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.