Re: [PATCH] strbuf: use designated initializers in STRBUF_INIT

From: Jeff King <peff@peff.net>
Date: Jul 15, 2017, 11:08 UTC
Message-ID: <20170715110839.hefdnf4eqhdwboxq@sigill.intra.peff.net>
In-Reply-To: <20170714224309.7p4a4vuuv3k2cdjg@glandium.org>

On Sat, Jul 15, 2017 at 07:43:09AM +0900, Mike Hommey wrote:

Show 13 quoted lines

> >  - There were occasional cases where we wished if variable-length
> >    arrays, flexible array members and variadic macros were available
> >    in our codebase during the course of this project.  We would
> >    probably want to add a similar test baloon patch for each of
> >    them to this series that is currently two-patch long.
> 
> FWIW, variadic macros have subtle implementation differences that can
> cause problems.
> For instance, MSVC only supports ##__VA_ARGS__ by way of
> ignoring ## somehow, but has the default behavior of dropping the comma
> when __VA_ARGS__ is empty, which means , __VA_ARGS__ *without* ## has a
> different behavior.
> See also https://connect.microsoft.com/VisualStudio/feedback/details/380090/variadic-macro-replacement

When there's any other non-optional argument, you can work around this by just lumping that argument in with the varargs. So don't do:

  #define FOO(fmt, ...) printf(fmt, __VA_ARGS__)

which fails when the format has no placeholders.  Instead do:

  #define FOO(...) printf(__VA_ARGS__)

That limits your interface for variadic macros, but in practice it doesn't usually come up (there are also disgusting macro tricks you can do to cover the true 0-arg case portably, but we haven't had to turn to them yet).

-Peff