Hi Fritz and Jens,
It makes total sense now. Out of the two options (NULL sentinel vs a number to
indicate the number of args), I would choose NULL because out of the two, it's
more foolproof. Is there a consensus about which option is considered best
practice?
Thanks again,
-- Tito
On Jul 4, 2012, at 4:43 PM, Fritz Anderson <fri...@manoverboard.org> wrote:
> On 4 Jul 2012, at 6:30 PM, Tito Ciuro wrote:
>
>> void blockStep(fooBlock firstBlock, ...)
>> {
>> va_list args;
>> va_start(args, firstBlock);
>> id result = nil;
>>
>> do {
>> result = firstBlock(result, nil);
>> NSLog(@"%@", result);
>> } while (nil != (firstBlock = va_arg(args, fooBlock)));
>>
>> va_end(args);
>> }
>>
>> The output looks like this:
>>
>>> 2012-07-04 16:18:40.000 BlockStep[12418:303] One!
>>> 2012-07-04 16:18:56.533 BlockStep[12418:303] One!, Two!
>>
>> I've eliminated the crash by adding a nil sentinel in blockStep():
>>
>> blockStep (
>> ^ (id result, NSError **error) {
>> NSMutableString *value = [NSMutableString new];
>> [value appendString:@"One!"];
>> return value;
>> },
>> ^ (id result, NSError **error) {
>> NSMutableString *value = [NSMutableString new];
>> if (nil != result) {
>> [value appendString:result];
>> [value appendString:@", "];
>> }
>> [value appendString:@"Two!"];
>> return value;
>> },
>> nil
>> );
>
>>
>> This allows it to work without crashing, but I'd like if possible to avoid
>> having to place the sentinel. Any ideas?
>
> Not possible.
>
> Notionally, all parameters are passed to C functions in memory on a stack,
> which is unformatted and could contain anything. A function has no way of
> knowing how many parameters have been pushed onto the stack, or where the
> memory trails off into saved processor state and the like, or the types, or
> the amounts of memory they subtend….
>
> Most variadic functions require sentinels (usually NULL) to tell them to stop
> looking for parameters. The best-known exceptions are printf()-family
> functions, which know what to find on the stack because the format string
> tells them.
>
> — F
>
> --
> Fritz Anderson
> Xcode 4 Unleashed: Don't bring your bathroom copy into the kitchen — were you
> raised in a barn?
> <http://x4u.manoverboard.org/>
>
>
>
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