我在网上找不到答案的简单问题。在可变参数宏中，如何找到参数的数量？如果有解决方案，我可以使用 boost 预处理器。
如果它有所作为，我正在尝试将可变数量的宏参数转换为提升预处理器序列、列表或数组以进行进一步的重新处理。
问问题
145917 次
12 回答
114
我通常使用这个宏来查找一些参数：
#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))
完整示例：
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))
#define SUM(...) (sum(NUMARGS(__VA_ARGS__), __VA_ARGS__))
void sum(int numargs, ...);
int main(int argc, char *argv[]) {
SUM(1);
SUM(1, 2);
SUM(1, 2, 3);
SUM(1, 2, 3, 4);
return 1;
}
void sum(int numargs, ...) {
int total = 0;
va_list ap;
printf("sum() called with %d params:", numargs);
va_start(ap, numargs);
while (numargs--)
total += va_arg(ap, int);
va_end(ap);
printf(" %d\n", total);
return;
}
它是完全有效的 C99 代码。不过，它有一个缺点——你不能在SUM()没有参数的情况下调用宏，但 GCC 有一个解决方案——见这里。
因此，在 GCC 的情况下，您需要定义如下宏：
#define NUMARGS(...) (sizeof((int[]){0, ##__VA_ARGS__})/sizeof(int)-1)
#define SUM(...) sum(NUMARGS(__VA_ARGS__), ##__VA_ARGS__)
它甚至可以使用空参数列表
于 2010-01-23T19:41:02.037 回答
105
这实际上取决于编译器，并且不受任何标准的支持。
但是，在这里您有一个可以计算的宏实现：
#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
/* 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,
1,2,3,4,5,6,7,8,9,0,
1,2,3,4,5,6,7,8,9,0,
1,2,3,4,5,6,7,8,9,0,
1,2,3,4,5,6,7,8,9,0,
1,2,3,4,5,6,7,8,9,0,
1,2,3) -> 63
于 2010-01-23T19:22:50.490 回答
51
如果您使用的是 C++11，并且需要将该值作为 C++ 编译时常量，那么一个非常优雅的解决方案是：
#include <tuple>
#define MACRO(...) \
std::cout << "num args: " \
<< std::tuple_size<decltype(std::make_tuple(__VA_ARGS__))>::value \
<< std::endl;
请注意：计数完全在编译时发生，并且可以在需要编译时整数时使用该值，例如作为 std::array 的模板参数。
于 2015-10-26T15:08:15.067 回答
28
为方便起见，这里有一个适用于 0 到 70 个参数的实现，适用于 Visual Studio、GCC 和 Clang。我相信它可以在 Visual Studio 2010 及更高版本中运行，但仅在 VS2013 中对其进行了测试。
#ifdef _MSC_VER // Microsoft compilers
# define GET_ARG_COUNT(...) INTERNAL_EXPAND_ARGS_PRIVATE(INTERNAL_ARGS_AUGMENTER(__VA_ARGS__))
# define INTERNAL_ARGS_AUGMENTER(...) unused, __VA_ARGS__
# define INTERNAL_EXPAND(x) x
# define INTERNAL_EXPAND_ARGS_PRIVATE(...) INTERNAL_EXPAND(INTERNAL_GET_ARG_COUNT_PRIVATE(__VA_ARGS__, 69, 68, 67, 66, 65, 64, 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))
# define INTERNAL_GET_ARG_COUNT_PRIVATE(_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, _64, _65, _66, _67, _68, _69, _70, count, ...) count
#else // Non-Microsoft compilers
# define GET_ARG_COUNT(...) INTERNAL_GET_ARG_COUNT_PRIVATE(0, ## __VA_ARGS__, 70, 69, 68, 67, 66, 65, 64, 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)
# define INTERNAL_GET_ARG_COUNT_PRIVATE(_0, _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, _64, _65, _66, _67, _68, _69, _70, count, ...) count
#endif
static_assert(GET_ARG_COUNT() == 0, "GET_ARG_COUNT() failed for 0 arguments");
static_assert(GET_ARG_COUNT(1) == 1, "GET_ARG_COUNT() failed for 1 argument");
static_assert(GET_ARG_COUNT(1,2) == 2, "GET_ARG_COUNT() failed for 2 arguments");
static_assert(GET_ARG_COUNT(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, 64, 65, 66, 67, 68, 69, 70) == 70, "GET_ARG_COUNT() failed for 70 arguments");
于 2016-03-15T15:12:46.797 回答
16
有一些 C++11 解决方案可以在编译时查找参数的数量，但令我惊讶的是，没有人提出任何如此简单的建议，例如：
#define VA_COUNT(...) detail::va_count(__VA_ARGS__)
namespace detail
{
template<typename ...Args>
constexpr std::size_t va_count(Args&&...) { return sizeof...(Args); }
}
这也不需要包含<tuple>标题。
于 2018-05-11T00:31:42.223 回答
9
这适用于 gcc/llvm 的 0 个参数。[链接是愚蠢的]
/*
* we need a comma at the start for ##_VA_ARGS__ to consume then
* the arguments are pushed out in such a way that 'cnt' ends up with
* the right count.
