朋友类声明和成员函数声明

Question

Why friend class need not forward declaration but member function need?

friend class:

class String0{     
friend class String;
private:
    int size = 0;
    string s;
};

class String {      
public:
    string combine(const string &s1);
private:
    int size = 0;
    string s;
};   

member friend:

class String {        
public:
    string combine(const string &s1);  //why need this?
private:
    int size = 0;
    string s;
};

class String0 {
friend string String::combine(const string &);
};

String String::combine(const string &s1){...}

Classes and nonmember functions need not have been declared before they are used in a friend declaration. But why member function need?

Answer 1

A friend class/function declaration IS a declaration in itself and forward-declares the class/function. You got that right.

Now if you could forward declare a member function of class A as a friend in class B , that means you are changing the interface of class A .

Imagine I did

class foo {
public:
    friend void std::vector<int>::fun();
};

Am I adding a function to std::vector<int> ?

Answer 2

Standard gives an example:

13.3 Friends [class.friend] 3/

 class C; typedef C Ct; class X1 { friend C; // OK: class C is a friend }; class X2 { friend Ct; // OK: class C is a friend friend D; // error: no type-name D in scope friend class D; // OK: elaborated-type-specifier declares new class }; 

friend class D both forward declares class D and a friend relationship with it.