Re: Selectors and categories in GNUstep (porting from Cocoa)

[Tom Hageman]

On Thu, 9 May 2002 00:25:20 +0200, Andreas Hoeschler wrote:
> if (sel == FBQualifierOperatorNotEqual)
> return [leftValue isNotEqualTo:rightValue];
> else if (sel == FBQualifierOperatorLessThan)
> return [leftValue isLessThan:rightValue];
> else if (sel == FBQualifierOperatorGreaterThan)
> return [leftValue isGreaterThan:rightValue];
> else if (sel == FBQualifierOperatorLessThanOrEqualTo)
> return [leftValue isLessThanOrEqualTo:rightValue];
> else if (sel == FBQualifierOperatorGreaterThanOrEqualTo)
> return [leftValue isGreaterThanOrEqualTo:rightValue];
> else if (sel == FBQualifierOperatorContains)
> return [leftValue doesContain:rightValue];
> else if (sel == FBQualifierOperatorLike)

Ouch, that's a very clumsy way to do things.  The very idea to pass down  
a selector instead of some random enum value is so you can perform it  
directly on a target object instead of having to hardwire a lot of  
decisions like above.

So conceptually, you'd like to do:

        return (BOOL)[leftValue performSelector:sel withValue:rightValue];

There's a slight complication, in that performSelector returns an (id),  
and the invoked selector returns a (BOOL).  It may not always be safe to  
do the (BOOL) -> (id) -> (BOOL) cast above (eg when doing DO), so a safer  
approach is:

        typedef BOOL (*BOOL_IMP)(id, SEL, ...);
        BOOL_IMP methodIMP;

        methodIMP = (BOOL_IMP)[leftValue methodForSelector:sel];
        NSParameterAssert(methodIMP != NULL);
        return methodIMP(leftValue, sel, rightValue);

(NB. compiled in mail only!)

Hope this helps,
Tom.

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