Circular dependency problems - c++

Question

While trying to make a simple game, I've run in to a circular dependency problem.
I searched on the internet and found that forward declaring could fix it, but... Both of my classes depend on a static value.
Is there any easy way to fix, perhaps to forward declare the static values, or do I have to rewrite the core of my game?
2ND EDIT: Looks like I was wrong, the error's still there even after removing almost everything:
main.cpp:
#include "App.h"
//Start the app
int main(int argc, char* args[]){
App App;
return App.on_execute();
}
App.h:
#ifndef APP_H
#define APP_H
#include "Object.h"
class App
{
public:
//Runs when the program starts
int on_execute();
};
#endif // APP_H
App.cpp:
#include "App.h"
int App::on_execute(){
return 0;
}
Object.h:
#ifndef OBJECT_H
#define OBJECT_H
#include <string>
#include <vector>
#include <stdio.h>
#include <SDL.h>
#include <math.h>
#include "Entity.h"
class Object
{
public:
Object(int character, int x, int y, std::string name, SDL_Color color, bool blocks);
//Object vector
static std::vector<Object*> objects;
};
#endif // OBJECT_H
Object.cpp:
#include "Object.h"
std::vector<Object*> Object::objects;
Object::Object(int character, int x, int y, std::string name, SDL_Color color, bool blocks){
}
Entity.h:
#ifndef ENTITY_H
#define ENTITY_H
#include "Object.h"
#include <sdl.h>
class Entity : public Object
{
public:
Entity(int character, int x, int y, std::string name, SDL_Color color, bool blocks, int hp, int power, int defense);
};
#endif // ENTITY_H
Entity.cpp:
#include "Entity.h"
Entity::Entity(int character, int x, int y, std::string name, SDL_Color color, bool blocks, int hp, int power, int defense) : Object(character, x, y, name, color, blocks){
}

Answer 1

I think the design of your code may need to be reworked.
First of all, I'd really discourage you from using non-primitive, non-trivial class statics where possible, whether class static or global static. Non-trivial static classes will have an explicit constructor call before main, and will need to register a destruction to be called after main. The relative ordering of these things is undefined. Worse, if you have a library structure later such that the assembly from the same .cpp file shows up twice, weird things can happen where two copies of the object get constructed, but the same one is destroyed twice.
Second, some of the information is unclear. For example, you claim that the Map class has a static member of type Map. I really don't think this is possible; the static Map member will also have an object of type map, and so on. Similarly with the vector of Objects declared inside Object. Maybe in both these cases you mean inside the file Map.cpp or Object.cpp, as opposed to literally inside the class?
Third, be clear on what forward declaration gives you. It makes the compile aware that something exists, nothing more. It lets you have pointers and references to that type. You cannot create an object or even declare a member variable of a forward declared type, because the compiler doesn't know the size of the forward declared object. You can't use its methods because the compiler doesn't know they exist.
Fourth, you didn't talk about your header files at all. It's only a circular dependency if Map.h requires Object.h, and vice versa. Two header files can't both include each other. On the other hand, the implementation is in Map.cpp and Object.cpp, both of these files can include both Map.h and Object.h. However, personally I prefer to avoid having mutually dependent classes.
What I'd probably suggest is that a Map should own the Objects present on that map. The pattern right now with Map accessing this global is not a good one. Instead of having objects be a global, make std::vector objects a member of the Map class. Notice that if you decide to have multiple Maps later, this works much better, each Map will own the Objects located on that map. The current design doesn't work well if there's more than one Map.
You can then implement move_to as a method not of Object, but rather of Map. Map::move_to(i, dx, dy) moves the ith Object.