3.5. Initializing Objects with Constructors

As mentioned in Section 3.4, when an object of class GradeBook (Fig. 3.5) is created, its data member courseName is initialized to the empty string by default. What if you want to provide a course name when you create a GradeBook object? Each class you declare can provide one or more constructors that can be used to initialize an object of the class when the object is created. A constructor is a special member function that must be defined with the same name as the class, so that the compiler can distinguish it from the class’s other member functions. An important difference between constructors and other functions is that constructors cannot return values, so they cannot specify a return type (not even void). Normally, constructors are declared public. In the early chapters, our classes will generally have one constructor—in later chapters, you’ll see how to create classes with more that one constructor using the technique of function overloading, which we introduce in Section 6.17.

C++ automatically calls a constructor for each object that’s created, which helps ensure that objects are initialized properly before they’re used in a program. The constructor call occurs when the object is created. If a class does not explicitly include constructors, the compiler provides a default constructor with no parameters. For example, when line 41 of Fig. 3.5 creates a GradeBook object, the default constructor is called. The default constructor provided by the compiler creates a GradeBook object without giving any initial values to the object’s fundamental type data members. For data members that are objects of other classes, the default constructor implicitly calls each data member’s default constructor to ensure that the data member is initialized properly. This is why the string data member courseName (in Fig. 3.5) was initialized to the empty string—the default constructor for class string sets the string’s value to the empty string.

In the example of Fig. 3.7, we specify a course name for a GradeBook object when the object is created (e.g., line 47). In this case, the argument "CS101 Introduction to C++ Programming" is passed to the GradeBook object’s constructor (lines 14–18) and used to initialize the courseName. Figure 3.7 defines a modified GradeBook class containing a constructor with a string parameter that receives the initial course name.

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Fig. 3.7. Instantiating multiple objects of the GradeBook class and using the GradeBook constructor to specify the course name when each GradeBook object is created.