So I have a problem when using a singleton Game class and the Story class.

Game.hpp

#ifndef GAME_HPP
#define GAME_HPP

#include <memory>

class Phase; // Forward declaration

class Game
{
  private:
    static inline Game* instance_reference_{nullptr};
    std::unique_ptr<Phase> current_phase_;
    bool is_running_{true};

    explicit Game() {};
public:
    virtual ~Game();
    const static Game* getInstance();

and of course the getInstance() method is defined in a .cpp file.

Story.hpp

#ifndef PHASE_HPP
#define PHASE_HPP

class Game; // Forward declaration

class Phase
{
  protected:
    const Game* game_;
  public:
    explicit Phase() : game_{ Game::getInstance() } {};
    virtual ~Phase() = default;
    virtual void handlePhase() = 0;
};
#endif

And some other classes inherit from the Phase class. Also, currently there is no Phase.cpp file.

Right now I cannot compile because the Game::getInstance() is nowhere declared.
but when I swap to #include "Game.hpp"instead of the forward declaration, when I compile I get this error:

/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/bits/unique_ptr.h:97:16: error: invalid application of 'sizeof' to an incomplete type 'Phase'
   97 |         static_assert(sizeof(_Tp)>0,
      |                       ^~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/bits/unique_ptr.h:404:4: note: in instantiation of member function 'std::default_delete<Phase>::operator()' requested here
  404 |           get_deleter()(std::move(__ptr));
      |           ^
./Game.hpp:36:14: note: in instantiation of member function 'std::unique_ptr<Phase>::~unique_ptr' requested here
   36 |     explicit Game() {};
      |              ^
./Game.hpp:13:7: note: forward declaration of 'Phase'
   13 | class Phase; // Forward declaration
      |       ^

So all in all, when I implement it how it is usually done, I have problems calling Game::getInstance() in the Phase constructor, but when I try to resolve this, it still won’t compile.