宏返回它在C中給出的參數的數量？

我想要的東西做：

foo(1) -> 1 
foo(cat, dog) -> 2 
foo(red, green, blue) -> 3

即使是它與##更好，如果這個宏可以定義這樣的方式讓

foo(1) -> bar1(1) 
foo(cat, dog) -> bar2(cat, dog) 
foo(red, green, blue) -> car3(red, green, blue)

謝謝！

編輯：我真的想要一個宏，不是一個函數。使用功能的建議將被降低。

來源

2010-02-22 anon

爲什麼不只是使用帶有可變長度參數列表的函數呢？ – 2010-02-22 01:14:07

如果你能以任何有意義的方式做到這一點，我們現在都會這樣做，你就會知道。 – jbcreix 2010-02-22 01:14:30

@jbcreix你不能用這種方法發明任何新東西:)這是可能的 - 請參閱下面的回覆。 – qrdl 2010-02-22 06:17:39

這是可以做到 - 該機制在comp.std.c新聞組解釋於2006年1月有關於這個問題的另一個最近在SO 2124339。

我藏匿的代碼了，萬一......

#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 <[email protected]> 
** 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 <[email protected]> 
** 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 */

它的工作原理，只要細如沒有超過64個參數。下面是測試代碼我使用：

#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); 
}

來源

2010-02-22 03:39:36

聰明...不幸的是（對我來說反正），它需要C99風格的'__VA_ARGS__'支持。還是很聰明的。 – 2010-02-22 05:37:22

這很巧妙。值得更多upvotes。 – anon 2010-02-22 08:47:59

@MichaelBurr我認爲要求對10年前的標準提供支持是合理的。 – 2013-10-06 01:22:33

我使用下面的宏：

#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))

請注意，它僅適用於C99，因爲複雜的宏沒有在C89支持。雖然它不適用於零參數。

但是，如果你正在使用gcc，你可以用略作修改宏：

#define NUMARGS(...) (sizeof((int[]){0, ##__VA_ARGS__})/sizeof(int)-1)

它正常工作，甚至爲零的參數，因爲GCC的預處理器時要粘貼空__VA_ARGS__刪除多餘的逗號。

來源

2010-02-22 06:15:21 qrdl

不幸的是，如果NUMARGS（）的參數未定義或者是字符串，則不起作用，如'int main（）{NUMARGS（「a」，b，c）; }' – 18446744073709551615 2014-05-15 07:58:02

我意識到這是一個很老的問題，但因爲從來沒有人回答了「獎金」添加到函數名的數量，這裏是部分與喬納森的回答減少到9個args作爲簡潔的例子。它假定你已經預定義了編號的函數，或者將它用作定義它們的基礎。

#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);

一個地方，我看到這些類型的功能是在Linux內核的系統調用，但由於第一個參數是系統調用號（通常定義爲__NR_something）數量將由一個被關閉，因此在這裏一個例子說明了這一點。

#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

來源

2013-02-18 08:16:30 technosaurus

宏返回它在C中給出的參數的數量？

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