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# friend declaration

From cppreference.com
< cpp‎ | language

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.

### [edit] Syntax

`friend` function-declaration | (1) |
`friend` function-definition | (2) |
`friend` type-specifier `;` | (3) |
`friend` elaborated-class-name `;` | (4) |

### [edit] Description

1) Designates a function or several functions as friends of this class

    class Y {
        int data; // private member
        // the non-member function operator<< will have access to Y's private members
        friend std::ostream& operator<<(std::ostream& out, const Y& o);
        friend char* X::foo(int); // members of other classes can be friends too
        friend X::X(char), X::~X(); // constructors and destructors can be friends
    };
    // friend declaration does not declare a member function
    // this operator<< still needs to be defined, as a non-member
    std::ostream& operator<<(std::ostream& out, const Y& y)
    {
        return out << y.data; // can access private member Y::data
    }

2) (only allowed in non-local class definitions) Defines a non-member function, and makes it a friend of this class at the same time. Such non-member function is always inline.

    class X {
        int a;
        friend void friend_set(X& p, int a) {
            p.a = i; // this is a non-member function
        }
     public:
        void member_set(int a) {
            a = i; // this is a member function
        }
    };

3) Designates the class, struct, or union named by the type-specifier as a friend of of this class. This means that the friend's member declarations and definitions can access private and protected members of this class and also that the friend can inherit from private and protected members of this class.
4) If the name of the class that is used in the friend declaration is not yet declared, it can be forward declared on the spot by using elaborated type name (the keyword `class`, `struct`, or `union` followed by class name).

    class Y {};
    class A {
        int data; // private data member
        class B { }; // private nested type
        enum { a = 100 }; // private enumerator
        friend Y; // friend class declaration
        friend class X; // friend class forward declaration
    };

    class X : A::B { // OK: A::B accessible to friend
        A::B mx; // OK: A::B accessible to member of friend
        class Y {
            A::B my; // OK: A::B accessible to nested member of friend
        };
        int v[A::a]; // OK: A::a accessible to member of friend
    };

### [edit] 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)

Storage class specifiers are not allowed in friend function declarations. A function that is defined in the friend declaration has external linkage, a function that was previously defined, keeps the linkage it was defined with.

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:

    class F {};
    int f();
    int main()
    {
        extern int g();
        class Local { // Local class in the main() function
            friend int f(); // Error, no such function declared in main()
            friend int g(); // OK, there is a declaration for g in main()
            friend class F; // friends a local F (defined later)
            friend class ::F; // friends the global F
        };
        class F {}; // local F
    }

### [edit] Template friends

Both function template and class template declarations may appear with the `friend` specifier in any non-local class or class template (although only function templates may be defined within the class or class template that is granting friendship). In this case, every specialization of the template becomes a friend, whether it is implicitly instantiated, partially specialized, or explicitly specialized.

    class A {
        template<typename T>
        friend class B; // every B<T> is a friend of A

        template<typename T>
        friend void f(T) {} // every f<T> is a friend of A
    };

Friend declarations cannot refer to partial specializations, but can refer to full specializations:

    template<class T> class A {}; // primary
    template<class T> class A<T*> {}; // partial
    template<> class A<int> {}; // full
    class X {
        template<class T> friend class A<T*>; // error!
        friend class A<int>; // OK
    };

When a friend declaration refers to a full specialization of a function template, the keyword `inline` and default arguments cannot be used.

    template<class T> void f(int);
    template<> void f<int>(int);

    class X {
        friend void f<int>(int x = 1); // error: default args not allowed
    };

If a member of a class template A is declared to be a friend of a non-template class B, the corresponding member of every specialization of A becomes a friend of B. If A is explicitly specialized, as long as there is a member of the same name, same kind (type, function, class template, function template), same parameters/signature, it will be a friend of B.

    template<typename T> // primary template
    struct A
    {
        struct C {};
        void f();
        struct D {
            void g();
        };
    };

    template<> // full specialization
    struct A<int>
    {
        struct C {};
        int f();
        struct D {
            void g();
        };
    };

    class B // non-template class
    {
        template<class T>
        friend struct A<T>::C; // A<int>::C is a friend, as well as all A<T>::C

        template<class T>
        friend void A<T>::f(); // A<int>::f() is not a friend, because the
                               // signatures do not match, but A<char>::f() is

        template<class T>
        friend void A<T>::D::g(); // A<int>::D::g() is not a friend: it is not a member
                                  // of A, and A<int>::D is not a specialization of A<T>::D
    };

### [edit] Template friend operators

A common use case for template friends is declaration of a non-member operator overload that acts on a class template, e.g. operator<<(std::ostream&, const Foo<T>&) for some user-defined Foo<T>

Such operator can be defined in the class body, which has the effect of generating a separate non-template `operator<<` for each `T` and makes that non-template `operator<<` a friend of its `Foo<T>`

Run this code

    #include <iostream>

    template<typename T>
    class Foo {
     public:
        Foo(const T& val) : data(val) {}
     private:
        T data;

        // generates a non-template operator<< for this T
        friend std::ostream& operator<<(std::ostream& os, const Foo& obj)
        {
            return os << obj.data;
        }
    };

    int main()
    {
        Foo<double> obj(1.23);
        std::cout << obj << '\n';
    }

Output:

    1.23

or the function template has to be declared as a template before the class body, in which case the friend declaration within `Foo<T>` can refer to the full specialization of `operator<<` for its `T`:

Run this code

    #include <iostream>

    template<typename T>
    class Foo; // forward declare to make function declaration possible

    template<typename T> // declaration
    std::ostream& operator<<(std::ostream&, const Foo<T>&);

    template<typename T>
    class Foo {
     public:
        Foo(const T& val) : data(val) {}
     private:
        T data;

        // refers to a full specialization for this particular T
        friend std::ostream& operator<< <> (std::ostream&, const Foo&);
        // (can also be written with "operator<< <T>")
    };

    // definition
    template<typename T>
    std::ostream& operator<<(std::ostream& os, const Foo<T>& obj)
    {
        return os << obj.data;
    }

    int main()
    {
        Foo<double> obj(1.23);
        std::cout << obj << '\n';
    }

### [edit] Example

stream insertion and extraction operators are often declared as non-member friends

Run this code

    #include <iostream>
    #include <sstream>

    class MyClass {
        int i;

        friend std::ostream& operator<<(std::ostream& out, const MyClass& o);
        friend std::istream& operator>>(std::istream& in, MyClass& o);
     public:
        MyClass(int i = 0) : i(i) {}
    };

    std::ostream& operator<<(std::ostream& out, const MyClass& mc)
    {
        return out << mc.i;
    }

    std::istream& operator>>(std::istream& in, MyClass& mc)
    {
        return in >> mc.i;
    }

    int main()
    {
        MyClass mc(7);
        std::cout << mc << '\n';
        std::istringstream("100") >> mc;
        std::cout << mc << '\n';
    }

Output:

    7
    100

### [edit] References

- C++11 standard (ISO/IEC 14882:2011):

  - 11.3 Friends [class.friend]

  - 14.5.4 Friends [temp.friend]

- C++98 standard (ISO/IEC 14882:1998):

  - 11.3 Friends [class.friend]

  - 14.5.3 Friends [temp.friend]

### [edit] See Also

Class declaration
Access specifiers

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