What is the purpose of the -self method in NSObject-conformant classes?

Question

That's it. Why would anyone want (at least as a public API) a method such as that? Is there any practical use for it?

Answer 1

The self method is useful for Key-Value Coding (KVC).

With KVC, you can treat an object somewhat like a dictionary. You can access a property of the object using a string containing the name of the property, like this: [view valueForKey:@"superview"]. You walk down a chain of properties using a string containing a key path, like this: [view valueForKeyPath:@"superview.superview.center"].

Since NSObject has a self method, you can use self as the key or key path: [view valueForKey:@"self"]. So if you're constructing your key paths programmatically, or reading them from a file, using "self" as a key may allow you to avoid writing a special case.

You can also use self in predicates, like this:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"self beginswith \"foo\""];
NSArray *filteredArray = [arrayOfStrings filteredArrayWithPredicate:predicate];

I don't know whether NSPredicate actually uses the self method (perhaps via KVC) in this case. It's certainly possible.

Answer 2

I'm not sure why "self" was added originally, but one thing it did come in handy for was protecting interior pointers to objects. Apple's official recommendation was to insert a [foo self] call after you're done with the interior pointer; the method call does nothing functionally but ensures the compiler keeps foo around until then.

Answer 3

I think it's to do with the ObjC runtime.

objc_msgSend(autoreleasePool, sel_registerName("drain"));
BOOL AppDel_didFinishLaunching(struct AppDel *self, SEL _cmd, void *application, void *options)

The first argument is self. I think it has something to do with that. In all honesty though as it would end up as:

id self(struct id *self, SEL _cmd) {
    return self;
}

....It made more sense before I started writing this response.