static Members Can Be Used in Ways Ordinary Members Can’t

As we’ve seen, static members exist independently of any other object. As a result, they can be used in ways that would be illegal for nonstatic data members. As one example, a static data member can have incomplete type (§ 7.3.3, p. 278). In particular, a static data member can have the same type as the class type of which it is a member. A nonstatic data member is restricted to being declared as a pointer or a reference to an object of its class:

class Bar {
public:
    // ...
private:
    static Bar mem1; // ok: static member can have incomplete type
    Bar *mem2;       // ok: pointer member can have incomplete type
    Bar mem3;        // error: data members must have complete type
};

Another difference between static and ordinary members is that we can use a static member as a default argument (§ 6.5.1, p. 236):

class Screen {
public:
    // bkground refers to the static member
    // declared later in the class definition
    Screen& clear(char = bkground);
private:
    static const char bkground;
};

A nonstatic data member may not be used as a default argument because its value is part of the object of which it is a member. Using a nonstatic data member as a default argument provides no object from which to obtain the member’s value and so is an error.