c++ call private function in container class

Question

If I have two classes:

class worker
{
   Top *mp_parent;

   worker(Top *parent) : mp_parent(parent) {};

   int doSomeWork()
   {
      int i = mp_parent->privateFunction(); // This is the function I want to call
   }
}

class Top
{
   private:
   worker m_Worker;

   int privateFunction() {return 1;}

}

Where Top class contains a instance of worker class. When worker is instantiated, a pointer to the parent class is passed in. Later, the function doSomeWork() is called which needs to get a value from the parent, so it calls mp_parent->privateFunction().

What is the best way to achieve this? - I don't really want to make privateFunction() a public function if I can avoid it, but it does not work as it is because it is private :o

Are there any other options?

Make Top a friend of worker. Alternatively, make worker a nested class in Top. — Neil Kirk, Nov 11 '14 at 15:07
@KarolyHorvath yes, sorry, this is only a minimal example I have not taken any care about other things then to describe the problem :) — code_fodder, Nov 11 '14 at 15:09
Why not just register a callback in the worker function instead of the actual parent's address? — doctorlove, Nov 11 '14 at 15:11
Actually I made a mistake. worker should be a friend of Top. — Neil Kirk, Nov 11 '14 at 15:13
@NeilKirk that's ok, I figured that :) ... also to fix my "example code" I would use a forward declare and put the implementation into c files and...etc... but its too long winded and serves little point for my question :( — code_fodder, Nov 11 '14 at 15:14

score 1 · Answer 1 · answered Nov 11 '14 at 15:08

Maybe you can use the 'friend' keyword:

class worker
{
   Top *mp_parent;

   worker(Top *parent) : mp_parent(parent) {};

   int doSomeWork()
   {
      int i = mp_parent->privateFunction(); // This is the function I want to call
   }
}

class Top
{
   friend class worker;
   private:
   worker m_Worker;

   int privateFunction() {return 1;}

}

score 0 · Answer 2 · answered Nov 11 '14 at 15:12

This is a classic use of friends:

class Top
{
   private:
   friend class Worker;
   worker m_Worker;

   int privateFunction() {return 1;}

}

You should be thoughtful about making friends, and consider whether you can redesign to avoid them, but sometimes it makes sense.

c++ call private function in container class

2 Answers2