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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:

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

ends up acting like

After posing, that’s just the same as

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


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.