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);
}
The following is the output of the above example:
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);
}
The following is the output of the above example:
a is 1
b is 2
You must use an elaborated type specifier when you declare a class as a friend. The following example demonstrates this:
class F;
class G;
class X {
friend class F;
friend G;
};
The compiler will warn you that the friend declaration of G must be an elaborated class name.
You cannot define a class in a friend declaration. For example, the compiler will not allow the following:
class F;
class X {
friend class F { };
};
However, you can define a function in a friend declaration. The class must be a non-local class, function, the function name must be unqualified, and the function has namespace scope. The following example demonstrates this:
class A {
void g();
};
void z() {
class B {
// friend void f() { };
};
}
class C {
// friend void A::g() { }
friend void h() { }
};
The compiler would not allow the function definition of f() or g(). The compiler will allow the definition of h().
You cannot declare a friend with a storage class specifier.
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