<Cannot find protocol declaration>

47down vote

Let me reduce the sample even further, and label the lines:

VC1.h

#import "VC2.h"  // A@class VC1;@protocol VC1Delegate // B@end@interface VC1 : UIViewController <VC2Delegate> // C@end

VC2.h

#import "VC1.h"  // D@class VC2;@protocol VC2Delegate // E@end@interface VC2 : UIViewController <VC1Delegate> // F@end

Consider what happens when something #imports VC1.h: It reaches line A, then the import is processed. Line D does nothing because VC1.h was already imported. Then line E is processed. Then line F, and we get an error because we haven't gotten to line B yet so the protocol is not declared!

Consider then what happens when something #imports VC2.h: It reaches line D, then the import is processed. Line A does nothing because VC2.h was already imported. Then line B is processed. Then line C, and we get an error because we haven't gotten to line E yet so the protocol is not declared!

The first step is to reconsider whether both of these classes really need to be each other's delegates. If you can break the cycle, that would probably be the way to go. If not, you'll need to restructure your headers. The most straightforward way is probably to put the delegates into their own headers:

VC1Delegate.h

@class VC1;@protocol VC1Delegate // B@end

VC1.h

#import "VC1Delegate.h"#import "VC2Delegate.h"@interface VC1 : UIViewController <VC2Delegate> // C@end

VC2Delegate.h

@class VC2;@protocol VC2Delegate // E@end

VC2.h

#import "VC1Delegate.h"#import "VC2Delegate.h"@interface VC2 : UIViewController <VC1Delegate> // F@end

If you trace through the imports now, you'll see that the appropriate protocols will now always be declared before the @interface lines try to use them.

http://stackoverflow.com/questions/6447573/cannot-find-protocol-declaration