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.
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Syntax
friend function-declaration
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(1) | ||||||||
friend function-definition
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(2) | ||||||||
friend elaborated-class-name ;
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friend simple-type-specifier ;
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(4) | (since C++11) | |||||||
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 } };
friend declaration is ignored. This declaration will not forward declare 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 friend Y; // friend class declaration (since c++11) }; class X : A::B // Error until C++11: the base-clause is not part of member declarations // allowed in C++11 { A::B mx; // OK: A::B accessible to member of friend class Y : A::B { // OK: A::B accessible to base-clause of 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)
Prior to C++11, member declarations and definitions inside the nested class of the friend of class T cannot access the private and protected members of class T, but some compilers accept it even in pre-C++11 mode.
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 class or class template X becomes a member of the innermost enclosing namespace of X, but is not accessible for lookup (except argument-dependent lookup that considers X) unless a matching declaration at the 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 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 };
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Default template arguments are only allowed on template friend declarations if the declaration is a definition. |
(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>
#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:
#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
#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
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]
See Also
| Class declaration | |
| Access specifiers |
