Friend declaration

The friend declaration appears in a class body and grants a function or another class access to private and protected members of the class where the friend declaration appears.

# Notes

Friendship is not transitive (a friend of your friend is not your friend).

Friendship is not inherited (your friend’s children are not your friends, and your friends are not your children’s friends).

Access specifiers have no effect on the meaning of friend declarations (they can appear in private: or in public: sections, with no difference).

A friend class declaration cannot define a new class (friend class X {}; is an error).

When a local class declares an unqualified function or class as a friend, only functions and classes in the innermost non-class scope are looked up, not the global functions:

A name first declared in a friend declaration within a class or class template X becomes a member of the innermost enclosing namespace of X, but is not visible for lookup (except argument-dependent lookup that considers X) unless a matching declaration at namespace scope is provided - see namespaces for details.

# Example

#include <iostream>
#include <sstream>
 
class MyClass
{
    int i;                   // friends have access to non-public, non-static
    static inline int id{6}; // and static (possibly inline) members
 
    friend std::ostream& operator<<(std::ostream& out, const MyClass&);
    friend std::istream& operator>>(std::istream& in, MyClass&);
    friend void change_id(int);
public:
    MyClass(int i = 0) : i(i) {}
};
 
std::ostream& operator<<(std::ostream& out, const MyClass& mc)
{
    return out << "MyClass::id = " << MyClass::id << "; i = " << mc.i;
}
 
std::istream& operator>>(std::istream& in, MyClass& mc)
{
    return in >> mc.i;
}
 
void change_id(int id) { MyClass::id = id; }
 
int main()
{
    MyClass mc(7);
    std::cout << mc << '\n';
//  mc.i = 333*2;  // error: i is a private member
    std::istringstream("100") >> mc;
    std::cout << mc << '\n';
//  MyClass::id = 222*3;  // error: id is a private member
    change_id(9);
    std::cout << mc << '\n';
}

# Defect reports

# See also

• Class types
• Access specifiers