Cannot inherit from NSMutableData

[Apple92]

Hi,

I have created a new class, from NSMutableData.

@interface MsgSetGame : NSMutableData {

}

@end

There is no implementation, I just want to used methods from the mother class.

Curiously, I cannot send methods to an object of this class:

const char Buffer[5] = {1, 2, 3, 4, 5};

NSLog(@"Avant création MsgSetGame");
MsgSetGame *myGame = [MsgSetGame alloc];
[myGame initWithLength:40];
NSLog(@"Avant append");
[myGame appendBytes:Buffer length:5];
NSLog(@"Avant release");
[myGame release];

The app crashes with the following message on the console:

2010-01-10 18:51:49.169 BMEngine[21024:207] updateStateMachine: STATE_BIDDING_GAME_UNCOMPLETED
2010-01-10 18:51:49.170 BMEngine[21024:207] Avant création MsgSetGame
2010-01-10 18:51:49.170 BMEngine[21024:207] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -initWithCapacity: only defined for abstract class. Define -[MsgSetGame initWithCapacity:]!'

Curiously, NSMutableData is not abstract, thus I do not have to implement the inherited methods. Might this be due to the fact that NSMutableData is a custer class?

Regards,
Franz

[_sjc_]

Technically, both NSData and NSMutableData are abstract. The magic for both is handled by NSCFData, which is the private object bridged to (and indentical with) CFData objects.

That said, I'm not sure how you would go about subclassing it. For other cluster classes (eg. NSString), Apple provides subclassing guidelines listing the atomic methods to provide.

If you don't want to add new ivars to the class, and only want to call existing NSData methods, you could always look at adding a category rather than subclassing.