Declaring two classes that contain instance variables of each others' types

Question

Let’s say you wish to have two classes like so, each declaring an instance variable that is of the other’s class.

@interface A : NSObject {
    B   *something;
}
@end

@interface B : NSObject {
    A   *something;
}
@end

It seems to be impossible to declare these classes with these instance variables. In order for A to include an IV of class B, B must already be compiled, and so its @interface must come before A’s. But A’s must be put before B’s for the same reason.

Putting the two class declarations in separate files, and #import-ing each other’s ‘.h’s doesn’t work either, for obvious reasons.

So, what is the solution? Or is this either (1) impossible or (2) indicative of a bad design anyway?

See also: http://stackoverflow.com/questions/322597/objective-c-class-vs-import — Felix Kling, Jun 16 '10 at 19:51
Thanks Felix. If I had only known to search for "forward class declaration!" — Benji XVI, Jun 16 '10 at 20:24

score 4 · Accepted Answer · answered Jun 16 '10 at 19:44

To do that you need to use forward class declaration:

//a.h
@class B;

@interface A : NSObject {
    B   *something;
}
@end

// b.h

@class A

@interface B : NSObject {
    A   *something;
}
@end

So compiler will know that A and B are classes. And in implementation files for both classes just include a.h and b.h headers.

score 3 · Answer 2 · answered Jun 16 '10 at 19:43

This is what the @class keyword is for. It tells the compiler that something is used as a class without the compiler necessarily knowing anything about it.

@class B;    
@interface A : NSObject {
    B   *something;
}
@end

@class A;
@interface B : NSObject {
    A   *something;
}
@end

Declaring two classes that contain instance variables of each others' types

2 Answers2