A friend of a class X is a function or class that is not a member of X, but is granted the same access to X as the members of X. Functions declared with the friend specifier in a class member list are called friend functions of that class. Classes declared with the friend specifier in the member list of another class are called friend classes of that class.
A class Y must be defined before any member of Y can be declared a friend of another class. In the following example, the friend function print is a member of class Y and accesses the private data members a and b of class X.
#include <iostream>
using namespace std;
class X;
class Y {
public:
void print(X& x);
};
class X {
int a, b;
friend void Y::print(X& x);
public:
X() : a(1), b(2) { }
};
void Y::print(X& x) {
cout << "a is " << x.a << endl;
cout << "b is " << x.b << endl;
}
int main() {
X xobj;
Y yobj;
yobj.print(xobj);
}
a is 1
b is 2
You can declare an entire class as a friend. Suppose class F is a friend of class A. This means that every member function and static data member definition of class F has access to class A.
In the following example, the friend class F has a member function print that accesses the private data members a and b of class X and performs the same task as the friend function print in the above example. Any other members declared in class F also have access to all members of class X.
#include <iostream>
using namespace std;
class X {
int a, b;
friend class F;
public:
X() : a(1), b(2) { }
};
class F {
public:
void print(X& x) {
cout << "a is " << x.a << endl;
cout << "b is " << x.b << endl;
}
};
int main() {
X xobj;
F fobj;
fobj.print(xobj);
}
a is 1
b is 2
class F;
class X {
friend class F { };
};
class A {
void g();
};
void z() {
class B {
friend void f() { }; // error
};
}
class C {
friend void A::g() { } // error
friend void h() { }
};
The compiler accepts the definition of h(),
but not the function definition of f() or g().You cannot declare a friend with a storage class specifier.
class F;
class G;
class X1 {
//C++98 friend declarations remain valid in C++11.
friend class F;
//Error in C++98 for missing the class-key.
friend G;
};
class X2 {
//Error in C++98 for missing the class-key.
//Error in C++11 for lookup failure (no previous class D declaration).
friend D;
friend class D;
};
class C;
template <typename T, typename U> class F {
//C++11 compiles sucessfully.
//Error in C++98 for missing the class-key.
friend T;
//Error in both C++98 and C++11: a template parameter
//must not be used in an elaborated type specifier.
friend class U;
};
F<C> rc;
F<char> Ri;
class Derived;
typedef Derived D;
class C;
typedef C Ct;
class Base{
public:
Base() : x(55) {}
//C++11 compiles sucessfully.
//Error in C++98 for missing the class-key.
friend D;
//Error in both C++98 and C++11: a typedef name
//must not be used in an elaborated type specifier.
friend class Ct;
private:
int x;
};
struct Derived : public Base {
int foo() { return this->x; }
};
int main() {
Derived d;
return d.foo();
}
struct T { };
namespace N {
struct A {
friend T;
};
}
In this example, if this feature is in effect, the friend
declaration statement does not declare a new entity T,
but looks for T. If there is no T found,
then the compiler issues an error. Consider another example:struct T { };
namespace N {
struct A {
friend class T; //fine, no error
};
}
In this example, the friend declaration statement does
not look for T outside namespace N,
nor does it find ::T. Instead, this statement declares
a new class T in namespace N.