表示静态的意思啊
可以加在变量前,
it is allocated when the program begins and deallocated when the program ends;and initializes it to 0 unless another value is specified.
In C++, when modifying a data member in a class declaration, the static keyword specifies that one copy of the member is shared by all the instances of the class.
加在函数前
When modifying a member function in a class declaration, the static keyword specifies that the function accesses only static members.