Re: Re: Checking a method for parameter types via SEL

FROM : Michael Ash
DATE : Tue Aug 01 01:09:47 2006

On 7/31/06, Tommy Nordgren <tommy.<email_removed>> wrote:
>
>        This, to be extended with further methods:
> @implementation NSCoder (ConditionalCoding)
>
> -(void) encodeObject:(id) obj onCondition:(SEL) cond withOwner: (id)
> owner
> {
>        BOOL strongCode = NO;
>        NSMethodSignature * sign = [obj methodSignatureForSelector:cond];
>        if (sign == nil) {
>                strongCode = YES;
>        } else {
>                if ([sign numberOfArguments] == 3 &&
>                        strcmp([sign getArgumentTypeAtIndex:2],@encode(id)) == 0 &&
>                        strcmp([sign methodReturnType] , @encode(BOOL)) == 0) {
>                        int i = (int)  objc_msgSend(obj,cond,owner);
>                        strongCode = i ? YES: NO;
>                }
>        }
>        if ( strongCode) {
>                [self encodeObject: obj];
>        } else {
>                [self encodeConditionalObject:obj];
>        }
> }
> @end


Why bother with this type checking only to have it fail silently
anyway? Much better would be to simply add this to the header:

// cond must return a BOOL and take a single id argument

If you insist on a runtime check then you should throw an exception or
something similar if the method signature does not match. Simply
taking a wrong signature as NO is going to create annoying and
difficult-to-find errors.

I should also note that your call to objc_msgSend is faulty. BOOL is
one byte, but int is four and id is also four. By using a function
declared to return id and casting its return value to int, you risk
picking up junk in the return value and getting false positives.
Instead, call it like this:

((BOOL (*)(id, SEL, id))objc_msgSend)(obj, cond, owner)

Mike