Question

I am fairly new to Objective-C. Currently porting my own library from C#/Java to objective C.

I now run into a very strange problem for me.

I have a NSArray with several Note objects. I want to transpose on of these notes:

 //Note.h
 - (Note *) transpose: (int) semitones; 

 //Main
 NSArray *notes = [get it from somewhere];
 Note *transposedNote = [[notes objectAtIndex:0]transpose:1]; //Doesn't compile
 Note *transposedNote = [(Note*)[notes objectAtIndex:0]transpose:1]//Does compile

Is this happening because there is already a transpose method available in the general libraries?

I thought due to the dynamic nature of objective-C at runtime it would be checked which class objectAtIndex returns and then sends the message to it?

Answer 1

Yes, the error is because there’s already a transpose: method in AppKit. And you’re also right that it normally doesn’t cause an error when you have two unrelated classes implementing methods with the same name. The reason you get an error is because the two methods either return incompatible types or take incompatible types as arguments. In your particular case, you’re seeing both problems:

• -[NSResponder transpose:] takes an id and returns void
• -[Note transpose:] takes an int and returns an id

These are totally incompatible types, and the compiler does need to know the types involved even if it doesn’t know what exact method is going to be called.