Question

When you retrieve the ID of a selector with @selector(), is the selector value different depending on the types of the arguments?

Here’s what I mean. I have a method that takes an object reference and a selector, then calls it with a parameter:

-(void)CallLater:(NSObject*) Obj Sel: (SEL)Sel
{
    //Some stuff, then:
    [Obj performSelector: Sel withObject: SomeOtherObject];
}

I’m using this method with a selector to a function that takes a typed object reference, not an id:

-(void)MyMethod: (MyObject*) a
{
}

[self CallLater: self Sel:@selector(MyMethod:)];

It seems to work, but my senses are tingling. In a statically typed language like C# this would be a foul, an upcast – CallLater is expecting a selector for a function that takes an id and I’m giving it a function that takes a MyObject.

On the other hand, the compiler does not complain, and both id and concrete object references seems to be mere pointers deep down, trivially castable to one another. Then again, there are many fouls that Objective C compiler does not complain about.

So the real question is – is it safe? Language lawyers welcome.

Answer 1

It’s safe; objects are objects. A selector parameter for an NSObject * is exactly the same as a selector parameter for a MyObject *.

If you want MyMethod to verify that it’s being called with an object of a particular type, it should do a NSParameterAssert on it:

NSParameterAssert([obj isKindOfClass: [MyObject class]]);

Personally, I rarely do this check. It’s enough that the actual object acts like the type I want it to be, and if it doesn’t I’ll get a runtime error (usually unrecognized selector). You’ll get a compiler warning in the simple cases, and it’s worth paying attention to this warning (and silencing it with an id cast when necessary).

I’m a bit confused here about your use of id in your question, so I want to make sure you understand this: An NSObject * is exactly as much an id as a MyObject * is. id is a generic instance pointer class, whereas NSObject * is a NSObject instance (or a subclass of NSObject). You can have objects that don’t descend from NSObject. But you’re unlikely to ever have to know this.

Other notes, re: convention:

• Selectors (both the name and parameters) start with lowercase letters, so CallLater:Sel: should be callLater:sel:.
• Variable and parameter names start with lowercase letters; Obj above should be obj.
• Class names do start with an uppercase letter. 🙂