是否可以编写一个返回其参数个数的C宏？
我想要做的事情：
foo(1) -> 1
foo(cat, dog) -> 2
foo(red, green, blue) -> 3
如果可以使用##以及
的方式定义此宏，那就更好了foo(1) -> bar1(1)
foo(cat, dog) -> bar2(cat, dog)
foo(red, green, blue) -> car3(red, green, blue)
谢谢！
编辑：我真的想要一个宏，而不是一个函数。使用功能的建议将被低估。答案 0 :(得分：68)
可以这样做 - 该机制在2006年1月的comp.std.c新闻组中进行了解释。最近在SO 2124339上还有另一个问题。
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 */
只要参数不超过64个，它就可以正常工作。这是我使用的测试代码：
#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);
}
答案 1 :(得分：4)
我意识到这是一个非常古老的问题，但由于没有人回答“奖励”来将数字添加到函数名称中，这里有一个例子，Jonathan的答案简化为9 args。它假定您已预定义编号的函数，或将其用作定义它们的基础。
#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);
#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
答案 2 :(得分：2)
我使用以下宏：
#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))
请注意，它仅适用于C99，因为C89不支持可变参数宏。虽然它不适用于零参数。
但是如果你使用的是GCC，你可以使用稍加修改的宏：
#define NUMARGS(...) (sizeof((int[]){0, ##__VA_ARGS__})/sizeof(int)-1)
即使是零参数也能正常工作，因为当你粘贴空__VA_ARGS__时，GCC的预处理器会删除额外的逗号。