Warnings with objective C protocols

Question

I can't work out how to post code here without the formatting going all screwy, so here's an English description.
I have a formal protocol declaring "MyFunction". I have a class heirarchy, and an object which is an instance of something in that heirarchy. I check to see if the object conformsToProtocol, and if it does, I call MyFunction on it.
This works, but I get a compiler warning that the class might not respond to MyFunction. I know it will, because I check - how do I give the compiler enough information that it stops bugging me?
Thanks
Message was edited by: SquintAndSquirm

Answer 1

SquintAndSquirm wrote:
A lot of variations on the theme of { code } have all failed for me. Clearly forum posting is beyond my little programming brains capacity :}
Just reply to my earlier post where I have the code block and click the "quote" button. You will be able to see the tags I used. It is brace, "code", brace, followed by all your code, then another brace, "code", brace.
I am explicitly typing because I like having the compiler check my function calls for me, rather than using id and doing unchecked type casts everywhere to remove warnings.
I am using a protocol because Objective-C provides me with protocols and, if I'm paying the price for dynamic binding anyway, I may as well make use of it. I'm also using them because I'm learning Objective-C. Please don't tell me there's no way to make non-trivial use of protocols that doesn't result in compiler warnings :/
You shouldn't have to live with compiler warnings. If you are getting them, it is usually because you aren't doing something correctly. Those warnings are your friends. They will help you figure out the correct way to structure the code. What you need to do is have the desire to structure your code correctly instead of just "get rid of the warnings". I think you are on the right path in this respect.
As I mentioned earlier, using "id" is not necessarily a bad way to go. You can restrict it like "id<MyProtocol>" where the type is an "id" that must conform to "MyProtocol". You can do this if your interface is 100% restricted to MyProtocol. If you need to call other functions too, then you will have to use a base class.
The Base class does not implement the protocol because the base class, in this case, is apples NSViewController, but even if it wasn't, I am not prepared to shuffle all of my functionality up into a single base class just to make the compiler happy - there wouldn't be any point having a protocol atall if I was doing that - if I didn't want to use OO I'd still be programming in C.
It would sure be helpful to see some code. You can always use the "Preview" tab to figure out the code tags.
In any event, I still disagree with your architecture. An NSViewController is just an NSViewController. If that is your base class then you must restrict your interface to that which NSViewController provides. If you need more than that, there are several ways to do it. You could create an NSViewController category and add the functionality you want right into the class. You could use composition to have a class that contains an instance of NSViewController. You could derive a new class from NSViewController, make it conform to your protocol, and use that as your base class. I'm pretty sure I'm missing at least 4-5 other methods too.
I'm coming from a C++ background, where I can arrange for my code to either be correct (the function will be called) or fail to compile. From your answers it seems I can't quite get that far in objective-C, even with static typing, so dynamically checking the type and then casting to it is probably the best bet.
I think you just need to learn a bit more about Objective-C. I worked in C++ for 15 years, but I don't think I ever will in the future. I now do GUIs in Objective-C and CLI in Perl.
That leaves me with another question (simple syntax in this case), which is how to cast an object of type Base * into a protocol. I can't cast it to Derived * because there is more than one class derived from Base and I don't know which it is.
You can do Base<MyProtocol> but it doesn't really make sense. I think the fundamental problem is that you want a base class to be a base+ class - a base with a little extra. That just isn't good OO. The only way to make that work is casting. A base class provides its base class functionality - nothing more.