undefined-key.m

// gcc -o undefined-key undefined-key.m -framework Foundation
#import <Foundation/Foundation.h>

@interface Foo : NSObject { }
@end


@implementation Foo

#if 1
- (id)valueForUndefinedKey:(NSString *)key
{
    NSLog(@"%@: undefined key: %@", [self className], key);
    fprintf(stderr, "%s: undefined key: %s\n", [[self className] UTF8String], [key UTF8String]);
    return nil;
}
#endif

@end

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

    Foo * foo = [[[Foo alloc] init] autorelease];
    id value = [foo valueForKey:@"bar"];
    NSLog(@"value: %@", value);

    value = [foo mutableSetValueForKey:@"bar"];
    NSLog(@"mutable value: %@", value);

    [pool drain];
    return 0;
}