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I'm trying to do some kind of Macro "Overloading", so that MACRO(something), gets expanded differently than MACRO(something, else).

Using a snippet I got from here (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

//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)

Now I'm still missing two other things that I want to do:

  1. (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.

  2. 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 __VA_ARGS__ 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?

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先回答你的问题2。是的，使用可变参数宏也可以检测空参数列表。解释有点长，我写到这里了。将这种方法与您正在使用的 boost 宏结合起来应该相对容易。

对于您的问题 1，是的，这也是可能的。Boost 有一些迭代器宏，我认为，但它们看起来有点可怕。如果我理解正确，您必须使用嵌套列表之类的东西(a, (b, (c,d)))，不太方便。

（我写了一组可以更直接实现这个的宏，可惜这个包还没有准备好发布，如果真的有兴趣可以私信我。）

编辑： P99包同时发布，包含很多关于宏“重载”和类型通用宏的内容。

于 2010-08-06T06:31:30.240 回答