Cannot find protocol declaration, duplicate declaration, delegate

[Dr.Lightning]

I have some wierdness going on that I don't understand. Perhaps its some "import" feature I don't understand. Perhaps it's cuz it's February (sarcasm).

Derived from the "first iPhone application" hello program, I have the following pertinent code:

ControlPanel.h:

...

#import <UIKit/UIKit.h>
#import "MainViewController.h"
#import "Connection.h"

@protocol ControlDelegate

- (BOOL)sendSetPoint: (float)setPoint Group: (int)groupNumber;

@end

...

mainViewController.h:

...

#import <UIKit/UIKit.h>
#import "ControlPanel.h"

#ifdef SeeForumText
@protocol ControlDelegate

- (BOOL)sendSetPoint: (float)setPoint Group: (int)groupNumber;

@end
#endif

@interface MainViewController : UIViewController <UITextFieldDelegate, DisplayDelegate, UIScrollViewDelegate> {

// Declare Display Delegate
id<ControlDelegate> controlDelegate;

...

Note that I've identified some lines above with #ifdef SeeForumText.

When those lines are ABSENT, as I believe SHOULD be the case, I get an error message on the id...controlDelegate line that reads "Cannot find protocol declaration for 'ControlDelegate'"

When those lines are PRESENT, I get a warning message within them that reads "Duplicate declaration for protocol 'ControlDelegate'. I make no other changes whatsoever.

It seems I'm d*mned if I do, and d*mned if I don't. This makes no sense to me. I've been moving forward with the warning, because it allows me to make progress on my development. But, at some point, I need to figure this out and get rid of the warning.

I see the circular import of .h files, but it's necessary because they both reference each other. I thought import took care of that. But am I getting this as a side effect?

Your help is greatly appreciated.

[ole]

You should remove the duplicate protocol declaration and yes, I'd say the circular import of the .h files is the problem. Instead of importing MainViewController.h in ControlPanel.h, just add a forward declaration of the class(es) you need to reference to the file:

// ControlPanel.h
#import <UIKit/UIKit.h>
#import "Connection.h"

@class MainViewController;

@protocol ControlDelegate
...

[Dr.Lightning]

Yep. It was the recursive imports.

I checked my reference example code, and it put each delegate declaration in a separate .h file, such as ControlDelegate.h. I suspect there was NO RESOLUTION to the recursion, without so separating the delegate declarations. I did this for my three delegates, and the problem went away.

The problem arose from having three logical layers in my app, and having each layer communicate with the other through a delegate. Each adjacent layer communicates with the next, and this led to the recursion. Therefore, each class had to include the definition of the other.

For classes, one can forward reference via "@class name;". However, I tried this for "@protocol name;" and it appears that the forward reference doesn't work. Therefore, I had to go the separate delegate declaration route.

Now, all layers that reference a delegate have a .h file that imports the delegate declaration .h file. The import's "don't double include" feature now does what it's supposed to do, because in the end these delegate declaration .h files are indeed included multiple times. But not circularly.

From now on, I'll just make it a standard of putting the delegate declarations in a separate .h. Again, I believe this is necessary because "@protocol name;" fails to act as a forward reference in the same way that "@class name;" does.

If I'm mistaken about the above, it ought to be a matter of syntax, because the exact text "@protocol name;" doesn't help. If I'm mistaken and somebody points out the correct, I *might* try. But then, the separate file solution is a fine one as well.

[ole]

Originally Posted by Dr.Lightning

For classes, one can forward reference via "@class name;". However, I tried this for "@protocol name;" and it appears that the forward reference doesn't work.

This is not true. Forward declaration of protocols work just like forward declarations of classes.

[Dr.Lightning]

Originally Posted by ole

This is not true. Forward declaration of protocols work just like forward declarations of classes.

Acknowledged. Dunno why it didn't work for me. Either way, I'll stick with putting the delegate decl in separate .h files. Thanks for your help.

[cynistersix]

Originally Posted by Dr.Lightning

Acknowledged. Dunno why it didn't work for me. Either way, I'll stick with putting the delegate decl in separate .h files. Thanks for your help.

Ran into something similar had to do with the protocol being in a header file and #include'ing it instead of #import'ing it. Watch out for this zinger.