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Re: Why doesn't __VA_LIST__ just eat the prior comma?

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On 2022-04-19, Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> Kaz Kylheku <480-992-1380@kylheku.com> writes:
>
>> There is a well-known problem that a macro like
>>
>> #define foo(x, ...) something(whatever, __VA_ARGS__)
>>
>> cannot work when the trailing argument list is empty because
>> you get  the expansion something(whatever, )
>>
>> GNU C provides ##__VA_ARGS__ which behaves like __VA_ARGS__ in
>> the nonempty case, and eats the prior comma in the empty case.
>>
>> C++20 provides __VA_OPT__ which is used like __VA_OPT__(,) to
>> conditionally eat the comma.  (GNU C has this also).
>>
>> The question is:  why wouldn't you just fix the semantics of
>> __VA_ARG__ so that this is not necessary?
>
> This idea is considered in section "Alternative Designs" of
>
>     http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2160.htm

Fantastic; thanks for that link!

The crux is that, for whatever reason, a macro like:

      #define ADD_COMMA(...) , __VA_ARGS__

is tabled as an important requirement, and for that reason, F()
expanding to f(10), though identified as desirable, is not considered;
the automatic comma eating would break ADD_COMMA.

But! I think that could be acceptably handled like this:

      #define FENCE
      #define ADD_COMMA(...) , FENCE __VA_ARGS__

The rule that __VA_ARGS__ consumes the comma would be carried out
prior to the macro replacement of the token replacement sequence,
during which time FENCE is still there. Since FENCE doesn't look like
a comma, all is cool.

(This seems natural because the substitution of __VA_ARGS__ should in fact
be taking place during argument replacement, and not during the
rescanning of the argument-substituted replacement sequence for more
macros.)

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Why doesn't __VA_LIST__ just eat the prior comma? Kaz Kylheku <480-992-1380@kylheku.com> - 2022-04-14 00:01 +0000
  Re: Why doesn't __VA_LIST__ just eat the prior comma? Ben <ben.usenet@bsb.me.uk> - 2022-04-14 14:17 +0100
    Re: Why doesn't __VA_LIST__ just eat the prior comma? Kaz Kylheku <480-992-1380@kylheku.com> - 2022-04-18 16:59 +0000
      Re: Why doesn't __VA_LIST__ just eat the prior comma? Ben <ben.usenet@bsb.me.uk> - 2022-04-18 20:43 +0100
  Re: Why doesn't __VA_LIST__ just eat the prior comma? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-19 02:04 -0700
    Re: Why doesn't __VA_LIST__ just eat the prior comma? Kaz Kylheku <480-992-1380@kylheku.com> - 2022-04-20 02:02 +0000
      Re: Why doesn't __VA_LIST__ just eat the prior comma? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-05-14 10:35 -0700

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