"Attempting to use the forward class 'Game' as superclass of 'MathGame'" in Cocos2d

Question

I'm making a Cocos2d game for iphone, and I have my main game mode, Game, which inherits from CCLayer.

I'm trying to make another game mode, MathGame, which inherits from Game, but when I try to compile, I get this error in MathGame.h:

Attempting to use the forward class 'Game' as superclass of 'MathGame'

I get the error even if the implementation and interface of MathGame are empty. And it only happens if I try to include MathGame.h in another file.

Here's the code for the Game class:

// Game.h
#import "cocos2d.h"
#import <GameKit/GameKit.h>
#import "SplashScreenLayer.h"

@interface Game : CCLayer
    // A bunch of stuff
@end

The new game type:

// MathGame.h
#import "Game.h"

@interface MathGame : Game
@end

And the main menu that includes both:

// SplashScreen.h
#import "cocos2d.h"
#import "Game.h"
#import "MathGame.h"
#import "HowToPlayLayer.h"
#import "AboutLayer.h"

@interface SplashScreenLayer : CCLayer
    // A bunch of stuff
@end

I can't find anything helpful online. Any ideas?

MathGame.h imports Game.h? (If that's not it, you may need to show some code.) — Phillip Mills, Aug 05 '12 at 20:27
Please show the relevant `@interface` sections. Usually this happens when you forget to #import headers and instead just use `@class` instead. — Chris Trahey, Aug 05 '12 at 20:27
I trust there's a `@end` somewhere after `@interface Game : CCLayer`? — Phillip Mills, Aug 05 '12 at 20:42
Do you do any old-school, C-style "#ifndef XXX #define XXX" in order to ensure the header is included only once? If so, this might cause that issue, and it's unnecessary with the #import keyword. — CodeSmile, Aug 05 '12 at 23:14

Mazyod · Accepted Answer · 2023-07-29T19:40:28.790

You simply have an import cycle:

Game imports SplashScreenLayer
SplashScreenLayer imports MathGame
MathGame imports Game

Your solution:

Leave the import inside the MathGame, and change the other imports to @class.

To sum it up:

// Game.h
#import "cocos2d.h"
#import <GameKit/GameKit.h>

@class SplashScreenLayer;
@interface Game : CCLayer
    // A bunch of stuff
@end

The new game type:

// MathGame.h
#import "Game.h"

@interface MathGame : Game
@end

And the main menu that includes both:

// SplashScreen.h
#import "cocos2d.h"
#import "HowToPlayLayer.h"
#import "AboutLayer.h"

@class Game;
@class MathGame;
@interface SplashScreenLayer : CCLayer
    // A bunch of stuff
@end

With your question answered above, let me explain a few things I already know from reading about forward declerations and import cycles:

First, go read about them! They are a very important part of Objective-C, and you don't want to miss it!

Secondly, use @class whenever you need that class for private variables or method parameters. Use imports for inheritance and strong properties.

Thirdly, don't forget to #import your forwarded classes in the implementation file!

HI. Regarding Thirdly, don't forget to #import your forwarded classes in the implementation file! - do you mean that in the .m files I have to import all the classes that I defined as @class ? why? — Dejell, Feb 21 '13 at 07:35
@Odelya Because using @ class only notifies your class that there is a class that exists with that name, but it doesn't expose the methods and properties it has — Mazyod, Feb 21 '13 at 08:29

score 1 · Answer 2 · answered Feb 26 '14 at 05:43

1

In my case,I user the xx class and use the @class but not #import the .h file.and the compile complain..

answered Feb 26 '14 at 05:43

frank

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"Attempting to use the forward class 'Game' as superclass of 'MathGame'" in Cocos2d

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