Creating an abstract class in Objective-C

I'm originally a Java programmer who now works with Objective-C. I'd like to create an abstract class, but that doesn't appear to be possible in Objective-C. Is this possible?

If not, how close to an abstract class can I get in Objective-C?

Typically, Objective-C class are abstract by convention only—if the author documents a class as abstract, just don't use it without subclassing it. There is no compile-time enforcement that prevents instantiation of an abstract class, however. In fact, there is nothing to stop a user from providing implementations of abstract methods via a category (i.e. at runtime). You can force a user to at least override certain methods by raising an exception in those methods implementation in your abstract class:

[NSException raise:NSInternalInconsistencyException 
            format:@"You must override %@ in a subclass", NSStringFromSelector(_cmd)];

If your method returns a value, it's a bit easier to use

@throw [NSException exceptionWithName:NSInternalInconsistencyException
                               reason:[NSString stringWithFormat:@"You must override %@ in a subclass", NSStringFromSelector(_cmd)]
                             userInfo:nil];

as then you don't need to add a return statement from the method.

If the abstract class is really an interface (i.e. has no concrete method implementations), using an Objective-C protocol is the more appropriate option.

No, there is no way to create an abstract class in Objective-C.

You can mock an abstract class - by making the methods/ selectors call doesNotRecognizeSelector: and therefore raise an exception making the class unusable.

For example:

- (id)someMethod:(SomeObject*)blah
{
     [self doesNotRecognizeSelector:_cmd];
     return nil;
}

You can also do this for init.

Just riffing on @Barry Wark's answer above (and updating for iOS 4.3) and leaving this for my own reference:

#define mustOverride() @throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:@"%s must be overridden in a subclass/category", __PRETTY_FUNCTION__] userInfo:nil]
#define methodNotImplemented() mustOverride()

then in your methods you can use this

- (void) someMethod {
     mustOverride(); // or methodNotImplemented(), same thing
}

Notes: Not sure if making a macro look like a C function is a good idea or not, but I'll keep it until schooled to the contrary. I think it's more correct to use NSInvalidArgumentException (rather than NSInternalInconsistencyException) since that's what the runtime system throws in response to doesNotRecognizeSelector being called (see NSObject docs).

The solution I came up with is:

1. Create a protocol for everything you want in your "abstract" class
2. Create a base class (or maybe call it abstract) that implements the protocol. For all the methods you want "abstract" implement them in the .m file, but not the .h file.
3. Have your child class inherit from the base class AND implement the protocol.

This way the compiler will give you a warning for any method in the protocol that isn't implemented by your child class.

It's not as succinct as in Java, but you do get the desired compiler warning.