- (id)retrieveObjectForClass:(Class)cls withSignature:(NSMethodSignature *)sig withKey:(id)key {
    if (_store == nil) {
        _store = [NSMutableDictionary dictionary];
    }

    id cached_copy = [_store objectForKey:key];
    if (cached_copy != nil)
        return cached_copy;

    NSInvocation *inv = [NSInvocation invocationWithMethodSignature:sig];
    [inv invoke];
    id new_copy;
    [inv getReturnValue:&new_copy];
    return new_copy;
}