*/
#define COUNT_ARGS(...) COUNT_ARGS_(,##__VA_ARGS__,6,5,4,3,2,1,0)
#define COUNT_ARGS_(z,a,b,c,d,e,f,cnt,...) cnt
#define C_ASSERT(test) \
switch(0) {\
case 0:\
case test:;\
}
int main() {
C_ASSERT(0 == COUNT_ARGS());
C_ASSERT(1 == COUNT_ARGS(a));
C_ASSERT(2 == COUNT_ARGS(a,b));
C_ASSERT(3 == COUNT_ARGS(a,b,c));
C_ASSERT(4 == COUNT_ARGS(a,b,c,d));
C_ASSERT(5 == COUNT_ARGS(a,b,c,d,e));
C_ASSERT(6 == COUNT_ARGS(a,b,c,d,e,f));
return 0;
}
Visual Studio 似乎忽略了用于使用空参数的 ## 运算符。你可能可以用类似的东西来解决这个问题
#define CNT_ COUNT_ARGS
#define PASTE(x,y) PASTE_(x,y)
#define PASTE_(x,y) x ## y
#define CNT(...) PASTE(ARGVS,PASTE(CNT_(__VA_ARGS__),CNT_(1,##__VA_ARGS__)))
//you know its 0 if its 11 or 01
#define ARGVS11 0
#define ARGVS01 0
#define ARGVS12 1
#define ARGVS23 2
#define ARGVS34 3
于 2013-06-04T20:08:59.037 回答
6
使用 msvc 扩展：
#define Y_TUPLE_SIZE(...) Y_TUPLE_SIZE_II((Y_TUPLE_SIZE_PREFIX_ ## __VA_ARGS__ ## _Y_TUPLE_SIZE_POSTFIX,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))
#define Y_TUPLE_SIZE_II(__args) Y_TUPLE_SIZE_I __args
#define Y_TUPLE_SIZE_PREFIX__Y_TUPLE_SIZE_POSTFIX ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,0
#define Y_TUPLE_SIZE_I(__p0,__p1,__p2,__p3,__p4,__p5,__p6,__p7,__p8,__p9,__p10,__p11,__p12,__p13,__p14,__p15,__p16,__p17,__p18,__p19,__p20,__p21,__p22,__p23,__p24,__p25,__p26,__p27,__p28,__p29,__p30,__p31,__n,...) __n
适用于 0 - 32 个参数。这个限制可以很容易地扩展。
编辑：简化版本（适用于 VS2015 14.0.25431.01 更新 3 和 gcc 7.4.0）多达 100 个要复制和粘贴的参数：
#define COUNTOF(...) _COUNTOF_CAT( _COUNTOF_A, ( 0, ##__VA_ARGS__, 100,\
99, 98, 97, 96, 95, 94, 93, 92, 91, 90,\
89, 88, 87, 86, 85, 84, 83, 82, 81, 80,\
79, 78, 77, 76, 75, 74, 73, 72, 71, 70,\
69, 68, 67, 66, 65, 64, 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 ) )
#define _COUNTOF_CAT( a, b ) a b
#define _COUNTOF_A( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9,\
a10, a11, a12, a13, a14, a15, a16, a17, a18, a19,\
a20, a21, a22, a23, a24, a25, a26, a27, a28, a29,\
a30, a31, a32, a33, a34, a35, a36, a37, a38, a39,\
a40, a41, a42, a43, a44, a45, a46, a47, a48, a49,\
a50, a51, a52, a53, a54, a55, a56, a57, a58, a59,\
a60, a61, a62, a63, a64, a65, a66, a67, a68, a69,\
a70, a71, a72, a73, a74, a75, a76, a77, a78, a79,\
a80, a81, a82, a83, a84, a85, a86, a87, a88, a89,\
a90, a91, a92, a93, a94, a95, a96, a97, a98, a99,\
a100, n, ... ) n
于 2011-04-22T13:47:04.823 回答
4
我假设每个参数都__VA_ARGS__将用逗号分隔。如果是这样，我认为这应该是一种非常干净的方法。
#include <cstring>
constexpr int CountOccurances(const char* str, char c) {
return str[0] == char(0) ? 0 : (str[0] == c) + CountOccurances(str+1, c);
}
#define NUMARGS(...) (CountOccurances(#__VA_ARGS__, ',') + 1)
int main(){
static_assert(NUMARGS(hello, world) == 2, ":(") ;
return 0;
}
