What actually is a @selector?

Question

There are some functions which take as an argument @selector(methodName). I used NSLog to find out what @selector is, and it returns an integer. It looks like a PID, but whe

Answer 1

@selector() is a compiler directive to turn whatever's inside the parenthesis into a SEL. A SEL is a type to indicate a method name, but not the method implementation. (For that you'd need a different type, probably an IMP or a Method) Under-the-hood, a SEL is implemented as a char*, although relying on that behavior is not a good idea. If you want to inspect what SEL you have, the best way to do it is to turn it into an NSString* like this:

NSLog(@"the current method is: %@", NSStringFromSelector(_cmd));

(Assuming you know that _cmd is one of the hidden parameters of every method call, and is the SEL that corresponds to the current method)

The Objective-C Programming Language Guide has much more information on the subject.

Answer 2

I think looking at the Objective-C implementation might be good for an understanding:

A selector is an integer value. But its type is different from normal C integer values so you can't assign them.

The selector name like "methodName" is the string that uniquely represents a name for this integer.

Other languages and systems call this unique program wide string to integer mapping an atom (Windows) or quark (GTK).

Objective-C keeps all functions for a class inside a hashtable. The hashtable key is an integer. The Objective-C runtime library looks up the hashtable on every method invocation. Without the unique integer number it would be much slower to do this critical lookup.

A selector is not an opaque pointer to a structure anymore. With MacOSX 10.6 the obj_send runtime function which implements the Objective-C method invocation is using an arithmetic operation on the selector at the beginning to find out if it is a retain, release, autorelease message and do something in this special cases. For example simply return in case you are using the garbage collector.