Is it possible to create warnings for methods that must be overridden?

Question

I was reading Creating an abstract class in Objective-C, and I wasn't terribly satisfied with the fact that the "abstractness" of the class is only enforced at runtime. I'd like to be able to show some warnings if the method is not implemented in a subclass. Ideally, this could be condensed down to one or two #defines.

Is there a clever use of __attribute((available,deprecated,etc))__ or a #warning with some careful #pragma clang diagnostic pushes that can accomplish this?

I think this is possible; I just don't know enough about Clang to figure it out.

EDIT:

There's no need to tell me I should be using protocols. I already use them for this purpose. I'm more curious to see if it can be done (to learn more about Clang) than whether it should be done.

I see this working the in a similar way to NSManagedObject requiring properties to be marked @synthesized or @dynamic. I've researched this, and I see that in NSManagedObject.h the class is marked NS_REQUIRES_PROPERTY_DEFINITIONS, which translates to __attribute__((objc_requires_property_definitions)) in NSObjCRuntime.h. Is there some creative use of these built-in #defines that could make this work?

EDIT #2:

For people saying that abstract superclasses are not the Objective-C way, I'll direct you to the documentation for UIMotionEffects:

Subclassing Notes

This class is abstract and cannot be instantiated directly.

Answer 1

I go for protocol

untested code, the idea is use protocol type (maybe combined with class type) instead of just class type.

@interface BaseClass : NSObject

- (void)foo;

@end

@protocol BaseClassProtocol : NSObject

@required
- (void)bar;

@end

@interface SubClass : BaseClass <BaseClassProtocol>

@end

@interface SubClass2 : BaseClass // no <BaseClassProtocol>

@end

// some code

BaseClass <BaseClassProtocol> * obj = [SubClass new]; // good
BaseClass <BaseClassProtocol> * obj = [SubClass2 new]; // warning

Answer 2

I prefer the NSException approach (which is a run-time, not a compile time, error).

In your superclass:

@interface NSSuperclass : NSObject

- (void)execute;

@end


@implementation NSSuperclass

- (void)execute
{
    @throw [NSException exceptionWithName:NSInternalInconsistencyException
                                   reason:[NSString stringWithFormat:@"Called %@ on superclass NSSuperclass, this should only be called on subclasses", NSStringFromSelector(_cmd)]
                                 userInfo:@{@"object": self}];
}

@end

And then override this method in your subclasses.