Friend declaration

From cppreference.com
< cpp‎ | language
 
C++
 
C++ language
General topics
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
decltype (C++11)
auto (C++11)
alignas (C++11)
Storage duration specifiers
Initialization
Expressions
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
Classes
Access specifiers
friend specifier
Class-specific function properties
explicit (C++11)
static
Special member functions
Templates
Miscellaneous
 
Classes
 

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

friend function-declaration (1)
friend function-definition (2)
friend elaborated-class-specifier ; (3)
friend simple-type-specifier ;

friend typename-specifier ;

(4) (since C++11)

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, unless it is attached to a named module (since C++20). 
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
    }
};
3) Designates the class, struct, or union named by the elaborated-class-specifier (see elaborated type specifier) as a friend 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. The name of the class that is used in this friend declaration does not need to be previously declared.
4) Designates the type named by the simple-type-specifier or typename-specifier as a friend of this class if that type is a (possibly cv-qualified) class, struct, or union; otherwise the friend declaration is ignored. This declaration will not forward declare a new type. 
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)
};
 
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).

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
};

Default template arguments are only allowed on template friend declarations if the declaration is a definition and no other declarations of this function template appear in this translation unit.

(since C++11)

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&);
 
    // 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

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

Run this code
#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';
}

Output: 

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.

DR Applied to Behavior as published Correct behavior
CWG 45 C++98 members of a class nested in a friend
class of T have no special access to T 		a nested class has the same
access as the enclosing class
CWG 500 C++98 friend class of T cannot inherit from private or
protected members of T, but its nested class can 		both can inherit
from such members
CWG 1439 C++98 the rule targeting friend declarations in non-local
classes did not cover template declarations 		covered
CWG 1477 C++98 a name first declared in a friend declaration within a class
or class template was not visible for lookup if the matching
declaration is provided in another namespace scope 		it is visible for
lookup in this case
CWG 1804 C++98 when a member of a class template is friended, the corresponding
member of specializations of partial specializations of the class
template was not a friend of the class granting friendship 		such members
are also friends
CWG 2379 C++11 friend declarations referring to full specializations
of function templates could be declared constexpr 		prohibited

References

  • C++20 standard (ISO/IEC 14882:2020):
  • 11.9.3 Friends [class.friend]
  • 13.7.4 Friends [temp.friend]
  • C++17 standard (ISO/IEC 14882:2017):
  • 14.3 Friends [class.friend]
  • 17.5.4 Friends [temp.friend]
  • C++14 standard (ISO/IEC 14882:2014):
  • 11.3 Friends [class.friend]
  • 14.5.4 Friends [temp.friend]
  • 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]

See also

Class types defines types holding several data members
Access specifiers defines visibility of class members
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&oldid=141405"