Path: csiph.com!eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.std.c
Subject: Re: Why doesn't __VA_LIST__ just eat the prior comma?
Date: Tue, 19 Apr 2022 02:04:36 -0700
Organization: A noiseless patient Spider
Lines: 22
Message-ID: <861qxta1gb.fsf@linuxsc.com>
References: <20220413165157.844@kylheku.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Injection-Info: reader02.eternal-september.org; posting-host="dca78489ef0b00bc0e84cad62da1ae52"; logging-data="20574"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18LHuRw5OrqTjaWqrVio5BrLCtWd1Ro6i0="
User-Agent: Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux)
Cancel-Lock: sha1:coSXZqNNssDjeDkUa1AqGGOG4Jg= sha1:rZG0t51L3MfwOmd/QFAEfrh/a04=
Xref: csiph.com comp.std.c:6434

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