Re: how to make a macro work as a single line if stmt without braces

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: how to make a macro work as a single line if stmt without braces
Date	2024-09-21 15:55 -0700
Organization	None to speak of
Message-ID	<87bk0gr50v.fsf@nosuchdomain.example.com> (permalink)
References	<PaWdnZ3R-9zI6nP7nZ2dnZfqn_GdnZ2d@brightview.co.uk> <86o74h88ak.fsf@linuxsc.com>

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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Mark Summerfield <mark@qtrac.eu> writes:
>> I have this macro:
>>
>> #define WARN(...)                                       \
>>     do {                                                \
>>         fprintf(stderr, "%s#%d: ", __FILE__, __LINE__); \
>>         fprintf(stderr, __VA_ARGS__);                   \
>>     } while (0);
>>
>> which I use like this:
>>
>> total++;
>> if (failed) {
>>     WARN("failed because...");
>> } else
>>     ok++;
>>
>> I would prefer to be able to write this instead:
>>
>> total++;
>> if (failed)
>>     WARN("failed because...");
>> else
>>     ok++;
>>
>> but doing so results in a compiler error:
>>
>> error: 'else' without a previous 'if'
>
> You can define WARN() this way
>
>    #define WARN(...) (                                  \
>       fprintf(stderr, "%s#%d: ", __FILE__, __LINE__),   \
>       fprintf(stderr, __VA_ARGS__)                      \
>    )
>
> and it will work as you want it to.

Good point.  To expand on that, the do/while trick (without the trailing
semicolon) is the usual way to write a macro that can be used anywhere a
statement can appear.  But it's not necessary if all you want is a
sequence of expression statements.  In that case, you can write the
expression statements (terminated by semicolons) as expressions
(separated by comma operators and enclosed in parentheses), and the
whole expansion is a single expression that can be used in an expression
statement.  The macro can then be used in more contexts, since its
expansion is an expression (though that's not likely to matter in this
case).

An if/else can be turned into a conditional operator, which also gives
you an expression, but there's no expression equivalent for most control
flow constructs (loops, return, etc.).  That's where the do/while trick
becomes necessary.

For this example (two fprintf calls), you can use either do/while or a
comma operator.  You might consider using the do/while in case you might
want to add control flow later.  If you have several similar macros,
there's something to be said for being consistent.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

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how to make a macro work as a single line if stmt without braces Mark Summerfield <mark@qtrac.eu> - 2024-09-21 07:35 +0000
  Re: how to make a macro work as a single line if stmt without braces Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-09-21 08:10 +0000
  Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-21 10:47 +0200
    Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-21 12:27 -0700
      Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-21 21:11 +0100
      Re: how to make a macro work as a single line if stmt without braces Opus <ifonly@youknew.org> - 2024-09-21 22:39 +0200
      Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 14:54 -0700
        Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-21 20:05 -0700
          Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 21:33 -0700
          Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-22 14:15 +0200
          Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-27 03:38 -0700
      Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-22 13:59 +0200
        Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-22 15:11 +0000
          Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-22 16:56 +0100
            Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-22 19:27 +0300
              Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-22 17:47 +0100
              Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-24 07:36 -0700
                Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-24 07:52 -0700
                Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-24 17:24 +0200
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 17:22 +0100
                Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-24 12:28 -0700
                Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-27 01:43 -0700
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 17:35 +0100
                Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-24 12:30 -0700
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 20:42 +0100
                Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-24 19:59 +0000
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 21:12 +0100
                Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-24 22:36 +0000
            Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-22 13:39 -0700
              Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-23 08:16 +0200
                Re: how to make a macro work as a single line if stmt without braces Richard Harnden <richard.nospam@gmail.invalid> - 2024-09-23 08:06 +0100
                Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-23 11:14 +0200
                Re: how to make a macro work as a single line if stmt without braces Richard Harnden <richard.nospam@gmail.invalid> - 2024-09-23 11:37 +0100
                Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-23 14:22 +0200
          Modern text editor - 'bout time someone paid attention to keeping the thread title relevant!!! (Was: how to make a macro work as a single line if stmt without braces) gazelle@shell.xmission.com (Kenny McCormack) - 2024-09-22 16:03 +0000
          Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-22 18:03 +0200
      Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 05:02 -0700
        Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-28 20:30 +0300
          Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 21:53 -0700
            Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-29 12:48 +0300
              Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-29 19:18 -0700
              Re: how to make a macro work as a single line if stmt without braces Alan Mackenzie <acm@muc.de> - 2024-09-30 15:05 +0000
        Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-28 21:02 -0700
          Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 22:47 -0700
            Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-29 07:41 -0700
              Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-29 18:04 +0300
              Re: how to make a macro work as a single line if stmt without braces scott@slp53.sl.home (Scott Lurndal) - 2024-09-29 18:30 +0000
              Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-29 16:39 -0700
                Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-30 02:29 +0000
                Re: how to make a macro work as a single line if stmt without braces Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-09-30 11:26 +0200
              Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-29 18:39 -0700
              Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-30 02:03 +0000
              Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-30 13:17 +0200
  Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-21 10:13 +0100
    Re: how to make a macro work as a single line if stmt without braces Mark Summerfield <mark@qtrac.eu> - 2024-09-21 09:53 +0000
  Re: how to make a macro work as a single line if stmt without braces Ike Naar <ike@sdf.org> - 2024-09-21 09:15 +0000
    Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 14:44 -0700
  Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-21 06:07 -0700
    Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 15:55 -0700
  Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-21 12:17 -0700
  Re: how to make a macro work as a single line if stmt without braces Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2024-09-21 21:18 +0000
    Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-24 06:18 -0700

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