Question

my xcode 4 just crashed and I get build errors:

Unknown type name
‘SecondViewController’

in the @interface code block:

SecondViewController *sms;

and

Property with ‘retain’ attribute must
be object type

in

@property(nonatomic,retain) SecondViewController *sms;

However I import SecondViewController.h. The same code worked before the crash.

FirstViewController.h: http://pastebin.com/jnPKBny7

SecondViewController.h: http://pastebin.com/2D058ZAK

Edit: I realized that this error occures because the classes import each other. Can anyone describe why this is wrong?

Any ideas?

Answer 1

You can’t have circular imports. I think it’s good practice to use forward class declarations with the @class directive whenever possible. For your case:

FirstViewController.h:

@class SecondViewController ;
@interface FirstViewController
{
    SecondViewController * _secondViewController ;
}

@property ( nonatomic, retain ) secondViewControlller ;

@end

SecondViewController.h:

@class FirstViewController ;
@interface SecondViewController
{
    FirstViewController * _firstViewController ;
}

@property ( nonatomic, retain ) firstViewControlller ;

@end

Then in your .m files, import the .h files for the classes you are using. The only reasons to import .h files into other .h files are:

• framework imports (i.e. )
• you need to use a protocol