Teaching

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C++ Programming: Program Design Including Data Structures, Fourth Edition
13-1
Chapter 13
Pointers, Classes, Virtual Functions, and Abstract Classes
At a Glance
Instructor’s Manual Table of Contents

Overview

Objectives

Teaching Tips

Quick Quizzes

Class Discussion Topics

Additional Projects

Additional Resources

Key Terms
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-2
Lecture Notes
Overview
Recall that C++ data types are classified into three categories: simple, structured, and
pointers. Chapter 13 introduces students to the pointer data type. Students will learn
how to declare and manipulate pointers. They will explore how to work with dynamic
variables and arrays, and examine the issues involved with using pointers as member
variables in classes. Students will also be introduced to additional object-oriented
techniques, including virtual functions and abstract classes.
Objectives
In this chapter, the student will:
 Learn about the pointer data type and pointer variables
 Explore how to declare and manipulate pointer variables
 Learn about the address of operator and the dereferencing operator
 Discover dynamic variables
 Explore how to use the new and delete operators to manipulate dynamic variables
 Learn about pointer arithmetic
 Discover dynamic arrays
 Become aware of the shallow and deep copies of data
 Discover the peculiarities of classes with pointer member variables
 Learn about virtual functions
 Examine the relationship between the address of operator and classes
 Become aware of abstract classes
Teaching Tips
Pointer Data Type and Pointer Variables
1. Define the term pointer variable and explain its purpose in C++.
Teaching
Tip
Plan to spend quite a bit of time introducing pointers. C++ offers low-level
programmer management of memory; students may not have had previous
experience with this unless they have worked with the C programming language.
Pointer terminology will most likely be confusing and initially frustrating for
your students.
C++ Programming: Program Design Including Data Structures, Fourth Edition
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Declaring Pointer Variables
1. Emphasize that there is no name associated with the pointer data type. Instead, you
identify a pointer variable with the type of data to be stored and an asterisk.
2. Examine the syntax for declaring a pointer variable. Note that an asterisk must precede
each pointer variable in a declaration.
Teaching
Tip
Emphasize the various ways pointers can be declared, including the use of white
space. Discuss the difference between syntax errors and conventions regarding
pointer declarations.
Address of Operator (&)
1. Explain the purpose of the address of operator and give an example of its use.
Teaching
Tip
Remind your students that the & symbol has been used in previous chapters to
denote a reference parameter. Relate the two by explaining that a reference
parameter holds the address of the argument that is passed to the calling function.
Dereferencing Operator (*)
1. Explain the purpose of the dereferencing operator and give an example of its use.
2. Illustrate how the address of and dereferencing operators access memory with Figures
13-1 through 13-4.
3. Step through the program in Example 13-1 to demonstrate the use of pointer variables.
Teaching
Tip
Students will likely encounter compiler or execution errors when coding with
pointers for the first time. Verify that they understand the use of the address of
and dereferencing operator with several examples in class.
C++ Programming: Program Design Including Data Structures, Fourth Edition
Teaching
Tip
13-4
Students will feel more comfortable with pointers as they learn to recognize and
debug errors in code with pointer variables. Ask the students to work in small
groups and create a small program such as the one in this section. They should
then introduce errors and make a note of them. Ask them to present their
program in class and see if other students pick up on their errors. At the end of
their presentation, they should debug and execute their code.
Quick Quiz 1
1. What is the general syntax to declare a pointer variable?
Answer: dataType *identifier;
2. The value of a pointer variable is always a(n) ____________________.
Answer: memory address
3. True or False: The address of operator is a binary operator.
Answer: False
4. True or False: There is no name associated with pointer data types.
Answer: True
Classes, Structs, and Pointer Variables
1. Explain how to declare pointer variables to classes and structs.
2. Describe the two methods of accessing member variables with pointer variables of a
class or struct type.
3. Examine the syntax of both of the above methods. Note the precedence rules of the
dereferencing and dot operators.
4. Use Example 13-3 to illustrate how pointers work with member functions. Clarify how
