gistfile1.m

+ (id)objectForKeyedSubscript:(id)key
{
    unsigned int outCount;
    if ([self instancesRespondToSelector:@selector(initWithCoder:)] && [key isKindOfClass:[NSCoder class]])
    {
        return [[self alloc] initWithCoder:key];
    }
    //todo: cache these results in an associated object
    Method *theseMethods = class_copyMethodList(self, &outCount);
    NSMutableIndexSet *methodArrayHits = [[NSMutableIndexSet alloc] init];
    for (unsigned int i = 0; i < outCount; i++) {
        NSString *initName = NSStringFromSelector(method_getName(theseMethods[i]));
        if ([initName isEqualToString:@"initWithCoder:"])
        {
            continue;
        }
        if ([[NSString reasonableTypeEncodingForMethod:theseMethods[i]] isEqualToString:@"@@:@"] && [initName hasPrefix:@"initWith"])
        {
            [methodArrayHits addIndex:i];
        }
    }
    if ([methodArrayHits count] == 1)
    {
        SEL sele = method_getName(theseMethods[[methodArrayHits firstIndex]]);
        free(theseMethods);
        return objc_msgSend(objc_msgSend([self alloc], sele, key), NSSelectorFromString(@"autorelease"));
    }
    return @"Foo!";
}

How about:

+ (id)objectForKeyedSubscript:(id)key
{
    return [[self alloc] initWithKeyedSubscript:key];
}

- (id)initWithKeyedSubscript:(id)key
{
    unsigned int outCount;
    if ([self instancesRespondToSelector:@selector(initWithCoder:)] && [key isKindOfClass:[NSCoder class]])
    {
        return [self initWithCoder:key];
    }
    //todo: cache these results in an associated object
    Method *theseMethods = class_copyMethodList(self, &outCount);
    SEL initMethod = NULL;
    NSInteger count = 0;
    for (unsigned int i = 0; i < outCount; i++) {
        SEL method = method_getName(theseMethods[i]);
        NSString *initName = NSStringFromSelector(method);
        if ([initName isEqualToString:@"initWithCoder:"])
        {
            continue;
        }
        if ([[NSString reasonableTypeEncodingForMethod:theseMethods[i]] isEqualToString:@"@@:@"] && [initName hasPrefix:@"initWith"])
        {
            initMethod = method;
            count++;
        }
    }
    free(theseMethods);
    if (count == 1)
    {
        return objc_msgSend(objc_msgSend(self, initMethod, key); //you might still need to use objc_msgSend to call NSSelectorFromString(@"autorelease") on this value - depending on how ARC handles values returned from init when it doesn't know their retaincount
    }
    return @"Foo!";
}