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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 when I ran ps ax that PID could not be found. What does that integer represent and why do we have to use @selector all the time instead of just passing the method name?

node ninja
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    `@selector` literals do not evaluate to integers. Printing anything as an integer will never fail, although if you try the wrong size of integer, it can print the wrong result (part of the actual value lopped off). More importantly, printing things that aren't integers as integers will “work” (it will print a number) but achieve nothing (the number will be useless to you). As Dave DeLong told you, a `@selector` literal evaluates to a `SEL`, which (in the current implementation) is actually a pointer. – Peter Hosey Aug 14 '10 at 07:55
  • No an selector is an integer. You see this when you disassemble the code. But from the Objective-C point of view it is not an integer. Well maybe saying that it is a scalar value might be more precise. – Lothar Aug 14 '10 at 13:15

2 Answers2

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@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.

Community
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Dave DeLong
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    `SEL` is not actually a C string anymore; it points to an opaque type defined in libobjc. The first member is a C string for compatibility purposes, but you're not supposed to rely on that; proper code uses `sel_getName` to actually access the C-string member, or `NSStringFromSelector` to create an NSString from it. – Peter Hosey Aug 14 '10 at 07:51
  • In fact, I would not be at all surprised if NSStringFromSelector(_cmd) starts crashing at some point. It should definitely be a compiler warning. – Catfish_Man Mar 05 '13 at 19:30
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    @Catfish_Man Why would NSStringFromSelector() crash if you are passing in a valid selector? It's a published API and there is no reason for it to go away – Jeff Laing Mar 06 '13 at 01:43
  • Sigh, I am really not doing well on reading comprehension today. My brain somehow parsed that as using _cmd as a C string. Please ignore me! – Catfish_Man Mar 06 '13 at 02:34
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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.

Lothar
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  • Um, the “integer operation” in question is simply a comparison to a known value, which is the same for integers and pointers. At the assembly level, they’re the same thing. In actuality, SELs are still C string pointers internally in all Apple runtimes. (They’re a struct in the gcc and GNUstep runtimes.) However, they are declared as `typedef struct objc_selector *SEL` to create an opaque type, and the implementation may change the meaning at any time. – Jens Ayton Aug 14 '10 at 15:18
  • In this case a selector should point to a valid memory location. Do they? And what GNUstep does isn't relevant it so out of the league. – Lothar Aug 14 '10 at 16:10
  • I can't find the article anymore but if they are simple pointers how can a single comparison be a fast path branch for 3 different selectors? There need to be more behind this in MacOSX 10.6 – Lothar Aug 14 '10 at 16:14
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    Lothar: If they didn't point to valid memory, you couldn't print them as C strings. As for how message dispatch works, see bbum's tour of objc_msgSend: http://www.friday.com/bbum/2009/12/18/objc_msgsend-part-1-the-road-map/ – Peter Hosey Aug 15 '10 at 01:06