为我在 godbolt 上为 clang 4 和 GCC 5.1 工作。这将在编译时计算，但不会评估预处理器。因此，如果您尝试做类似制作FOR_EACH的事情，那么这将行不通。
于 2018-07-24T16:51:50.283 回答
3
这是计算VA_ARGS的 0 个或多个参数的简单方法，我的示例假设最多 5 个变量，但您可以根据需要添加更多。
#define VA_ARGS_NUM_PRIV(P1, P2, P3, P4, P5, P6, Pn, ...) Pn
#define VA_ARGS_NUM(...) VA_ARGS_NUM_PRIV(-1, ##__VA_ARGS__, 5, 4, 3, 2, 1, 0)
VA_ARGS_NUM() ==> 0
VA_ARGS_NUM(19) ==> 1
VA_ARGS_NUM(9, 10) ==> 2
...
于 2016-03-14T11:58:09.763 回答
2
我在这里找到的答案仍然不完整。
我从这里找到的最接近的可移植实现是： C++ preprocessor __VA_ARGS__ number of arguments
但它不适用于 GCC 中的零参数，至少没有-std=gnu++11命令行参数。
所以我决定将此解决方案与该解决方案合并： https ://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments/
#define UTILITY_PP_CONCAT_(v1, v2) v1 ## v2
#define UTILITY_PP_CONCAT(v1, v2) UTILITY_PP_CONCAT_(v1, v2)
#define UTILITY_PP_CONCAT5_(_0, _1, _2, _3, _4) _0 ## _1 ## _2 ## _3 ## _4
#define UTILITY_PP_IDENTITY_(x) x
#define UTILITY_PP_IDENTITY(x) UTILITY_PP_IDENTITY_(x)
#define UTILITY_PP_VA_ARGS_(...) __VA_ARGS__
#define UTILITY_PP_VA_ARGS(...) UTILITY_PP_VA_ARGS_(__VA_ARGS__)
#define UTILITY_PP_IDENTITY_VA_ARGS_(x, ...) x, __VA_ARGS__
#define UTILITY_PP_IDENTITY_VA_ARGS(x, ...) UTILITY_PP_IDENTITY_VA_ARGS_(x, __VA_ARGS__)
#define UTILITY_PP_IIF_0(x, ...) __VA_ARGS__
#define UTILITY_PP_IIF_1(x, ...) x
#define UTILITY_PP_IIF(c) UTILITY_PP_CONCAT_(UTILITY_PP_IIF_, c)
#define UTILITY_PP_HAS_COMMA(...) UTILITY_PP_IDENTITY(UTILITY_PP_VA_ARGS_TAIL(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0))
#define UTILITY_PP_IS_EMPTY_TRIGGER_PARENTHESIS_(...) ,
#define UTILITY_PP_IS_EMPTY(...) UTILITY_PP_IS_EMPTY_( \
/* test if there is just one argument, eventually an empty one */ \
UTILITY_PP_HAS_COMMA(__VA_ARGS__), \
/* test if _TRIGGER_PARENTHESIS_ together with the argument adds a comma */ \
UTILITY_PP_HAS_COMMA(UTILITY_PP_IS_EMPTY_TRIGGER_PARENTHESIS_ __VA_ARGS__), \
/* test if the argument together with a parenthesis adds a comma */ \
UTILITY_PP_HAS_COMMA(__VA_ARGS__ ()), \
/* test if placing it between _TRIGGER_PARENTHESIS_ and the parenthesis adds a comma */ \
UTILITY_PP_HAS_COMMA(UTILITY_PP_IS_EMPTY_TRIGGER_PARENTHESIS_ __VA_ARGS__ ()))
#define UTILITY_PP_IS_EMPTY_(_0, _1, _2, _3) UTILITY_PP_HAS_COMMA(UTILITY_PP_CONCAT5_(UTILITY_PP_IS_EMPTY_IS_EMPTY_CASE_, _0, _1, _2, _3))
#define UTILITY_PP_IS_EMPTY_IS_EMPTY_CASE_0001 ,
#define UTILITY_PP_VA_ARGS_SIZE(...) UTILITY_PP_IIF(UTILITY_PP_IS_EMPTY(__VA_ARGS__))(0, UTILITY_PP_VA_ARGS_SIZE_(__VA_ARGS__, UTILITY_PP_VA_ARGS_SEQ64()))
#define UTILITY_PP_VA_ARGS_SIZE_(...) UTILITY_PP_IDENTITY(UTILITY_PP_VA_ARGS_TAIL(__VA_ARGS__))
#define UTILITY_PP_VA_ARGS_TAIL(_0,_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,_11,_12,_13,_14, x, ...) x
#define UTILITY_PP_VA_ARGS_SEQ64() 15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0
#define EATER0(...)
