C 预处理器，宏“重载"； [英] C Preprocessor, Macro "Overloading"

问题描述

我正在尝试执行某种宏重载"，以便 MACRO(something) 的扩展方式与 MACRO(something, else) 不同.

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

使用我从 这里获得的片段(我是不确定它是否 100% 可移植)和 Boost PP 库中的一些功能，我能够让它工作:D

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)

现在我仍然想念另外两件我想做的事情:

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

1. (这个我真的不在乎我是否永远不会解决它)我相信可以编写一个宏，当占用变体"的数量时，其对应的输出"会生成类似的代码上面那个.像 TEMPLATE(3, function_A(...), function_B(...), function_C(...)) 这样的东西来生成上面的例子.

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.

在没有参数的情况下调用 TEST() 会发生什么?好吧，VA_NARGS 展开为 1.但第一个参数是"(无).我正在尝试找到一种方法来检测 __VA_ARGS__ 中的零"参数或区分空"参数和真实参数，以便扩展重载"功能以响应这个情况.有什么想法吗?

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?

推荐答案

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，我已经写了 here.将这种方法与您正在使用的 boost 宏结合起来应该相对容易.

To answer your question 2 first. Yes, with variadic macros it is also possible to detect an empty argument list. The explanation is a bit lengthy, I have written it up here. It should be relatively easy to combine this approach with the boost macros that you are using.

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.如果我理解正确你必须使用类似嵌套列表的东西(a, (b, (c,d)))，不太方便.

For your question 1, yes this is also possible. Boost has some iterator macros that come close to this, I think, but they look a bit scary to use. If I understand correctly you have to use something like nested lists (a, (b, (c,d))), not too convenient.

(我写了一组可以更直接实现这个的宏，但不幸的是，该软件包尚未准备好发布.如果您真的对此感兴趣，请私下联系我.)

(I wrote a set of macros that can achieve this more directly, but unfortunately the package is not yet ready for release. Contact me in private if you are really interested in it.)

P99 包同时发布并且包含很多关于宏重载"和类型通用宏的东西.

The P99 package is published in the mean time and contains a lot of stuff over macro "overloading" and type generic macros.

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