Declaring static Members

We say a member is associated with the class by adding the keyword static to its declaration. Like any other member, static members can be public or private. The type of a static data member can be const, reference, array, class type, and so forth.

As an example, we’ll define a class to represent an account record at a bank:

class Account {
public:
    void calculate() { amount += amount * interestRate; }
    static double rate() { return interestRate; }
    static void rate(double);
private:
    std::string owner;
    double amount;
    static double interestRate;
    static double initRate();
};

The static members of a class exist outside any object. Objects do not contain data associated with static data members. Thus, each Account object will contain two data members—owner and amount. There is only one interestRate object that will be shared by all the Account objects.

Similarly, static member functions are not bound to any object; they do not have a this pointer. As a result, static member functions may not be declared as const, and we may not refer to this in the body of a static member. This restriction applies both to explicit uses of this and to implicit uses of this by calling a nonstatic member.