Question

So my Idea is simple – to have a class that can have its own public methosds and a nested class that would forvard only some of that public methods (pseudocode):

class API
{
public:
    go();
    stop();
    friend class B
    {
    public:
        void public_request()
        {
            request();
        }
        void public_go()
        {
            go()
        }
    };
private:
    void request(){}
};

Is it possible to implement such nested class in C++ and how?

Answer 1

Yes, but you have to provide an instance of the outer class when you create the inner one… the two types are [mostly] independant, so B is not associated with any specific instance of API.

More pseudo-code

struct API {
   void go();
   struct B {
       B(API& api) : api(api) { }
       void go() { api.go(); }
   };
};

API a;
B b(a);
b.go();

Alternatively, you can pass an instance of API to B‘s function [B::go(API&)]

struct API {
   void go();
   struct B {
       void go(API& api) { api.go(); }
   };
};

API a1, a2;
B b;
b.go(a1);
b.go(a2);

Further, API does not contain an instance of B unless you explicitely add an instance.

struct API {
  struct B { };
  B b;
};

Also, note that B does not need to be granted friendship by API. As an inner class, B can already access APIs private/protected members. The converse is not true, however… API can not access Bs private/protected members unless B grants it permission.