c++ - Forward declaration & circular dependency

Question

I've got two classes, Entity and Level. Both need to access methods of one another. Therefore, using #include, the issue of circular dependencies arises. Therefore to avoid this, I attempted to forward declare Level in Entity.h:

class Level { };

However, as Entity needs access to methods in Level, it cannot access such methods, since it does not know they exist. Is there a way to resolve this without re-declaring the majority of Level in Entity?

Answer 1

A proper forward declaration is simply:

class Level;

Note the lack of curly braces. This tells the compiler that there's a class named Level, but nothing about the contents of it. You can then use pointers (Level *) and references (Level &) to this undefined class freely.

Note that you cannot directly instantiate Level since the compiler needs to know the class's size to create variables.

class Level;

class Entity
{
    Level &level;  // legal
    Level level;   // illegal
};

To be able to use Level in Entity's methods, you should ideally define Level's methods in a separate .cpp file and only declare them in the header. Separating declarations from definitions is a C++ best practice.

// entity.h

class Level;

class Entity
{
    void changeLevel(Level &);
};


// entity.cpp

#include "level.h"
#include "entity.h"

void Entity::changeLevel(Level &level)
{
    level.loadEntity(*this);
}