为什么Clang抱怨我所有的自动释放物品？

I have the following function:

我有以下功能:

- (NSString *)urlEncodedValue {
NSString *result = (NSString *)CFURLCreateStringByAddingPercentEscapes(
                   kCFAllocatorDefault, 
                   (CFStringRef)self, 
                   NULL, 
                   CFSTR("?=&+/\r\n"), 
                   kCFStringEncodingUTF8
);

return [result autorelease];

}
- (N