macros返回在C中给出的参数的数量？
是否有可能写一个Cmacros返回其参数的数量？
我想要的东西：
foo(1) -> 1 foo(cat, dog) -> 2 foo(red, green, blue) -> 3 更好的是，如果这个macros可以用##这样的方式来定义
foo(1) -> bar1(1) foo(cat, dog) -> bar2(cat, dog) foo(red, green, blue) -> car3(red, green, blue)
谢谢！
编辑：我真的想要一个macros，而不是一个function。 使用函数的build议将被低估。
可以这样做 – 这个机制在2006年1月的comp.std.c新闻组中解释过。关于SO 2124339最近有另外一个问题。
我把代码藏起来，以防万一
#ifndef JLSS_ID_NARG_H #define JLSS_ID_NARG_H /* ** http://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5?pli=1 ** ** Newsgroups: comp.std.c ** From: Laurent Deniau <laurent.deniau@cern.ch> ** Date: Mon, 16 Jan 2006 18:43:40 +0100 ** Subject: __VA_NARG__ ** ** A year ago, I was asking here for an equivalent of __VA_NARG__ which ** would return the number of arguments contained in __VA_ARGS__ before its ** expansion. In fact my problem at that time (detecting for a third ** argument) was solved by the solution of P. Mensonides. But I was still ** thinking that the standard should have provided such a facilities rather ** easy to compute for cpp. ** ** This morning I had to face again the same problem, that is knowing the ** number of arguments contained in __VA_ARGS__ before its expansion (after ** its expansion can always be achieved if you can do it before). I found a ** simple non-iterative solution which may be of interest here as an answer ** to who will ask in the future for a kind of __VA_NARG__ in the standard ** and I post it for archiving. May be some more elegant-efficient solution ** exists? ** ** Returns NARG, the number of arguments contained in __VA_ARGS__ before ** expansion as far as NARG is >0 and <64 (cpp limits): ** ** #define PP_NARG( ...) PP_NARG_(__VA_ARGS__,PP_RSEQ_N()) ** #define PP_NARG_(...) PP_ARG_N(__VA_ARGS__) ** #define PP_ARG_N(_1,_2,_3,_4,_5,_6,_7,_8,_9,[..],_61,_62,_63,N,...) N ** #define PP_RSEQ_N() 63,62,61,60,[..],9,8,7,6,5,4,3,2,1,0 ** ** [..] stands for the continuation of the sequence omitted here for ** lisibility. ** ** PP_NARG(A) -> 1 ** PP_NARG(A,B) -> 2 ** PP_NARG(A,B,C) -> 3 ** PP_NARG(A,B,C,D) -> 4 ** PP_NARG(A,B,C,D,E) -> 5 ** PP_NARG(A1,A2,[..],A62,A63) -> 63 ** ** ====== ** ** Newsgroups: comp.std.c ** From: Roland Illig <roland.il...@gmx.de> ** Date: Fri, 20 Jan 2006 12:58:41 +0100 ** Subject: Re: __VA_NARG__ ** ** Laurent Deniau wrote: ** > This morning I had to face again the same problem, that is knowing the ** > number of arguments contained in __VA_ARGS__ before its expansion (after ** > its expansion can always be achieved if you can do it before). I found a ** > simple non-iterative solution which may be of interest here as an answer ** > to who will ask in the future for a kind of __VA_NARG__ in the standard ** > and I post it for archiving. May be some more elegant-efficient solution ** > exists? ** ** Thanks for this idea. I really like it. ** ** For those that only want to copy and paste it, here is the expanded version: ** ** // Some test cases ** PP_NARG(A) -> 1 ** PP_NARG(A,B) -> 2 ** PP_NARG(A,B,C) -> 3 ** PP_NARG(A,B,C,D) -> 4 ** PP_NARG(A,B,C,D,E) -> 5 ** PP_NARG(1,2,3,4,5,6,7,8,9,0, // 1..10 ** 1,2,3,4,5,6,7,8,9,0, // 11..20 ** 1,2,3,4,5,6,7,8,9,0, // 21..30 ** 1,2,3,4,5,6,7,8,9,0, // 31..40 ** 1,2,3,4,5,6,7,8,9,0, // 41..50 ** 1,2,3,4,5,6,7,8,9,0, // 51..60 ** 1,2,3) -> 63 ** **Note: using PP_NARG() without arguments would violate 6.10.3p4 of ISO C99. */ /* The PP_NARG macro returns the number of arguments that have been ** passed to it. */ #define PP_NARG(...) \ PP_NARG_(__VA_ARGS__,PP_RSEQ_N()) #define PP_NARG_(...) \ PP_ARG_N(__VA_ARGS__) #define