CODEWITHSUNDEEP
c++
GMHDBJD
GMHDBJD

Reputation: 149

friend class declaration and member function declaration

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?

Upvotes: 2

Views: 119

Answers (2)

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36401

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.

Upvotes: 1

Ivaylo Valchev
Ivaylo Valchev

Reputation: 10425

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>?

Upvotes: 2

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