[cfe-dev] _Generic constant string as controlling expression

[Richard Smith]

That's http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1693.pdf page 19.


On Mon, Dec 9, 2013 at 12:29 PM, Richard Smith <richard at metafoo.co.uk>wrote:

> On Mon, Dec 9, 2013 at 12:26 PM, Zhihao Yuan <zy at miator.net> wrote:
>
>> On Mon, Dec 9, 2013 at 3:17 AM, Richard Smith <richard at metafoo.co.uk>
>> wrote:
>> >> Since this clause has no mention for _Generic, shouldn't the type of
>> >> "hello"
>> >> be converted from char[6] to char * when it's used as a controlling
>> >> expression ?
>> >> Or am I missing something ?
>> >
>> >
>> > Yes, I think it should -- this looks like a bug. Please file a bug
>> report!
>>
>> I don't think so.  It looks more like an overlook to me. CC'ed Clark (the
>> author of _Generic).
>>
>
> See also http://gcc.gnu.org/ml/gcc-patches/2013-07/msg00971.html and in
> particular:
>
>   "The controlling expression of a generic selection was very carefully
>   not added to the list of contexts in which lvalue conversion is not
>   done and type qualification is discarded", the minutes [from the C
>   committee meeting] say
>
> At a first glance, performing an array-to-pointer conversion make things
>> easier, but imagine something like this:
>>
>>   #define F(e) _Generic(e, char *: sizeof(e))
>>
>> So what, sizeof a pointer is the array length?  This leads very obscure
>> programs.  Please don't.  In unevalued context (stealing C++ term here),
>> no convert should be performed.
>>
>> Here is an solution (no matter this is a bug in which side):
>>
>>
>> http://stackoverflow.com/questions/18857056/c11-generic-how-to-deal-with-string-literals
>>
>> --
>> Zhihao Yuan, ID lichray
>> The best way to predict the future is to invent it.
>> ___________________________________________________
>> 4BSD -- http://4bsd.biz/
>>
>
>
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