So I have a class Gamestate:

class GameState
{
public:
	virtual void HandleEvents() = 0;
	virtual void logic() = 0;
	virtual void render() = 0;
	virtual ~GameState(){};

	void SetNextState(GameStates newState);	// State status manager
	void ChangeState();		// State changer

private:
};

With the functions:

void GameState::ChangeState()
{
	if (nextState != STATE_NULL)
	{
		if (nextState != STATE_EXIT)
			delete currentState;

		switch (nextState)
		{
		case STATE_INTRO: break;
		case STATE_MENU: currentState = new MainMenu(); break;
		case STATE_GAME: currentState = new Game(32, 32); break;
		}

		nextState = STATE_NULL;
	}
}

void GameState::SetNextState(GameStates newState)
{
	nextState = newState;
}

Things to note here are MainMenu, and Game - each in their own header file. This is a problem because MainMenu and Game inherit from GameState:

class Game : public GameState
{
   //Game stuff...
};

class MainMenu : public GameState
{
   //MainMenu stuff...
};

Gamestate needs to be able to create a new instance of MainMenu, Game, or watever other states I add, while these objects need to inherit from GameState.

See, these header files need to reference each other. Of course, this causes an infinite loop, so how do i fix this?

Help is greatly appreciated!

to Ashaman, thanks a heap! that fixed my problems, and I will use this technique for the rest of my life.

to EWClay, my main game loop looks like this:

while (stateID != STATE_EXIT)
	{
		// Events
		currentState->HandleEvents();

		currentState->ChangeState();

		// Logic
		currentState->logic();

		// Rendering
		currentState->render();

		drawCursor();

		SDL_GL_SwapBuffers();
	}

If they all derive from gamestate, I can just change the state, and the loop will continue normally. (A state machine I think they call this?)

to Nusakan, I have NO IDEA how to make a class like that. The idea of "Manager" classes are too abstract for me, but if you want to show me how you would write it, I would really appreciate it, and implement it for sure.

to Ashaman, thanks a heap! that fixed my problems, and I will use this technique for the rest of my life.

to EWClay, my main game loop looks like this:

while (stateID != STATE_EXIT)
	{
		// Events
		currentState->HandleEvents();

		currentState->ChangeState();

		// Logic
		currentState->logic();

		// Rendering
		currentState->render();

		drawCursor();

		SDL_GL_SwapBuffers();
	}

Sorry if I'm completely misunderstanding, but is currentState not a member of GameState? If not, why is ChangeState() a member of GameState? Game and MainMenu inherit ChangeState(), but they don't seem to need it. Something doesn't look right.

The link from Nusakan shows a good way to implement this.

3) The class contains (including a pointer) something from the standard library. (i.e std::string, std::string*)

Also... beware of smart pointers. The deleter function in it will need more than just a forward declare.