How do I declare a child class as a property of a parent class to be used in related child classes?

Question

I have a small class hierarchy where I would like to have a child class be a property of it's parent class and all related subclasses. Essentially,

• I have AbstractClass with a property of GroupClass.
• GroupClass is a child of AbstractClass.
• UsableObjectClass is a child of AbstractClass and uses GroupClass.

If I do the following...

#import <Cocoa/Cocoa.h>
#import "GroupingClass.h" // I've bounced this between @class as well.
@interface myAbstractClass : NSObject {
    GroupingClass* parentGroup;
}
@property (readwrite, assign) GroupingClass* parentGroup;
@end

#import "myAbstractClass.h" // ERROR LOCATION
@implementation myAbstractClass
@synthesize parentGroup;
@end

#import <Cocoa/Cocoa.h>
@interface GroupingClass : myAbstractClass {
}
@end

#import <Cocoa/Cocoa.h>  // ERROR LOCATION IN ALL CHILD CLASSES OF AbstractClass
@interface GroupingClass : myAbstractClass {
}
@end

...I get this funky "Line Location GroupingClass.h:3: error: cannot find interface declaration for 'myAbstractClass', superclass of 'GroupingClass'" at the noted error locations.

I've tried various ways of getting this to work (yes, I know the above is totally wrong), Categories and Protocols seems the be right direction but this is becoming only so much flailing around now, and I really just need a shove in the right direction. I think I'm missing something grossly fundamental (frankly, I'm a little embarrassed to be asking this question).

I'm currently porting this application from REALbasic and something like this was a breeze; I could just add the property as the child class and it just worked. Objective-C...not so much.

Any help, even just a hint, is greatly appreciated!

Answer 1

You have what is called a circular reference or dependency. Your base class is dependent upon its child class, which is in turn dependent upon the base class.

I suggest you heavily reconsider your class structure.

Answer 2

You should be able to use a @class declaration in your header and import the subclass's header in your implementation file.

Answer 3

What you're looking for is a "forward class declaration". At the top of the AbstractClass header, add

@class GroupingClass;

This tells the (unfortunately slightly archaic) C compiler that there will be a class called GroupingClass and that it can allocate space for variables of that type appropriately.