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James Kuyper <> wrote:
>
> I hadn't considered that issue, but taking a look at 6.10.3.3p3, I found
> that it says "each each instance of a ## preprocessing token
> in the replacement list (not from an argument) is deleted and the
> preceding preprocessing token is concatenated with the following
> preprocessing token. ... If the result is not a valid preprocessing
> token, the behavior is undefined." It seems to me ambiguous whether "the
> result" it refers to is the result of each concatenation, or the
> combined result of all of the concatenations.

I think it's pretty clear from context that it's talking about each
replacement individually. For a token-based preprocessor, you either
need each result to be a token or you need the concept of executing a
bunch of operators in parallel (which we don't have in C).
--
Larry Jones

Hey! What's the matter? Can't you take a joke?! It was a JOKE! -- Calvin

On Nov 7, 11:53*pm, lawrence.jo...@siemens.com wrote:
> James Kuyper <jameskuy...@verizon.net> wrote:
> > #include <stdlib.h>
> > #define TRICAT(a, b, c) a ## b ## c
> > #define CHAR(a, b, c) TRICAT(a, b, c)
>
> > #define CASE1 x
>
> ...
> > * * case CHAR('
> > * * * * , CASE1, '
> > * * * * ): break;
>
> Nice try, but no cigar. *The ## operator requires that the result be a
> valid pp-token and, depending on the order in which the operators are
> evaluated, you either end up with 'x or x', neither of which are valid
> pp-tokens.

Even if ## were redefined so that intermediate results of a ## b ## c
are ignored, the behaviour is explicitly undefined.

6.4:
preprocessing-token:
[...]
each non-white-space character that cannot be one of the above

6.4p3:
If a ' or a " character matches the last category, the behavior is
undefined.

On 11/08/2011 12:01 PM, Harald van Dijk wrote:
> On Nov 7, 11:53 pm, lawrence.jo...@siemens.com wrote:
>> James Kuyper <jameskuy...@verizon.net> wrote:
>>> #include <stdlib.h>
>>> #define TRICAT(a, b, c) a ## b ## c
>>> #define CHAR(a, b, c) TRICAT(a, b, c)
>>
>>> #define CASE1 x
>>
>> ...
>>> case CHAR('
>>> , CASE1, '
>>> ): break;
>>
>> Nice try, but no cigar. The ## operator requires that the result be a
>> valid pp-token and, depending on the order in which the operators are
>> evaluated, you either end up with 'x or x', neither of which are valid
>> pp-tokens.
>
> Even if ## were redefined so that intermediate results of a ## b ## c
> are ignored, the behaviour is explicitly undefined.
>
> 6.4:
> preprocessing-token:
> [...]
> each non-white-space character that cannot be one of the above
>
> 6.4p3:
> If a ' or a " character matches the last category, the behavior is
> undefined.

Yes, I forgot about that item, too.