Creating an abstract class in Objective C

Question

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?

Answer 1

from http://www.omnigroup.com/mailman/archive/macosx-dev/2004-August/053887.html

Objective-C doesn't have the abstract compiler construct like Java at this time.

So all you do is define the abstract class as any other normal class and implement methods stubs for the abstract methods that either are empty or report non-support for selector. For example...

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

I also do the following to prevent the initialization of the abstract class via the default initializer.

- (id)init
{
     [self doesNotRecognizeSelector:_cmd];
     [self release];
     return nil;
}

Answer 2

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.

Answer 3

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

Answer 4

Instead of trying to create an abstract base class, consider using a protocol (similar to a Java interface). This allows you to define a set of methods, and then accept all objects that conform to the protocol and implement the methods. For example, I can define an Operation protocol, and then have a function like this:

- (void)performOperation:(id<Operation>)op
{
   // do something with operation
}

Where op can be any object implementing the Operation protocol.

If you need your abstract base class to do more than simply define methods, you can create a regular Objective-C class and prevent it from being instantiated. Just override the - (id)init function and make it return nil or assert(false). It's not a very clean solution, but since Objective-C is fully dynamic, there's really no direct equivalent to an abstract base class.

Answer 5

so have a look at the CocoaDev site which gives it a java comparison

http://www.cocoadev.com/index.pl?AbstractSuperClass