C++: static member funcion

整理自：Thinking in C++

和 java 的 static method 基本一致。

A static member function is designed to be conceptually associated with the class as a whole.
A static member function cannot access ordinary data members, only static data members.
A static member function can call only other static member functions.
Normally, the address of the current object (this) is quietly passed in when any member function is called, but a static member has no this, which is the reason it cannot access ordinary members or call ordinary member function.