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C++ A Beginner’s Guide by Herbert Schildt Module8
Classes and Objects
Table of Contents
CRITICAL SKILL 8.1: The General Form of a Class .......................................................................................... 2
CRITICAL SKILL 8.2: Defining a Class and Creating Objects ........................................................................... 2
CRITICAL SKILL 8.3: Adding Member Functions to a Class ............................................................................ 6
Project 8-1 Creating a Help Class .................................................................................................................. 9
CRITICAL SKILL 8.4: Constructors and Destructors ..................................................................................... 14
CRITICAL SKILL 8.5: Parameterized Constructors ........................................................................................ 17
CRITICAL SKILL 8.6: Inline Functions ........................................................................................................... 22
CRITICAL SKILL 8.7: Arrays of Objects ......................................................................................................... 31
CRITICAL SKILL 8.8: Initializing Object Arrays .............................................................................................. 32
CRITICAL SKILL 8.9: Pointers to Objects ...................................................................................................... 34

Up to this point, you have been writing programs that did not use any of C++’s object-oriented
capabilities. Thus, the programs in the preceding modules reflected structured programming, not
object-oriented programming. To write object-oriented programs, you will need to use classes. The class
is C++’s basic unit of encapsulation. Classes are used to create objects. Classes and objects are so
fundamental to C++ that much of the remainder of this book is devoted to them in one way or another.
Class Fundamentals
Let’s begin by reviewing the terms class and object. A class is a template that defines the form of an
object. A class specifies both code and data. C++ uses a class specification to construct objects. Objects
are instances of a class. Thus, a class is essentially a set of plans that specify how to build an object. It is
important to be clear on one issue: a class is a logical abstraction. It is not until an object of that class
has been created that a physical representation of that class exists in memory.
When you define a class, you declare the data that it contains and the code that operates on that data.
While very simple classes might contain only code or only data, most real-world classes contain both.

information into the same class will quickly destructure your code!
Let’s review: In C++, a class creates a new data type that can be used to create objects.
Specifically, a class creates a logical framework that defines a relationship between its members. When
you declare a variable of a class, you are creating an object. An object has physical existence and is a
specific instance of a class. That is, an object occupies memory space, but a type definition does not.
CRITICAL SKILL 8.2: Defining a Class and Creating Objects
To illustrate classes, we will be evolving a class that encapsulates information about vehicles, such as
cars, vans, and trucks. This class is called Vehicle, and it will store three items of information about a
vehicle: the number of passengers that it can carry, its fuel capacity, and its average fuel consumption
(in miles per gallon).
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C++ A Beginner’s Guide by Herbert Schildt The first version of Vehicle is shown here. It defines three instance variables: passengers, fuelcap, and
mpg. Notice that Vehicle does not contain any functions. Thus, it is currently a data-only class.
(Subsequent sections will add functions to it.)

The instance variables defined by Vehicle illustrate the way that instance variables are declared in
general. The general form for declaring an instance variable is shown here:
type var-name;
Here, type specifies the type of variable, and var-name is the variable’s name. Thus, you declare an
instance variable in the same way that you declare other variables. For Vehicle, the variables are
preceded by the public access specifier. As explained, this allows them to be accessed by code outside of
Vehicle.
A class definition creates a new data type. In this case, the new data type is called Vehicle. You will use
this name to declare objects of type Vehicle. Remember that a class declaration is only a type
description; it does not create an actual object. Thus, the preceding code does not cause any objects of
type Vehicle to come into existence.
To actually create a Vehicle object, simply use a declaration statement, such as the following:

own copy of passengers, fuelcap, and mpg, and the contents of these can differ between the two
objects. The following program demonstrates this fact:
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C++ A Beginner’s Guide by Herbert Schildt The output produced by this program is shown here:
Minivan can carry 7 with a range of 336
Sportscar can carry 2 with a range of 168
As you can see, minivan’s data is completely separate from the data contained in sportscar. Figure 8-1
depicts this situation.
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C++ A Beginner’s Guide by Herbert Schildt

