Objective-C use of #import and inheritance

Question

I have a hypothetical UIViewController class named "foo". foo inherits from class bar and class bar #import's "Class A", a class which foo uses extensively. The problem is, when I'm using an instance of class A in foo, I don't get any compiler errors, but I do get a warning for instance, that an instance of Class A does not respond to a particular method. Do I have to explicitly #import ClassA.h into class 'foo'? even though class foo extends extends bar, which already imports it?

Hope that's not too confusing. Let me know if I need to clear anything up.

Answer 1

It sounds like you have a circular dependency issue. In order to resolve it, yes, each imlementation file (.m) needs to #import the proper header file. However, if you try to have the header files #import each other, you'll run into problems.

In order to use inheritance, you need to know the size of the superclass, which means you need to #import it. For other things, though, such as member variables which are pointers, or methods which take as a parameter or return the other type, you don't actually need the class definition, so you can use a forward reference to resolve the compiler errors.

// bar.h
@class A;  // forward declaration of class A -- do not to #import it here

@interface bar : UIViewController
{
    A *member;  // ok
}

- (A) method:(A)parameter;  // also ok
@end

// bar.m
#import "bar.h"
#import "A.h"

// can now use bar & A without any errors or warnings