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POC Preprocessor, Macro "Overloading"
 

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    <p>I'm trying to do some kind of Macro "Overloading", so that MACRO(something), gets expanded differently than MACRO(something, else).</p> <p>Using a snippet I got from <a href="https://stackoverflow.com/questions/2124339/c-preprocessor-va-args-number-of-arguments">here</a> (I'm not sure if it's 100% portable) and some functions from the Boost PP Library, I was able to make it work :D</p> <pre><code>//THESE TWO COUNT THE NUMBER OF ARGUMENTS #define VA_NARGS_IMPL(_1, _2, _3, _4, _5, N, ...) N #define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 5, 4, 3, 2, 1) //THIS ONE RETURNS THE PARAMETER AT POSITION _i FROM A LIST OF __VA_ARGS__ #define VA_ARG(_i, ...) BOOST_PP_ARRAY_ELEM(_i, (VA_NARGS(__VA_ARGS__), (__VA_ARGS__))) //AND THIS ONE IS THE 'OVERLOADED' MACRO ;) #define TEST(...) BOOST_PP_IF(BOOST_PP_EQUAL(1, VA_NARGS(__VA_ARGS__)), function_A(VA_ARG(0, __VA_ARGS__)), \ //1 parameter BOOST_PP_IF(BOOST_PP_EQUAL(2, VA_NARGS(__VA_ARGS__)), function_B(VA_ARG(0, __VA_ARGS__) + VA_ARG(1, __VA_ARGS__)), \ //2 parameters BOOST_PP_IF(BOOST_PP_EQUAL(3, VA_NARGS(__VA_ARGS__)), function_C(VA_ARG(1, __VA_ARGS__) + VA_ARG(2, __VA_ARGS__)), BOOST_PP_EMPTY())) // 3 parameters and so on ... So TEST(a) = function_A(a) TEST(a, b) = function_B(a + b) TEST(a, b, c) = function_C(b + c) </code></pre> <p>Now I'm still missing two other things that I want to do:</p> <ol> <li><p>(This one I don't really care if I never solve it) I believe that a MACRO can be written that when taking up the number of 'variants' and its correspondent 'output' generates a code similar like the one above. Something like TEMPLATE(3, function_A(...), function_B(...), function_C(...)) to generate the example above.</p></li> <li><p>What happens when TEST() is called without arguments? Well, VA_NARGS expands to 1. But the first argument is ""(nothing). I'm trying to find a way to either detect 'zero' arguments in <code>__VA_ARGS__</code> or to differentiate between a 'null' argument and a real one, in order to extend the 'overloading' function to react to this situation. Any ideas?</p></li> </ol>
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    1. COVote question up if you can! :)
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    2. COBoost -> C++ -> if you are already using C++, do not mess with pre-processor, use plain functions (inlined if you wish). C preprocessor is intentionally dumb so that people wouldn't abuse it.
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    3. COIs this for C or C++ ? Also note that using varargs with macros is not portable.
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