Defining static Members

As with any other member function, we can define a static member function inside or outside of the class body. When we define a static member outside the class, we do not repeat the static keyword. The keyword appears only with the declaration inside the class body:

void Account::rate(double newRate)
{
    interestRate = newRate;
}

Because static data members are not part of individual objects of the class type, they are not defined when we create objects of the class. As a result, they are not initialized by the class’ constructors. Moreover, in general, we may not initialize a static member inside the class. Instead, we must define and initialize each static data member outside the class body. Like any other object, a static data member may be defined only once.

Like global objects (§ 6.1.1, p. 204), static data members are defined outside any function. Hence, once they are defined, they continue to exist until the program completes.

We define a static data member similarly to how we define class member functions outside the class. We name the object’s type, followed by the name of the class, the scope operator, and the member’s own name:

// define and initialize a static class member
double Account::interestRate = initRate();

This statement defines the object named interestRate that is a static member of class Account and has type double. Once the class name is seen, the remainder of the definition is in the scope of the class. As a result, we can use initRate without qualification as the initializer for interestrate. Note also that although initRate is private, we can use it to initialize interestRate. Aswith any other member definition, a static data member definition may access the private members of its class.