#define EATER1(...) ,
#define EATER2(...) (/*empty*/)
#define EATER3(...) (/*empty*/),
#define EATER4(...) EATER1
#define EATER5(...) EATER2
#define MAC0() ()
#define MAC1(x) ()
#define MACV(...) ()
#define MAC2(x,y) whatever
static_assert(UTILITY_PP_VA_ARGS_SIZE() == 0, "1");
static_assert(UTILITY_PP_VA_ARGS_SIZE(/*comment*/) == 0, "2");
static_assert(UTILITY_PP_VA_ARGS_SIZE(a) == 1, "3");
static_assert(UTILITY_PP_VA_ARGS_SIZE(a, b) == 2, "4");
static_assert(UTILITY_PP_VA_ARGS_SIZE(a, b, c) == 3, "5");
static_assert(UTILITY_PP_VA_ARGS_SIZE(a, b, c, d) == 4, "6");
static_assert(UTILITY_PP_VA_ARGS_SIZE(a, b, c, d, e) == 5, "7");
static_assert(UTILITY_PP_VA_ARGS_SIZE((void)) == 1, "8");
static_assert(UTILITY_PP_VA_ARGS_SIZE((void), b, c, d) == 4, "9");
static_assert(UTILITY_PP_VA_ARGS_SIZE(UTILITY_PP_IS_EMPTY_TRIGGER_PARENTHESIS_) == 1, "10");
static_assert(UTILITY_PP_VA_ARGS_SIZE(EATER0) == 1, "11");
static_assert(UTILITY_PP_VA_ARGS_SIZE(EATER1) == 1, "12");
static_assert(UTILITY_PP_VA_ARGS_SIZE(EATER2) == 1, "13");
static_assert(UTILITY_PP_VA_ARGS_SIZE(EATER3) == 1, "14");
static_assert(UTILITY_PP_VA_ARGS_SIZE(EATER4) == 1, "15");
static_assert(UTILITY_PP_VA_ARGS_SIZE(MAC0) == 1, "16");
// a warning in msvc
static_assert(UTILITY_PP_VA_ARGS_SIZE(MAC1) == 1, "17");
static_assert(UTILITY_PP_VA_ARGS_SIZE(MACV) == 1, "18");
// This one will fail because MAC2 is not called correctly
//static_assert(UTILITY_PP_VA_ARGS_SIZE(MAC2) == 1, "19");
c++11,msvc 2015,gcc 4.7.1,clang 3.0
于 2020-06-03T22:23:04.607 回答
0
您可以对令牌进行字符串化和计数：
int countArgs(char *args)
{
int result = 0;
int i = 0;
while(isspace(args[i])) ++i;
if(args[i]) ++result;
while(args[i]) {
if(args[i]==',') ++result;
else if(args[i]=='\'') i+=2;
else if(args[i]=='\"') {
while(args[i]) {
if(args[i+1]=='\"' && args[i]!='\\') {
++i;
break;
}
++i;
}
}
++i;
}
return result;
}
#define MACRO(...) \
{ \
int count = countArgs(#__VA_ARGS__); \
printf("NUM ARGS: %d\n",count); \
}
于 2015-03-20T12:15:18.793 回答
0
Boost Preprocessor 实际上在 Boost 1.49 中具有此功能，如BOOST_PP_VARIADIC_SIZE(...). 它的最大尺寸为 64。
于 2018-12-20T16:27:54.303 回答