Differences between messages and methods

methods

In Objective C you have the concept of sending messages to other objects, and, well this is very similar to method calling in languages like C# and Java.

But what exactly are the subtle differences? How should I think of messaging when thinking about my code?

Note: Just a bit of background here, I'm a C#/Java developer trying to understand some concepts about Objective C.

Best Answer

A message is the name of a selector, and the parameters for that selector.

A selector is a symbol.

A method is a piece of code in a class identified by a selector.

In other words, [foo bar: baz] says "send the message called @selector(bar:) with parameter baz to object foo. You could send that message to many different objects.

In contrast, the method bar: for a Foo might look like

-(int)bar:(int)n {
  return n + 1;
}

but for a FooTwo might look like

-(int)bar:(int)n {
  return n + 2;
}

(I hope I have the syntax right; it's been a while since I last touched Objective-C.)

When you send the message, the Objective-C kernel dispatches the message to foo which decides whether it understands the message. It decides this based on whether it can find a method identified by that selector.

Two methods with the same name, and one message.

It's also possible for an object to simply forward a particular message (or set of messages) to another object for processing. In this case, you send a message to this proxy object, which has no methods to match that message, and the proxy forwards the message to its wrapped object.

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