How to avoid circular dependency in this project?

Question

My project at the moment is set up something like this

game.cpp

#include "game.h"
#include "enemy.h"

class Game {
    public:
    
    ...
    ...
    std::vector<Enemy> enemies;
    ...
};

enemy.cpp

#include "game.h"

class Enemy {
    public:

    ...
    ...
    void turn(Game* game);
    ...
};

I obviously understand that in this case, this creates a circular dependency. The Game object needs a reference to Enemy to keep a list of enemies and the Enemy has a function for doing its turn that needs a reference to the Game object. I'm not really sure how to break up this circular dependency. I tried to make a header file with forward class declarations but I was getting hit with "invalid use of incomplete type" errors.

[https://stackoverflow.com/questions/625799/resolve-build-errors-due-to-circular-dependency-amongst-classes](https://stackoverflow.com/questions/625799/resolve-build-errors-due-to-circular-dependency-amongst-classes) — drescherjm, Feb 15 '21 at 22:47

score 1 · Accepted Answer · answered Feb 15 '21 at 22:43

If those are the only dependent functions, you could declare and initialize in the following order:

class Game;

class Enemy {
public:
    void turn(Game* game);
};

class Game {
public:
    std::vector<Enemy> enemies;
};

Live example: https://ideone.com/g6Vmwi

How to avoid circular dependency in this project?

1 Answers1