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


  • Subject: Re: Re: Checking a method for parameter types via SEL
  • From: "Michael Ash" <email@hidden>
  • Date: Mon, 31 Jul 2006 19:09:47 -0400

On 7/31/06, Tommy Nordgren <email@hidden> 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
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References: 
 >Checking a method for parameter types via SEL (From: Tommy Nordgren <email@hidden>)
 >Re: Checking a method for parameter types via SEL (From: Bill Bumgarner <email@hidden>)
 >Re: Checking a method for parameter types via SEL (From: Tommy Nordgren <email@hidden>)

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