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POStandard alternative to GCC's ##__VA_ARGS__ trick?
 

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  1. POStandard alternative to GCC's ##__VA_ARGS__ trick?
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    <p>There is a <a href="https://stackoverflow.com/questions/4054085/gcc-appending-to-va-args">well-known</a> <a href="http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html" rel="noreferrer">problem</a> with empty args for variadic macros in C99.</p> <p>example:</p> <pre><code>#define FOO(...) printf(__VA_ARGS__) #define BAR(fmt, ...) printf(fmt, __VA_ARGS__) FOO("this works fine"); BAR("this breaks!"); </code></pre> <p>The use of <code>BAR()</code> above is indeed incorrect according to the C99 standard, since it will expand to:</p> <pre><code>printf("this breaks!",); </code></pre> <p>Note the trailing comma - not workable.</p> <p>Some compilers (eg: Visual Studio 2010) will quietly get rid of that trailing comma for you. Other compilers (eg: GCC) support putting <code>##</code> in front of <code>__VA_ARGS__</code>, like so:</p> <pre><code>#define BAR(fmt, ...) printf(fmt, ##__VA_ARGS__) </code></pre> <p>But is there a standards-compliant way to get this behavior? Perhaps using multiple macros?</p> <p>Right now, the <code>##</code> version seems fairly well-supported (at least on my platforms), but I'd really rather use a standards-compliant solution.</p> <p>Pre-emptive: I know I could just write a small function. I'm trying to do this using macros.</p> <p><strong>Edit</strong>: Here is an example (though simple) of why I would want to use BAR():</p> <pre><code>#define BAR(fmt, ...) printf(fmt "\n", ##__VA_ARGS__) BAR("here is a log message"); BAR("here is a log message with a param: %d", 42); </code></pre> <p>This automatically adds a newline to my BAR() logging statements, assuming <code>fmt</code> is always a double-quoted C-string. It does NOT print the newline as a separate printf(), which is advantageous if the logging is line-buffered and coming from multiple sources asynchronously.</p>
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      1. POStandard alternative to GCC's ##__VA_ARGS__ trick?
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    1. COWhy use `BAR` instead of `FOO` in the first place?
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    2. CO@GMan: I added an example at the end
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    3. COIn that case, you can just do a multi-statement macro with a `printf("\n");` at the end.
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