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Referring to Other Protocols

When working on complex applications, you occasionally find yourself writing code that looks like this:

#import "B.h"

@protocol A

- foo:(id <B>)anObject;

@end

where protocol B is declared like this:

#import "A.h"

@protocol B

- bar:(id <A>)anObject;

@end

In such a situation, circularity results and neither file will compile correctly. To break this recursive cycle, you must use the @protocol directive to make a forward reference to the needed protocol instead of importing the interface file where the protocol is defined. The following code excerpt illustrates how you would do this:

@protocol B;

@protocol A

- foo:(id <B>)anObject;

@end

Note that using the @protocol directive in this manner simply informs the compiler that “B” is a protocol to be defined later. It doesn’t import the interface file where protocol B is defined.

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