PP_ARG_N( \ _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \ _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \ _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \ _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \ _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \ _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \ _61,_62,_63, N, ...) N #define PP_RSEQ_N() \ 63,62,61,60, \ 59,58,57,56,55,54,53,52,51,50, \ 49,48,47,46,45,44,43,42,41,40, \ 39,38,37,36,35,34,33,32,31,30, \ 29,28,27,26,25,24,23,22,21,20, \ 19,18,17,16,15,14,13,12,11,10, \ 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 #endif /* JLSS_ID_NARG_H */
只要不超过64个参数，它就可以正常工作。 这是我使用的testing代码：
#include "narg.h" #include <stdio.h> #define PRINT(pp_narg) printf("%2d = %s\n", pp_narg, # pp_narg) #ifndef lint /* Prevent over-aggressive optimizers from eliminating ID string */ extern const char jlss_id_narg_c[]; const char jlss_id_narg_c[] = "@(#)$Id: narg.c,v 1.2 2010/01/24 18:12:05 jleffler Exp $"; #endif /* lint */ int main(void) { PRINT(PP_NARG(A)); PRINT(PP_NARG(A, B)); PRINT(PP_NARG(A, B, C)); PRINT(PP_NARG(A, B, C, D)); PRINT(PP_NARG(A, B, C, D, E)); PRINT(PP_NARG(1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 1..10 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 11..20 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 21..30 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 31..40 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 41..50 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 51..60 1, 2, 3)); /** ** If the number of arguments to PP_NARG() is greater than 63, the ** 64th argument is returned. This is well-defined behaviour, but ** not exactly what was intended. */ PRINT(PP_NARG(1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 1..10 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 11..20 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 21..30 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 31..40 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 41..50 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 51..60 1, 2, 3, -123456789)); PRINT(PP_NARG(1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 1..10 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 11..20 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 21..30 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 31..40 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 41..50 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, // 51..60 1, 2, 3, -123456789, -987654321)); return(0); }
我意识到这是一个相当古老的问题，但是由于没有人回答将“数字”添加到函数名称的“奖金”，下面是乔纳森的答案简化为9个参数的一个例子。 它假定你已经预定义了编号的函数，或者将它作为基础来定义它们。
#define MKFN(fn,...) MKFN_N(fn,##__VA_ARGS__,9,8,7,6,5,4,3,2,1,0)(__VA_ARGS__) #define MKFN_N(fn,n0,n1,n2,n3,n4,n5,n6,n7,n8,n,...) fn##n #define myfunc(...) MKFN(myfunc,##__VA_ARGS__) myfunc(); myfunc(a,b,c,d,e,f); //gcc -E this.c //myfunc0(); //myfunc6(a,b,c,d,e,f);
我看过这些types函数的地方在Linux内核的系统调用中，但是数字会被忽略掉，因为第一个参数是系统调用号（通常定义为__NR_something），所以这里是一个例子。
#define MKFN(fn,...) MKFN_N(fn,##__VA_ARGS__,9,8,7,6,5,4,3,2,1,0)(__VA_ARGS__) #define MKFN_N(fn,NR,n0,n1,n2,n3,n4,n5,n6,n7,n8,n,...) fn##n #define syscall(...) MKFN(syscall,##__VA_ARGS__) syscall(__NR_fork); syscall(77,a,b,c,d,e,f);//fake example
我使用下面的macros：
#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))
请注意，它仅适用于C99，因为可变macros在C89中不受支持。 虽然它不适用于零参数。
但是如果你使用GCC，你可以使用稍微修改过的macros：
#define NUMARGS(...) (sizeof((int[]){0, ##__VA_ARGS__})/sizeof(int)-1)
即使对于零参数，它也能正常工作，因为GCC的预处理器在粘贴空的__VA_ARGS__时会删除额外的逗号。