memory is accessed with Figures 13-6 and 13-7.
Teaching
Tip
Verify that students understand how to use the member access operator arrow
when accessing pointer variables of a class or struct type. Emphasize that this
is the most commonly used access technique and discuss the advantages of using
this operator rather than the combination of the dot/dereferencing operators.
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-5
Initializing Pointer Variables
1. Reiterate that C++ does not initialize variables.
2. Explain that pointer variables are generally initialized with the constant NULL or the
integer 0.
Teaching
Tip
Emphasize that the majority of errors your students will encounter when
programming with pointers involve the lack of initialization or updating of the
pointer value, thereby causing the pointer to point to an unintended location.
Teaching
Tip
Note that although pointer variables point to memory locations, they cannot be
used as integer types; the assignment of the number 0 is only allowed because it
has the same effect as using the constant NULL.
Dynamic Variables
1. Define the term dynamic variable and discuss how pointers are a valuable tool for
creating and manipulating data dynamically.
2. Briefly introduce the new and delete operators and explain their purpose.
Teaching
Tip
Reiterate that the programmer is responsible for handling pointer memory and
emphasize the importance of allocating and deallocating memory properly.
Operator new
1. Describe the two forms of the operator new and examine the syntax for each.
2. Use the code snippets in this section to illustrate how memory is allocated dynamically
and then manipulated for primitive types, strings, and arrays.
Teaching
Tip
Illustrate the results of attempting to access pointer memory for a string or array
without the new operator.
Operator delete
1. Describe a memory leak using Figures 13-8 through 13-11.
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-6
2. Explain how to use the operator delete to avoid memory leaks.
3. Examine the syntax of the two forms of the delete operator.
4. Define the term dangling pointer and explain how to avoid this problem.
Teaching
Tip
This section is difficult to grasp, especially the portions involving arrays.
Emphasize the two-step process in deallocating memory – deleting the memory
and setting the pointer to NULL.
Quick Quiz 2
1. What is the purpose of the member access operator arrow?
Answer: The member access operator arrow simplifies the accessing of class or
struct members with a pointer.
2. Write an equivalent statement to the following: studentPtr.gpa = 3.9;.
Answer: (*studentPtr).gpa = 3.9;
3. Variables that are created during program execution are called ____________________
variables.
Answer: dynamic
4. True or False: Any integer can be assigned to a pointer variable.
Answer: False
Operations on Pointer Variables
1. Discuss the operations that are allowed on pointer variables.
2. Illustrate the effect of using assignment and relational operators on pointer variables
with the code snippets in this section.
3. Explain how arithmetic operations on pointer variables differ from those on numbers.
4. Demonstrate how the increment and decrement operators are used with pointer
variables.
Teaching
Tip
Warn your students about the hazards of using pointer arithmetic and encourage
them to be patient when debugging errors involving pointer arithmetic.
C++ Programming: Program Design Including Data Structures, Fourth Edition
Teaching
Tip
13-7
Verify that your students understand how integer types are stored in memory so
that they may easily visualize how to add pointer values.
Dynamic Arrays
1. Describe the differences between static and dynamic arrays.
2. Discuss situations in which dynamic arrays are preferable to static arrays.
3. Examine the syntax for declaring and initializing a dynamic array.
4. Use Figures 13-12 and 13-13 to describe what takes place in memory when a dynamic
array is allocated.
5. Illustrate how a dynamic array is used in a program using Example 13-4.
Teaching
Tip
Explain that dynamic arrays are more flexible than static arrays because the user
can define the size of the array at run time. Emphasize, however, that the array
must still be initialized with a fixed number of items before it is used. In other
words, it is dynamically allocated, but is not a truly dynamic data structure.
Review the vector class as an example of a dynamic data structure. Students
may get a brief overview of this topic here:
http://en.wikipedia.org/wiki/Dynamic_array
Functions and Pointers
1. Explain that pointers can be passed by value or by reference.
2. Examine the syntax for passing a pointer as a value parameter and as a reference
parameter.
Teaching
Tip
The terminology for using a pointer as a reference parameter in a function header
may be a bit baffling to students. Step through a few function headers slowly and
remind students of the meaning of the * and & operators. Emphasize why the
