Programmer interview book-youyuan Function

The class has the characteristics of encapsulation and Information Hiding. Only the member functions of the class can access the private members of the class,ProgramOther functions in cannot access private members. A non-member function can be a public member of the callback class.

Youyuan is a common function defined outside the class, but it needs to be described in the class. to distinguish it from other member functions, the keyword friend should be added during the description, youyuan is not a member function, but it can be a private member of the member class. youyuan is used to improve the running efficiency of the program, but destroys the encapsulation and hiding of the class, so that non-member functions can also be Private Members of the callback class.