unknown type name in objective c

Question

I\'m pretty new to objective c, and having some basic problems.

I wrote a simple program using a navigator, and everything worked fine. then I added few lines of cod

Answer 1

@JustAStranger and @NathanielSymer, both are correct!

Anyway, worth remember that this case, below, has the same problem too:

Class_A.h: #import "Class_B.h"

Class_B.h: #import "Class_C.h"

Class_C.h: #import "Class_A.h"

This problem reveal to us how important is take care about owners at our Class relationships. Is very easy creates cycle problems using ObjC headers.

Answer 2

This problem looks like a typo because class names usually start with an uppercase character. Therefore, mainController could/should be MainController. Check the class name to see if the error is indeed a typo, because the compiler is telling you it cannot find any class called mainController.

Answer 3

Check the target and the files it is compiling. Perhaps mainController has some how been removed from that target. If so, when building, you would get the message that it cannot be found.

Answer 4

I figured out, that the same error appears if you have an import-cycle:

Class_A.h: #import "Class_B.h"

Class_B.h: #import "Class_A.h"

To fix: look for any imports of the offending class (the error tab is your friend, expand the relevant error for a list of imports). Remove #import's accordingly

Answer 5

As others have mentioned, this is indeed caused by cyclic imports. To fix this remove the imports in one of the classes. But sometimes this is not sufficient. If the classes depend on each other, simply forward-declare the one class in the other:

Class A:

#import <UIKit/UIKit.h>
@class B; //<- this is essential here

@interface A: NSObject

@property(nonatomic, strong) B *b;
//...

In class B we have:

#import "A.h"
@interface B: NSObject

@property(nonatomic, strong) A *a;

Answer 6

This problem happen to me once.

I was importing the "APPDelegate.h" in my h file and in my APPDelegate.h I was importing the file too (it shouldn't be a problem but...)

What I did: I changed the Import from my own .h to .m and it worked :)