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

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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?
>
> When would you ever want to interpolate __VA_ARGS__ into a replacement
> sequence such that if it is empty, and placed after a comma, that
> comma does not disappear?

You might not want it (as a design feature), but there is probably code
out there that now depends on it.  The C committee is very reluctant to
break existing code, especially where the fix is more complex than
"search for 'restrict' to check it's not used".

-- 
Ben.

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Thread

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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