including collision between two files c++ - c++

Question

I 've got a collision issue. I mean that in my A.h in need to include B.h but in B.h I need to include A.h so I can't figure out how to fixe it.
Interface.h
#ifndef _INTERFACE_H
#define _INTERFACE_H
#include <SDL.h>
#include <vector>
#include "Widget.h"
class Interface
{
public:
Interface(SDL_Rect &r);
~Interface();
private:
SDL_Rect m_rect;
std::vector<Widget*> m_widgets;
};
#endif
Widget.h
#ifndef _WIDGET_H
#define _WIDGET_H
#include <SDL.h>
#include "Interface.h"
class Widget
{
public:
Widget(Interface *main, SDL_Rect &r);
~Widget();
private:
SDL_Rect m_rect;
Interface* m_master;
};
#endif

Answer 1

Since you rely on pointers, you can declare (rather than define) the classes, and include the header files in the cpp files:
#ifndef _INTERFACE_H
#define _INTERFACE_H
#include <SDL.h>
#include <vector>
class Widget; //See the swap from include to declaration?
class Interface
{
public:
Interface(SDL_Rect &r);
~Interface();
private:
SDL_Rect m_rect;
std::vector<Widget*> m_widgets;
};
#endif
Do a similar swap in the other header.

Answer 2

That's not a "collosion" but a circular dependency.
For your case it's solved very easily by not including the header files at all, and only use forward declarations of the classes:
File Interface.h:
#ifndef INTERFACE_H
#define INTERFACE_H
#include <SDL.h>
#include <vector>
// No inclusion of Widget.h
// Forward declare the class instead
class Widget;
class Interface
{
public:
Interface(SDL_Rect &r);
~Interface();
private:
SDL_Rect m_rect;
std::vector<Widget*> m_widgets;
};
#endif
File Widget.h:
#ifndef WIDGET_H
#define WIDGET_H
#include <SDL.h>
// Don't include Interface.h
// Forward declare it instead
class Interface;
class Widget
{
public:
Widget(Interface *main, SDL_Rect &r);
~Widget();
private:
SDL_Rect m_rect;
Interface* m_master;
};
#endif
You of course needs to include the header files in your source files.
Also note that I changed the symbols for your include guards. Symbols with a leading underscore followed by an upper-case letter are reserved in all scopes by the "implementation" (the compiler and standard library). See this old question and its answers for details.

Answer 3

EDIT: Doctorlove was faster.
use forward declaration in one of the files:
#ifndef _INTERFACE_H
#define _INTERFACE_H
#include <SDL.h>
#include <vector>
#include "Widget.h"
class Widget;
class Interface
{.....
#endif