Software Engineering with Set and Get Functions

A class’s private data members can be manipulated only by member functions of that class (and by “friends” of the class as you’ll see in Chapter 9). So a client of an object—that is, any statement that calls the object’s member functions from outside the object—calls the class’s public member functions to request the class’s services for particular objects of the class. This is why the statements in function main call member functions setCourseName, getCourseName and displayMessage on a GradeBook object. Classes often provide public member functions to allow clients of the class to set (i.e., assign values to) or get (i.e., obtain the values of) private data members. These member function names need not begin with set or get, but this naming convention is common. In this example, the member function that sets the courseName data member is called setCourseName, and the member function that gets the value of the courseName data member is called getCourseName. Set functions are sometimes called mutators (because they mutate, or change, values), and get functions are also called accessors (because they access values).

Recall that declaring data members with access specifier private enforces data hiding. Providing public set and get functions allows clients of a class to access the hidden data, but only indirectly. The client knows that it’s attempting to modify or obtain an object’s data, but the client does not know how the object performs these operations. In some cases, a class may internally represent a piece of data one way, but expose that data to clients in a different way. For example, suppose a Clock class represents the time of day as a private int data member time that stores the number of seconds since midnight. However, when a client calls a Clock object’s getTime member function, the object could return the time with hours, minutes and seconds in a string in the format "HH:MM:SS". Similarly, suppose the Clock class provides a set function named setTime that takes a string parameter in the "HH:MM:SS" format. Using string capabilities presented in Chapter 19, the setTime function could convert this string to a number of seconds, which the function stores in its private data member. The set function could also check that the value it receives represents a valid time (e.g., "12:30:45" is valid but "42:85:70" is not). The set and get functions allow a client to interact with an object, but the object’s private data remains safely encapsulated (i.e., hidden) in the object itself.

The set and get functions of a class also should be used by other member functions within the class to manipulate the class’s private data, even though these member functions can access the private data directly. In Fig. 3.5, member functions setCourseName and getCourseName are public member functions, so they’re accessible to clients of the class, as well as to the class itself. Member function displayMessage calls member function getCourseName to obtain the value of data member courseName for display purposes, even though displayMessage can access courseName directly—accessing a data member via its get function creates a better, more robust class (i.e., a class that’s easier to maintain and less likely to malfunction). If we decide to change the data member courseName in some way, the displayMessage definition will not require modification—only the bodies of the get and set functions that directly manipulate the data member will need to change. For example, suppose we want to represent the course name as two separate data members—courseNumber (e.g., "CS101") and courseTitle (e.g., "Introduction to C++ Programming"). Member function displayMessage can still issue a single call to member function getCourseName to obtain the full course name to display as part of the welcome message. In this case, getCourseName would need to build and return a string containing the courseNumber followed by the courseTitle. Member function displayMessage could continue to display the complete course title “CS101 Introduction to C++ Programming.” The benefits of calling a set function from another member function of the same class will become clearer when we discuss validation in Section 3.8.