C++ Patterns: The Badge(awesomekling.github.io)

    template <typename Registry>
    class DeviceManager;

    class Device {
        template<typename Registry> friend class DeviceManager;
        int y;
    };

    class VFS {
        friend struct DeviceManager<VFS>;
        int x;
    };

    template <typename Registry>
    struct DeviceManager {
        static void register_device (Registry& registry, Device& device) {
            registry.x = 1; // accessing privates
            device.y = 2;   // accessing privates
        }
    };

    int main() {
        VFS vfs;
        Device dev;
        DeviceManager<VFS>::register_device (vfs, dev);
    }

    //private:

    template<typename T>
    Badge<T> FakeBadge() {
      struct Stub {}
      return reinterpret_cast<Badge<T>>(Stub());
    }

  class Device;

  class VFS;

  class Device {
  public:  // we want this private!
     static void registrationHelper(VFS &v, Device &d);
  private:
     void registerWith(VFS &vfs) { registrationHelper(vfs, *this); }
  };

  class VFS {
  private:
     void registerDevice(const Device &);
     friend void Device::registrationHelper(VFS &, Device &);
  };

  void Device::registrationHelper(VFS &v, Device &d)
  {
     v.registerDevice(d);
  }

  class Device;
  class VFS {
  public:
   class {
   friend Device;
   void operator()(int y) {/* do stuff with y here*/}}     register_device;
  };
  class Device {
  public:
   void foo(VFS fs) {
    fs.register_device(1);
   }
  };
  
  int main(int argc, char *argv[]) {
  // fails:
  // VFS().register_device(2);
  // works:
   Device().foo(VFS());
   return 0;
  }