Why doesn't __VA_LIST__ just eat the prior comma?

From	Kaz Kylheku <480-992-1380@kylheku.com>
Newsgroups	comp.std.c
Subject	Why doesn't __VA_LIST__ just eat the prior comma?
Date	2022-04-14 00:01 +0000
Organization	A noiseless patient Spider
Message-ID	<20220413165157.844@kylheku.com> (permalink)

Show all headers | View raw

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?

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal

Back to comp.std.c | Previous | Next — Next in thread | Find similar | Unroll thread

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

csiph-web