Class GradeBook

Before function main (lines 19–23) can create a GradeBook object, we must tell the compiler what member functions and data members belong to the class. The GradeBook class definition (lines 8–16) contains a member function called displayMessage (lines 12–15) that displays a message on the screen (line 14). We need to make an object of class GradeBook (line 21) and call its displayMessage member function (line 22) to get line 14 to execute and display the welcome message. We’ll soon explain lines 21–22 in detail.

The class definition begins in line 8 with the keyword class followed by the class name GradeBook. By convention, the name of a user-defined class begins with a capital letter, and for readability, each subsequent word in the class name begins with a capital letter. This capitalization style is often referred to as Pascal case, because the convention was widely used in the Pascal programming language. The occasional uppercase letters resemble a camel’s humps. More generally, camel case capitalization style allows the first letter to be either lowercase or uppercase (e.g., myGradeBook in line 21).

Every class’s body is enclosed in a pair of left and right braces ({ and }), as in lines 9 and 16. The class definition terminates with a semicolon (line 16).

Recall that the function main is always called automatically when you execute a program. Most functions do not get called automatically. As you’ll soon see, you must call member function displayMessage explicitly to tell it to perform its task.

Line 10 contains the keyword public, which is an access specifier. Lines 12–15 define member function displayMessage. This member function appears after access specifier public: to indicate that the function is “available to the public”—that is, it can be called by other functions in the program (such as main), and by member functions of other classes (if there are any). Access specifiers are always followed by a colon (:). For the remainder of the text, when we refer to the access specifier public in the text, we’ll omit the colon as we did in this sentence. Section 3.4 introduces the access specifier private. Later in the book we’ll study the access specifier protected.

When you define a function, you must specify a return type to indicate the type of the value returned by the function when it completes its task. In line 12, keyword void to the left of the function name displayMessage is the function’s return type. Return type void indicates that displayMessage will not return any data to its calling function (in this example, line 22 of main, as we’ll see in a moment) when it completes its task. In Fig. 3.5, you’ll see an example of a function that does return a value.

The name of the member function, displayMessage, follows the return type (line 12). By convention, our function names use the camel case style with a lowercase first letter. The parentheses after the member function name indicate that this is a function. An empty set of parentheses, as shown in line 12, indicates that this member function does not require additional data to perform its task. You’ll see an example of a member function that does require additional data in Section 3.3.

We declared member function displayMessage const in line 12 because in the process of displaying "Welcome to the Grade Book!" the function does not, and should not, modify the GradeBook object on which it’s called. Declaring displayMessage const tells the compiler, “this function should not modify the object on which it’s called—if it does, please issue a compilation error.” This can help you locate errors if you accidentally insert code in displayMessage that would modify the object. Line 12 is commonly referred to as a function header.

Every function’s body is delimited by left and right braces ({ and }), as in lines 13 and 15. The function body contains statements that perform the function’s task. In this case, member function displayMessage contains one statement (line 14) that displays the message "Welcome to the Grade Book!". After this statement executes, the function has completed its task.