Question

Can i put definition of friend function / class inside another class? I mean something like this:

class Foo
{
    friend void foo() {} // 1
    friend class Bar {}; // 2
};

gcc compiles friend function, but can’t compile friend class.

Answer 1

You can define a friend function in the friend declaration, and it has interesting behavior that cannot be obtained any other way (in the case of the enclosing type being a template).

You cannot define a friend class in the friend declaration, and there is no need for that. If you want to create a new type inline with full access, you can just create a nested type. Being a member it will have full access to the enclosing type. The only difference is that the type will not be found at namespace level, but you can add a typedef if needed (or alternatively, define the class at namespace level and just declare the friendship inside the class).

class Outer {
   int x;
   class Inner {
      static void f( Outer& o ) { o.x = 5; } // fine
   };
};