1. A class can contain what two things?

2. What operator is used to access the members of a class through an object?

3. Each object has its own copies of the class’ _____________.

CRITICAL SKILL 8.3: Adding Member Functions to a Class
So far, Vehicle contains only data, but no functions. Although data-only classes are perfectly valid, most
classes will have function members. In general, member functions manipulate the data defined by the
class and, in many cases, provide access to that data. Typically, other parts of your program will interact

The body of range( ) consists solely of this line:
return mpg * fuelcap;
This statement returns the range of the vehicle by multiplying fuelcap by mpg. Since each object of type
Vehicle has its own copy of fuelcap and mpg, when range( ) is called, the range computation uses the
calling object’s copies of those variables.
Inside range( ) the instance variables fuelcap and mpg are referred to directly, without preceding them
with an object name or the dot operator. When a member function uses an instance variable that is
defined by its class, it does so directly, without explicit reference to an object and without use of the dot
operator. This is easy to understand if you think about it. A member function is always invoked relative
to some object of its class. Once this invocation has occurred, the object is known. Thus, within a
member function, there is no need to specify the object a second time. This means that fuelcap and mpg
inside range( ) implicitly refer to the copies of those variables found in the object that invokes range( ).
Of course, code outside Vehicle must refer to fuelcap and mpg through an object and by using the dot
operator.
A member function must be called relative to a specific object. There are two ways that this can happen.
First, a member function can be called by code that is outside its class. In this case, you must use the
object’s name and the dot operator. For example, this calls range( ) on minivan:
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C++ A Beginner’s Guide by Herbert Schildt range = minivan.range();
The invocation minivan.range( ) causes range( ) to operate on minivan’s copy of the instance variables.
Thus, it returns the range for minivan.
The second way a member function can be called is from within another member function of the same
class. When one member function calls another member function of the same class, it can do so directly,
without using the dot operator. In this case, the compiler already knows which object is being operated
upon. It is only when a member function is called by code that does not belong to the class that the
object name and the dot operator must be used.
The program shown here puts together all the pieces and missing details, and illustrates the range( )

Help class. Let’s examine why this is a good idea. First, the help system defines one logical unit. It simply
displays the syntax for the C++ control statements. Thus, its functionality is compact and well defined.
Second, putting help in a class is an esthetically pleasing approach. Whenever you want to offer the help
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C++ A Beginner’s Guide by Herbert Schildt system to a user, simply instantiate a help-system object. Finally, because help is encapsulated, it can be
upgraded or changed without causing unwanted side effects in the programs that use it.
Step by Step
1. Create a new file called HelpClass.cpp. To save you some typing, you might want to copy the file from
Project 3-3, Help3.cpp, into HelpClass.cpp.

2. To convert the help system into a class, you must first determine precisely what constitutes the help
system. For example, in Help3.cpp, there is code to display a menu, input the user’s choice, check for a
valid response, and display information about the item selected. The program also loops until q is
pressed. If you think about it, it is clear that the menu, the check for a valid response, and the display of
the information are integral to the help system. How user input is obtained, and whether repeated
requests should be processed, are not. Thus, you will create a class that displays the help information,
the help menu, and checks for a valid selection. These functions will be called helpon( ), showmenu(
),and isvalid( ), respectively.

3. Declare the Help class, as shown here: Notice that this is a function-only class; no instance variables are needed. As explained, data-only and
code-only classes are perfectly valid. (Question 9 in the Mastery Check adds an instance variable to the
Help class.)

4. Create the helpon( ) function, as shown here:

When you try the program, you will find that it is functionally the same as in Module 3. The advantage to
this approach is that you now have a help system component that can be reused whenever it is needed.

CRITICAL SKILL 8.4: Constructors and Destructors
In the preceding examples, the instance variables of each Vehicle object had to be set manually by use
of a sequence of statements, such as: