolution operator ::
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Implementation of Member
Functions: Example
Class name followed by scope resolution operator
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This code is outside of the class definition, so you must precede the function's name with clockType:: to indicate that this is a member function of the clockType class, rather than just a regular function.

Separating the Definition from the
Implementation
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Though not required, in a program that uses classes we usually separate the code into the following files:
1.
A header file that holds the class definition.
2.
3.
An implementation file that holds the implementations of the class’s member functions.
A file that holds the client program, which uses the class defined in the other two files.
Here is where you’ll put your program’s main() function.
For a program that uses more than one class, files 1 and 2 will be repeated for each class.

Separating the Definition from the
Implementation: Example

Getting Started with a Program that Uses ClockType
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Create a new project named clockPractice and start a new source-code file named
clockPractice.cpp.
Download the files named clockType.h and
clockType.cpp from the website and place them in your project’s folder that contains clockPractice.cpp.
In your project’s Solution Explorer, right-click
Source Files and select Add > Existing
Item…. Then select clockType.cpp and click
Add.
In clockPractice.cpp, type the code shown on the next slide.

Getting Started with a Program that Uses ClockType (cont’d.)
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Run the program.
Modify main to call myClock.incrementSeconds() and then myClock.printTime() again.

• Once a class has been defined and implemented, you can declare variables of that class type: clockType myClock;
• A class variable is called a class object or class instance, or simply an object.
• Each object has its own copy of the data members ( hr , min , and sec ).
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• Code outside of the member functions can access the object's public members, using the syntax:
– The dot ( .
) is the member access operator (same as for struct s).
• But code outside of the member functions cannot access the object's private members.
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Accessing Class Members From
Outside: Example
Accessing myClock's public members: okay!
Trying to access myClock's private members: no good!
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This code is in the program's main(), so it is outside of the class's member functions.
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Of course, the class and its member functions are defined elsewhere in this program.

• Code within a member function of the class can access the object's public and private members directly by name (no dot operator needed).
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Accessing Class Members Within the Class Code: Example
Accessing an object's private members: okay!
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This code is in the member function printTime(), so it can access public and private members.
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Note that we don’t need to precede the member name with the object's name.

• A constructor is a special member function that runs automatically when a new class object is created. Its purpose is to initialize the object’s data members.
• A constructor’s name is always the same as the name of the class.
• A constructor has no type (not even void ).
• Unlike other functions, constructors cannot be explicitly called in user-written code.
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Constructors: Example
In the class’s header file:
In the class’s implementation file:

• A class can have more than one constructor.
– This is an example of function overloading, which we looked at briefly in Unit 7.
– Recall from Unit 7 that each overloaded function must have a different formal parameter list.
• Example of overloaded constructor functions: clockType(); clockType(int hours, int minutes, int seconds);
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Multiple Constructors: Example
In the class’s header file:
In the class’s implementation file:

• A constructor is automatically executed when a class object is declared.
• Which of the constructors is executed depends on whether you specify parameters in parentheses after the class object’s name when you declare it.
• Examples: clockType yourClock; clockType myClock(6, 30, 56);
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• To invoke the default constructor, don’t specify any parameters after the class object’s name when you declare it:
• Example: clockType yourClock;
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• Syntax:
• Example: clockType myClock(6, 30, 56);
• The number and type of arguments should match the formal parameters (in the order given) of one of the constructors.
– Otherwise, C++ uses type conversion and looks for the best match.
– Any ambiguity causes a compile-time error.
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• A constructor can have default parameters.
– The rules for declaring formal parameters are the same as for declaring default formal parameters in any function.
– Actual parameters are passed according to same rules for any functions.
• Default constructor: a constructor with no parameters or with all default parameters.
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• As with struct s, aggregate arithmetic, relational, and input/output operations are not allowed on class objects, but aggregate assignment is allowed: clockType myClock, yourClock; myClock = yourClock + 1; //Error if (myClock >= yourClock) //Error cout << myClock; //Error cin >> myClock; //Error myClock = yourClock; //Okay!!
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• Objects can be passed as parameters to functions by value or by reference.
• If an object is passed by value, a copy must be made of the object’s data members. For large objects, this may require a lot of time and memory, degrading performance.
• If an object is passed by reference, no copy is made—may result in better performance.
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• Passing by reference is an efficient way to pass an object as a parameter.
– Potential Problem: when passing by reference, the actual parameter is changed if the formal parameter is changed.
– Solution: use const in the formal parameter list.
This prevents the function from changing the parameter’s value. Example from clockType:
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• The previous slide showed how to use const to prevent a function from changing the value of a reference parameter.
• Another use of const is to prevent a function from changing the object on which the function is invoked. Example from clockType:
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Two Uses of const : Example
This is an error because of this const .
This is an error because of this const .

• Accessor function: member function that only accesses the values of member variables.
• Mutator function: member function that modifies the values of member variables.
• Constant function:
– Member function that cannot modify member variables.
– Use const at the end of the function heading, as we saw previously:
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• Unified Modeling Language (UML) notation is widely used by programmers to graphically describe a class and its members. UML has many special symbols, including:
+ for public members
for private members
# for protected members
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• Abstraction
– Separating design details from usage
– Separating the logical properties from the implementation details
• Abstraction can also be applied to data
• Abstract data type (ADT): data type that separates the logical properties from the implementation details
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• Information hiding: hiding the details of the operations on the data.
• Interface (header) file: contains the specification details.
• File extension is .h
• Implementation file: contains the implementation details.
• File extension is .cpp
• In header file, include function prototypes and comments that briefly describe the functions.
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• Implementation file must include header file via include statement
• In include statement:
– User-defined header files are enclosed in double quotes
– System-provided header files are enclosed between angular brackets
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