Question

Common examples for a ObjC object are like this (for the header file):

@interface A: B {
    int x;
    int y;
}
@end

Is it possible to avoid the inheritance specification (i.e. B here) in the header file?

---

In my case, the framework A defines that class A and another (sub-)framework B defines the class B (which is a subclass of NSView). A links to B. In my application, I link to A and I don’t need to know anything about B except that it is a subclass of NSView. And I want to avoid to link to B. But if B is in the header file, I think I cannot avoid it, that’s why I was asking about how to avoid it.

Answer 1

no.

you must often work around this with a class cluster, hold a private implementation, or create an object factory. then you can minimize the dependencies across modules.

you’ll still ultimately need to link to the sub library at some stage if you intend to use it (e.g. create an instance of).

Update – Demonstrate Private Implementations

Private implementations can be entirely opaque. If you do expose them, here are two ways to implement private implementations which are visible to clients:

via protocol:

// MONDrawProtocol.h
// zero linkage required
// needs to be visible to adopt
// may be forwarded
@protocol MONDrawProtocol
- (void)drawView:(NSView *)view inRect:(NSRect)rect;
@end

// MONView.h
@protocol MONDrawProtocol;

@interface MONView : NSView
{
    NSObject<MONDrawProtocol>* drawer;
}

@end

// MONView.m

#include "MONDrawProtocol.h"

@implementation MONView

- (void)drawRect:(NSRect)rect
{
    [self.drawer drawView:self inRect:rect];
}

@end

via base:

// MONDrawer.h
// base needs to be visible to subclass and types which use MONDrawer
// may be forwarded
@interface MONDrawer : NSObject
- (void)drawView:(NSView *)view inRect:(NSRect)rect;
@end

// MONView.h
@class MONDrawer;
@interface MONView : NSView
{
    MONDrawer * drawer;
}

@end

// MONView.m

#include "MONDrawer.h"

@implementation MONView

- (void)drawRect:(NSRect)rect
{
    [self.drawer drawView:self inRect:rect];
}

@end