PosingWithCategoriesSuperGotcha

That’s a gotcha with calls to super, not a really special gotcha. To read up on class posing and categories see ExtendingClasses.

Summary: Messages to super have the same effect as messages to self in a category on a posing subclass.

I just ran into something that surprised me. Suppose you have a class MyClass and a subclass MyClassPoser that poses for it. Now suppose you have a category MyClassPoser (Private) containing the following method:

(id)valueForKey:(id)aKey { [super valueForKey:aKey]; }

What do you think would happen if you called the method after posing?

id object = [[MyClass alloc] init];

id dum = [object valueForKey:@”dummy”];

I’d expect that you’d get the method defined by -[MyClassPoser(Private) valueForKey:], which would immediately call through to the method originally named by -[MyClass valueForKey:], which would either look up the key dummy, or throw an exception or something.
This is what happens if we move MyClassPoser’s valueForKey: method from the category to the main implementation.

As it stands we’re going to get an infinite recursion and blow the stack. It turns out that

(id)valueForKey:(id)aKey { [super valueForKey:aKey]; }

ends up acting like

(id)valueForKey:(id)aKey { [MyClass class] instanceMethodForSelector:@selector(valueForKey:); }

After posing, that’s just the same as

(id)valueForKey:(id)aKey { [self valueForKey:aKey]; }

so we go into a tailspin. I guess this is a bug? Anyway, I’m submitting it to Apple.

-KenFerry

Hi Ken. Thanks for putting a note up. Here’s a test case you can include with your bug report. Open Xcode and create a “Foundation Tool” project. Copy and paste the code below into the file where main is defined. Build and Run.

#import <Foundation/Foundation.h>

@interface MyClass : NSObject {} -(void)foo; @end

@implementation MyClass -(void)foo { static unsigned int call = 1; if (call < 3) { NSLog(@”-[MyClass foo]\n”); call++; } else { assert(0); } } @end

@interface MyClassPoser : MyClass {} @end

@implementation MyClassPoser @end

@interface MyClassPoser (Category) -(void)foo; @end

@implementation MyClassPoser (Category) -(void)foo { static unsigned int call = 1; if (call < 3) { NSLog(@”-[MyClassPoser foo]\n”); call++; [super foo]; } else { assert(0); } } @end

int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

[MyClassPoser poseAsClass: [MyClass class]];
id object = [[MyClass alloc] init];
[object foo];

[object release];
[pool release];
return 0; }

Best, jd

Thanks for the code jd. I already included a test foundation project with my original submission, so it would probably be overkill to include more code (though your code is a little better than mine - mine doesn’t log and crashes instead of failing an assertion).

I think your test makes this page easier to understand though.

-KenFerry

CocoaDev