ordering of the operators is important.
Pointers and Function Return Values
1. Explain how to return a pointer from a function.
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-8
Dynamic Two-Dimensional Arrays
1. Describe the two techniques presented in this section to declare two-dimensional arrays.
2. Illustrate the use of a two-dimensional array in a program using Example 13-5.
Teaching
Tip
In this section, the two-dimensional array is passed to the fill function both as
a value parameter and as a pointer. Does the function change the actual values of
the array? Explain that pointers are, in essence, reference types. The implications
of this will be clear later on in the chapter.
Quick Quiz 3
1. List the operations that can be performed on pointer variables.
Answer: Assignment, relational, and the following arithmetic operations: addition,
subtraction, increment, and decrement
2. If p is a pointer of type int, what is the result of the statement p++;?
Answer: It increments the value of p by four bytes.
3. True or False: The arithmetic operations that are allowed on pointer variables differ
from those allowed on numbers.
Answer: True
4. To create a dynamic array, you use the second form of the ____________________
operator.
Answer: new
Shallow versus Deep Copy and Pointers
1. Explain the difference between a shallow and deep copy of a variable.
2. Illustrate the issue involving pointers and a shallow copy of an array with Figures 13-16
through 13-19 and the accompanying code. Explain how creating a deep copy resolves
this issue with Figure 13-20 and the accompanying code.
Teaching
Tip
Verify that students understand the code involved in making a deep copy of an
array.
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-9
Classes and Pointers: Some Peculiarities
1. Examine the class presented in this section using Figure 13-21 and the accompanying
code. It will be used to discuss the following sections.
Destructor
1. Discuss the purpose of a destructor, particularly as it relates to dynamic memory
allocation.
2. Examine the syntax of a destructor function with the example in this section.
Teaching
Tip
Explain that the classes in previous chapters did not need destructors because the
programmer did not explicitly allocate memory with the new operator.
Assignment Operator
1. Describe the limitations of the built-in assignment operators for classes with pointer
variables. Use Figures 13-23 through 13-25 to illustrate how pointer variable class
assignments are handled in memory.
2. Note that the use of assignment overloading to avoid shallow copying will be discussed
in Chapter 14.
Teaching
Tip
Discuss why it is necessary to extend, or overload, operations for pointer
variables.
Copy Constructor
1. Describe the purpose of a copy constructor.
2. Explain the C++ default member-wise initialization when an object is initialized with
another object (of the same type). Illustrate its limitations using Figures 13-26 through
13-29.
3. Explain how to write a copy constructor function using Example 13-6.
Teaching
Tip
Note that the copy constructor will resolve the parameter problem described in
the Teaching Tip of the two-dimensional array discussion. Emphasize again the
reality of passing pointer arrays by value; that, in fact, the programmer must
ensure that a deep copy is made.
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-10
Inheritance, Pointers, and Virtual Functions
1. Emphasize that C++ allows an object of a derived class to be passed to a formal
parameter of a base class type. Explain how this might create unintended results with
the program in this section.
2. Explain the difference between static and dynamic binding.
3. Describe how C++ provides dynamic binding through the use of virtual functions.
4. Examine the syntax of virtual function prototypes and definitions.
5. Describe how to use virtual functions with value parameters of a class type.
Teaching
Tip
Discuss when it might be useful to pass a derived object to a formal parameter of
the base class type; for example, when an array is declared with objects of the
base type.
Classes and Virtual Destructors
1. Explain when and why the destructor function of a base class should be declared virtual.
Abstract Classes and Pure Virtual Functions
1. Define the terms abstract class and pure virtual functions.
2. Describe the situations in which using pure virtual functions and abstract classes is
useful. Use the shape class presented in this section as an illustration.
3. Step through the program in Example 13-7 to demonstrate the use of an abstract class.
Teaching
Tip
Explain how a virtual function and a pure virtual function are related, namely, by
their run-time binding. Then explain how they are different, namely, in that a
pure virtual function is not implemented, but can be used to enforce an
implementation in a nonabstract derived class.
Teaching
Tip
