Difference between revisions of "cpp/language/friend"

Revision as of 03:44, 24 April 2024

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.

Syntax

Description

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
}

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

class Y {};
 
class A
{
    int data; // private data member
 
    class B {}; // private nested type
 
    enum { a = 100 }; // private enumerator
 
    friend class X; // friend class forward declaration (elaborated class specifier)
    friend Y; // friend class declaration (simple type specifier) (since C++11)
 
    // the two friend declarations above can be merged since C++26:
    // friend class X, Y;
};
 
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
};

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).

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
}

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.

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 keywords inline/constexpr(since C++11)/consteval(since C++20) 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
};

A template friend declaration can name a member of a class template A, which can be either a member function or a member type (the type must use elaborated-type-specifier). Such declaration is only well-formed if the last component in its nested-name-specifier (the name to the left of the last ::) is a simple-template-id (template name followed by argument list in angle brackets) that names the class template. The template parameters of such template friend declaration must be deducible from the simple-template-id.

In this case, the member of any specialization of either A or partial specializations of A becomes a friend. This does not involve instantiating the primary template A or partial specializations of A: the only requirements are that the deduction of the template parameters of A from that specialization succeeds, and that substitution of the deduced template arguments into the friend declaration produces a declaration that would be a valid redeclaration of the member of the specialization:

// primary template
template<class T>
struct A
{ 
    struct B {};
 
    void f();
 
    struct D { void g(); };
 
    T h();
 
    template<T U>
    T i();
};
 
// full specialization
template<>
struct A<int>
{
    struct B {};
 
    int f();
 
    struct D { void g(); };
 
    template<int U>
    int i();
};
 
// another full specialization
template<>
struct A<float*>
{
    int *h();
};
 
// the non-template class granting friendship to members of class template A
class X
{
    template<class T>
    friend struct A<T>::B; // all A<T>::B are friends, including A<int>::B
 
    template<class T>
    friend void A<T>::f(); // A<int>::f() is not a friend because its signature
                           // does not match, but e.g. A<char>::f() is a friend
 
//  template<class T>
//  friend void A<T>::D::g(); // ill-formed, the last part of the nested-name-specifier,
//                            // D in A<T>::D::, is not simple-template-id
 
    template<class T>
    friend int* A<T*>::h(); // all A<T*>::h are friends:
                            // A<float*>::h(), A<int*>::h(), etc
 
    template<class T> 
    template<T U>       // all instantiations of A<T>::i() and A<int>::i() are friends, 
    friend T A<T>::i(); // and thereby all specializations of those function templates
};

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>:

#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';
}

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:

#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&);
 
    // note: this relies on template argument deduction in declarations
    // can also specify the template argument 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';
}

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';
}

MyClass::id = 6; i = 7
MyClass::id = 6; i = 100
MyClass::id = 9; i = 100

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

References

See also