C宏:如何将另一个宏映射到可变参数?

编程入门行业动态更新时间:2024-10-09 09:12:03

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我想知道如何将一元函数(或另一个宏)应用于宏的可变参数，例如

I'd like to know how to apply a unary function (or another macro) to variadic arguments of a macro, like

int f(int a); #define apply(args...) <the magic> apply(a, b, c)

可展开

f(a) f(b) f(c)

请注意，参数数量未知.

Note that the number of arguments is unknown.

推荐答案

下面的代码适用于您要求的内容，最多包含1024个参数，并且没有使用了boost等其他内容.它定义了一个EVAL(...)和一个MAP(m, first, ...)宏来进行递归，并为每次迭代使用带有下一个参数first的宏m.

The code below is working for what you've asked for with up to 1024 arguments and without using additional stuff like boost. It defines an EVAL(...) and also a MAP(m, first, ...) macro to do recursion and to use for each iteration the macro m with the next parameter first.

使用它，您的apply(...)看起来像:#define apply(...) EVAL(MAP(apply_, __VA_ARGS__)).

With the use of that, your apply(...) looks like: #define apply(...) EVAL(MAP(apply_, __VA_ARGS__)).

它主要是从 C预处理器魔术复制而来.那里的解释也很棒.您也可以在 git存储库，实际代码中也有很多说明.它是可变参数的，因此它需要您想要的参数数量.

It is mostly copied from C Pre-Processor Magic. It is also great explained there. You can also download these helper macros like EVAL(...) at this git repository, there are also a lot of explanation in the actual code. It is variadic so it takes the number of arguments you want.

但是我更改了FIRST和SECOND宏，因为它使用了Gnu扩展名，就像我从中复制源代码一样. @ HWalters 在下面的评论中这样说:

But I changed the FIRST and the SECOND macro as it uses a Gnu extension like it is in the source I've copied it from. This is said in the comments below by @HWalters:

具体地说，是6.10.3p4:否则，[标识符列表以...结尾".在调用中，参数应比宏定义中的参数(而不是...)要多."

主要功能部分:

int main() { int a, b, c; apply(a, b, c) /* Expands to: f(a); f(b); f(c); */ return 0; }

宏定义:

#define FIRST_(a, ...) a #define SECOND_(a, b, ...) b #define FIRST(...) FIRST_(__VA_ARGS__,) #define SECOND(...) SECOND_(__VA_ARGS__,) #define EMPTY() #define EVAL(...) EVAL1024(__VA_ARGS__) #define EVAL1024(...) EVAL512(EVAL512(__VA_ARGS__)) #define EVAL512(...) EVAL256(EVAL256(__VA_ARGS__)) #define EVAL256(...) EVAL128(EVAL128(__VA_ARGS__)) #define EVAL128(...) EVAL64(EVAL64(__VA_ARGS__)) #define EVAL64(...) EVAL32(EVAL32(__VA_ARGS__)) #define EVAL32(...) EVAL16(EVAL16(__VA_ARGS__)) #define EVAL16(...) EVAL8(EVAL8(__VA_ARGS__)) #define EVAL8(...) EVAL4(EVAL4(__VA_ARGS__)) #define EVAL4(...) EVAL2(EVAL2(__VA_ARGS__)) #define EVAL2(...) EVAL1(EVAL1(__VA_ARGS__)) #define EVAL1(...) __VA_ARGS__ #define DEFER1(m) m EMPTY() #define DEFER2(m) m EMPTY EMPTY()() #define IS_PROBE(...) SECOND(__VA_ARGS__, 0) #define PROBE() ~, 1 #define CAT(a,b) a ## b #define NOT(x) IS_PROBE(CAT(_NOT_, x)) #define _NOT_0 PROBE() #define BOOL(x) NOT(NOT(x)) #define IF_ELSE(condition) _IF_ELSE(BOOL(condition)) #define _IF_ELSE(condition) CAT(_IF_, condition) #define _IF_1(...) __VA_ARGS__ _IF_1_ELSE #define _IF_0(...) _IF_0_ELSE #define _IF_1_ELSE(...) #define _IF_0_ELSE(...) __VA_ARGS__ #define HAS_ARGS(...) BOOL(FIRST(_END_OF_ARGUMENTS_ __VA_ARGS__)()) #define _END_OF_ARGUMENTS_() 0 #define MAP(m, first, ...) \ m(first) \ IF_ELSE(HAS_ARGS(__VA_ARGS__))( \ DEFER2(_MAP)()(m, __VA_ARGS__) \ )( \ /* Do nothing, just terminate */ \ ) #define _MAP() MAP #define apply_(x) f(x); #define apply(...) EVAL(MAP(apply_, __VA_ARGS__))

要测试宏扩展，将gcc与命令行参数-E一起使用会很有用:

To test macro expansion it is useful to use gcc with the command line argument -E: