[cfe-dev] Objc, cannot cast 'super' error

Dave Keck davekeck at gmail.com
Sun Jul 3 04:15:26 PDT 2011

Hey list,

Clang doesn't allow super to be casted; I'd like to present my
use-case demonstrating why I think this should be allowed, or get some
feedback on why Clang is right and my design is wrong:

I have two classes, ClassA and ClassB. ClassB inherits from ClassA,
and ClassB alone implements MyProtocol:

    @protocol MyProtocol
    - (void)handleToken: (id)token;

The contract for MyProtocol is that if you don't recognize 'token',
you send -handleToken: to super. If no superclass implements
-handleToken:, then a runtime exception is correctly generated since
no one could handle the token. (This is exactly the same as KVO's
-observeValueForKeyPath: contract.)

ClassB's implementation of -handleToken: triggers the "'ClassA' may
not respond to 'handleToken:'" warning on the call to super:

    - (void)handleToken: (id)token
        if (token == myToken)
            ... handle token ...
            [super handleToken: token];

Without the ability to cast super, I have the following options, none
of which are optimal:

    1. Add the following code to ClassB.m:

        @interface ClassA (Stub) <MyProtocol>

       This option is bad because it'll need changing if ClassB's
superclass changes, and will have to be separated from the code that
triggers the warning.

    2. Omit the [super handleToken: call], since (at present) no other
superclass actually implements -handleToken:.

        This option is bad because it'll break if a superclass starts
implementing -handleToken:.

    3. Make MyProtocol an informal protocol on NSObject instead.

        This option is bad because a formal protocol is the correct
abstraction for my use-case.

    4. Use objc_msgSendSuper directly.

        This option is just ugly and inconsistent with the rest of my code.

Why is my design wrong?



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