Clang 3.1: interesting warning...

gistfile1.m

@protocol MyProtocol <NSObject>

@end

int main(int argc, char *argv[])
{
    @autoreleasepool
    {
        id <MyProtocol> foo = nil;
        __unused id value = [foo valueForKey:@"bar"];
        
        return EXIT_SUCCESS;
    }
}

When I compile, I got this warning:

/Users/alex/Desktop/Test/Test/main.m:10:29: warning: instance method '-valueForKey:' not found (return type defaults to 'id')
        __unused id value = [foo valueForKey:@"bar"];
                                      ^~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

I don't understand why Clang generates a warning for the previous code? The variable foo is an instance of NSObject. The method valueForKey: is defined in an information protocol of NSObject (NSKeyValueCoding). So why Clang gives me this warning?

There is two ways to remove the warning:

• Cast foo into an id ([(id)foo valueForKey:@"bar"]).
• Remove the protocol from the definition (id foo = nil;). It's not an acceptable solution because my object does not conform the protocol anymore.

Does someone knows why this generate a warning?

PS1: The code itself does not have any sense. I wrote the shortest code that generates this warning :).
PS2: I used Xcode 4.3.3 + Clang 3.1.

It's because MyProtocol does not declare "valueForKey:"

In this line:

@protocol MyProtocol <NSObject>

You are extending the NSObject protocol, not the NSObject class. The NSObject protocol does not declare "valueForKey:"

One way to fix this is:

@protocol MyProtocol <NSObject>

- (id)valueForKey:(NSString *)key;

@end

The protocol that I have wrote does not declare valueForKey:, but this method should still be accessible through the category NSKeyValueCoding on NSObject. The following code does not generate warning:

id foo = nil;
id value = [foo valueForKey:@"bar"];

This one does:

id < MyProtocol > foo = nil;
id value = [foo valueForKey:@"bar"];

It is as if the compiler does not consider the type 'id < MyProtocol >' of type 'id'. Which is really odd. And so if you force the cast of foo into an id, the code compile without any warning.

id < MyProtocol > foo = nil;
id value = [(id)foo valueForKey:@"bar"];

I understood :) http://unixjunkie.blogspot.com/2008/03/id-vs-nsobject-vs-id.html