Handling circular dependencies in C++

Question

So, I know that there are many questions on stack overflow out there that are trying to handle circular dependencies. But none of those could really answer my question, if it is possible to have two classes know each other, and, more importantly, access information from each other. So, basically, I have read that you could use forward decleration, but with forward decleration, I couldn't access any fields.

Maybe I should also add that I am really new to C++.

But enough talking, let's get to an example:

Let's say we have a class called Scene and a class called EntityBase, which are defined as following:

EntityBase.h

#pragma once
#include <string>
#include "Scene.h"

class EntityBase
{
public:
    std::string entityId;
    Scene scene;


    EntityBase(std::string entityId);

    /* 
    This method will be called when this object is added to the scene 
    */
    void onAddedToScene(Scene* scene);

    /*
    This method will be called when this object is removed from the scene
    */
    void onRemovedFromScene(Scene* scene);
};

Scene.h

#pragma once
#include <vector>
#include <string>
#include "EntityBase.h"

class Scene
{
public:
    std::vector<EntityBase> entities;
    std::string name;

    Scene(std::string name);

    void addToScene(EntityBase& entityBase);
};

The question arises,

How can I print out the name of Scene(and use every method of Scene) while EntityBase can fully access Scene too? So, I'd really appreciate if you'd tell how to do this, because I will probably need to access every field and/or method later.

Answer 1

Since the C++17 standard you don't need the full EntityBase class definition for the Scene class, only a forward declaration:

#pragma once
#include <vector>
#include <string>

class EntityBase;  // Forward declaration

class Scene
{
public:
    std::vector<EntityBase> entities;
    std::string name;

    Scene(std::string name);

    void addToScene(EntityBase& entityBase);
};

You of course need to include EntityBase.h anywhere where the entities vector is used, most notable in the Scene.cpp source file.

---

Since EntityBase is using an actual instance of Scene you can't do the same with that class and the EntityBase.h header file. Here you must include the Scene.h header file.

---

If you build targeting an older C++ standard (C++14 or earlier) then you must have the full definition of the EntityBase class for the vector.

As a possible workaround either make entities a vector of pointers to EntityBase; Or make EntityBase::scene a pointer and use forward declaration in the EntityBase.h header file instead. Or a combination of both.