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From: Blue-Maned_Hawk <bluemanedhawk@invalid.invalid>
Newsgroups: comp.lang.c
Subject: Re: how to make a macro work as a single line if stmt without braces
Date: Sat, 21 Sep 2024 21:18:07 -0000 (UTC)
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Mark Summerfield wrote:

> 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'

Since the initial problem has already been solved by other messages, 
here's some extra completely unsolicited advice that you are free to 
ignore:

If you intend for this macro to be invoked like how one would invoke the 
printf function (i.e. the arguments consisting of a format string literal 
followed by arguments to be formatted into it), you could get away with 
defining it like this instead:

#define WARN(msg, ...) (fprintf(stderr, "%s#%d: " msg, __FILE__, __LINE__ 
__VA_OPT__(,) __VA_ARGS__))

This would break if you try to invoke it with a nonliteral string, because 
this takes advantage of preprocessor adjacent string literal 
concatenation, but modern compilers tend to complain mightily if you try 
that anyway.  It would have the advantage of expanding to an expression 
instead, which is often more beneficial for macros because it permits more 
flexibility in placement (something that i find unlikely to be relevant 
for this particular one, but which can't really particularly hurt to have 
anyway).

-- 
Blue-Maned_Hawk│shortens to Hawk│/blu.mɛin.dʰak/│he/him/his/himself/Mr.
blue-maned_hawk.srht.site
How do i run in these marrowy clogs?