Negative tive C ++ clause 19: differentiate member functions, non-member functions, and friend Functions

These terms draw the following conclusions:

Suppose f is a function to be correctly declared, and C is a class related to it:

1. The virtual function must be a member function. If F must be a virtual function, make it a member function of C.

2. Operator> and operator <must not be member functions. If F is Operator> or operator <, make f a non-member function. If F also needs to access non-public members of C, make f a friend function of C.

3. Only non-member functions perform type conversion on the leftmost parameters. If f needs to convert the type of the leftmost parameter to make f a non-member function. If F also needs to access non-public members of C, make f a friend function of C.

4. In other cases, it is declared as a member function. If none of the above is true, make f a member function of C.