What are the digits in an ObjC method type encoding string?

Question

I'm reading Apple's article about Objective-C runtime type encoding strings and some methods have numbers in their type strings.

What do the numbers in v12@0:4@8 mean?

Answer 1

This looks like an encoding of a setter method like this:

- (void) setSomething:(id) anObject

To break it down:

• v means void return type
• 12 means the size of the argument frame (12 bytes)
• @0 means that there is an Objective-C object type at byte offset 0 of the argument frame (this is the implicit self object in each Objective-C method)
• :4 means that there is a selector at byte offset 4 (this is the implicit _cmd in every method, which is the selector that was used to invoke the method).
• @8 means that there is another Objective-C object type at byte offset 8.