Note that an abstract class cannot be instantiated, but that it may contain
implementations and values for member functions that are inherited.
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-11
Address of Operator and Classes
1. Describe the additional uses of the address of operator in C++.
2. Explain how using the address of operator to manipulate private member variables can
be prevented. Use the two versions of the testAdd class presented in this section to
illustrate.
Quick Quiz 4
1. What is meant by a shallow copy?
Answer: In a shallow copy, two pointer variables of the same type point to the same
memory.
2. Dynamic memory must be deallocated with the operator ____________________.
Answer: delete
3. C++ provides default member-wise initialization of objects that are initialized with the
value of an existing object with the ____________________.
Answer: copy constructor
4. True or False: In dynamic binding, the necessary code to call a specific function is
generated by the compiler.
Answer: False
Class Discussion Topics
1. The section on classes, structs, and pointers had an example of declaring a variable
of a class as a pointer type. What are some reasons to declare classes as pointer
variables?
2. Note that the concept of pointers in C++ is inherited from the C language, which relies
extensively on the use of pointers. What are the advantages and disadvantages of having
the functionality of pointers in a programming language? Do students mind the
additional responsibility that comes along with the power of manipulating memory? Are
there situations in which manipulating computer memory directly is advantageous?
C++ Programming: Program Design Including Data Structures, Fourth Edition
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Additional Projects
1. Rewrite the birthday program you created in Chapter 11 to use a dynamic array of
gift objects. Allow the user to determine the maximum number of gifts he or she will
add to the list. Include a copy constructor and a destructor in your class. Write a
program to test the new implementation of the class.
2. Rewrite the capitals program you created in Chapter 11 to use a dynamic “continent”
array that contains countryType classes. You will also create a continentType
class that holds this array (if necessary, review the composition of classes from Chapter
12). Allow the user to determine the size of the array. Make the necessary modifications
for accessing the countryType objects, and then create a new test program that
creates a dynamic array of continentType objects.
Additional Resources
1. Pointers:
www.cplusplus.com/doc/tutorial/pointers.html
2. C++ Tutorial - About Pointers:
http://cplus.about.com/od/learning1/ss/pointers.htm
3. Dynamic Memory Allocation:
http://en.wikipedia.org/wiki/Dynamic_memory_allocation
4. Abstract Classes (C++):
http://msdn2.microsoft.com/en-us/library/c8whxhf1.aspx
Key Terms
 Abstract class: class that contains one or more pure virtual functions
 Address of operator (&): unary operator that returns the address of its operand
 Compile-time binding (static binding): binding in which the necessary code to call a
specific function is generated by the compiler
 Copy constructor: performs the default member-wise initialization provided by the
compiler; can be overridden by the programmer to ensure deep copies of a class
 Dangling pointers: pointer variables that contain the addresses of deallocated memory
spaces
 Deep copy: two or more pointers of the same type that have their own data
 Dereferencing operator (indirection operator): refers to the object to which its
operand (a pointer) points
 Dynamic array: array created during the execution of a program from a pointer
variable and the new operator
 Dynamic variables: variables that are created during program execution
C++ Programming: Program Design Including Data Structures, Fourth Edition
13-13
 Member access operator arrow (->): operator -> consists of two consecutive symbols:
a hyphen and the greater than symbol; used to simplify the accessing of class
components with a pointer
 Memory leak: when an unused memory space cannot be allocated
 Null pointer: pointer that has the predefined constant value 0
 Pointer variable: variable whose content is a memory address
 Pure virtual function: functions with no definition
 Run-time binding (dynamic binding): binding of virtual functions that occurs at
program execution time
 Shallow copy: two or more pointers of the same type point to the same memory; that is,
they point to the same data; this occurs when you do not override the default copy
constructor for a class containing pointers
 Virtual destructor: destructor that is marked as virtual in a base class; automatically
makes the destructor of a derived class virtual
 Virtual functions: functions that are dynamically bound, allowing the compiler to
generate code that selects the correct function at run time
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