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POHow can C++ and C variadic arguments be used together?
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copied!<p>Generally, using the C++11 variadic template feature with functions requires the variadic-based function arguments to be the last in the function argument list. There is one exception; they are the next-to-last arguments if there are C-level variadic arguments, which must be dead last.</p> <pre><code>template < typename ...Args > int super_printf( Something x, Args &&...a, ... ); </code></pre> <p>I sometimes randomly think about C++, and I wondered how such a function can be implemented. I first thought about the usual recursive peeling of arguments from <em>a</em>, then I remembered that the C-level varargs don't cascade. I have to turn them to a definitive va_list right away.</p> <pre><code>template < typename ...Args > int super_vaprintf( Something x, std::va_list &aa, Args &&...a ); // Note that "aa" is passed by reference. template < typename ...Args > int super_printf( Something x, Args &&...a, ... ) { std::va_list args2; int result; va_start( args2, XXX ); // (A) try { result = super_vaprintf( x, args2, std::forward<Args>(a)... ); } catch ( ... ) { va_end( args2 ); // (1) throw; } va_end( args2 ); // (2) return result; // Can (1) and (2) be compacted with RAII using a custom deleter lambda // in std::unique_ptr or something? Remember that "va_end" is a macro! } </code></pre> <p>The usual C++ variadic recursive peeling happens in the <code>super_vaprintf</code> call. At line (A), what goes in the place of <code>XXX</code>, "a" or "a..."? What happens if <em>a</em> is empty, does <em>x</em> go there instead? If that last question's true, are we screwed if there's no <em>x</em>; that there's no arguments besides the variadic ones? (And if it's true, how do we conditionalize the code to use <em>x</em> when <em>a</em> is empty, and <em>a</em> otherwise?)</p> <p>...</p> <p>I just looked at my copy of the C++11 standard for any assistance here. There doesn't seem to be any. This would prompt a request for the C++ committee to come back to fix this, but I'm not sure that there's any way such a function could be called without the C++ varargs taking everything. Am I wrong; can a function call be made to use both C++ and C varargs? Or is mixing only useful for declarations, in terms of Stupid (Template) Instantiation Tricks?</p>

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