Question

This is the code:

#include <map>
class Hidden {
private:
  friend class Visible;
  Hidden(); { /* nothing */ }
};
class Visible {
public:
  void f() {
    std::map<int, Hidden> m;
    m[1] = Hidden(); // compilation error, class Hidden is private
  }
};

The code doesn’t compile because the constructor of class Hidden is private for class std::map. Obviously, I don’t want to make class std::map a friend of Hidden. But what should I do here? Thanks in advance!

Answer 1

Add the map as a friend class:

#include <map>
class Hidden {
private:
  friend class Visible;
  friend class std::map<int, Hidden> ;
  Hidden() {}
};
class Visible {
public:
  void f() {
    std::map<int, Hidden> m;
    m[1] = Hidden(); // compilation error, class Hidden is private
  }
};

Of course, it means you have to declare all Hidden users inside Hidden, but that’s exactly the point of the “private class” pattern you’re using…