问题
I have a variadic function from a third-party C library:
int func(int argc, ...);
argc indicates the number of passed optional arguments.
I'm wrapping it with a macro that counts the number of arguments, as suggested here. For reading convenience, here's the macro:
#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
#define PP_NARG_(...) PP_ARG_N(__VA_ARGS__)
#define PP_NARG(...) PP_NARG_(__VA_ARGS__, PP_RSEQ_N())
and I'm wrapping it like so:
#define my_func(...) func(PP_NARG(__VA_ARGS__), __VA_ARGS__)
The PP_NARG macro works great for functions accepting one or more arguments. For instance, PP_NARG("Hello", "World") evaluates to 2.
The problem is that when no arguments are passed, PP_NARG() evaluates to 1 instead of 0.
I understand how this macro works, but I can't come up with an idea to modify it so that it behaves correctly for this case as well.
Any ideas?
EDIT:
I have found a workaround for PP_NARG, and posted it as an answer.
I still have problems with wrapping the variadic function though. When __VA_ARGS__ is empty, my_func expands to func(0, ) which triggers a compilation error.
回答1:
Another possibility, which does not use sizeof nor a GCC extension is to add the following to your code
#define PP_COMMASEQ_N() \
1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 0, 0
#define PP_COMMA(...) ,
#define PP_HASCOMMA(...) \
PP_NARG_(__VA_ARGS__, PP_COMMASEQ_N())
#define PP_NARG(...) \
PP_NARG_HELPER1( \
PP_HASCOMMA(__VA_ARGS__), \
PP_HASCOMMA(PP_COMMA __VA_ARGS__ ()), \
PP_NARG_(__VA_ARGS__, PP_RSEQ_N()))
#define PP_NARG_HELPER1(a, b, N) PP_NARG_HELPER2(a, b, N)
#define PP_NARG_HELPER2(a, b, N) PP_NARG_HELPER3_ ## a ## b(N)
#define PP_NARG_HELPER3_01(N) 0
#define PP_NARG_HELPER3_00(N) 1
#define PP_NARG_HELPER3_11(N) N
The result is
PP_NARG() // expands to 0
PP_NARG(x) // expands to 1
PP_NARG(x, 2) // expands to 2
Explanation:
The trick in these macros is that PP_HASCOMMA(...) expands to 0 when called with zero or one argument and to 1 when called with at least two arguments. To distinguish between these two cases, I used PP_COMMA __VA_ARGS__ (), which returns a comma when __VA_ARGS__ is empty and returns nothing when __VA_ARGS__ is non-empty.
Now there are three possible cases:
__VA_ARGS__is empty:PP_HASCOMMA(__VA_ARGS__)returns 0 andPP_HASCOMMA(PP_COMMA __VA_ARGS__ ())returns 1.__VA_ARGS__contains one argument:PP_HASCOMMA(__VA_ARGS__)returns 0 andPP_HASCOMMA(PP_COMMA __VA_ARGS__ ())returns 0.__VA_ARGS__contains two or more arguments:PP_HASCOMMA(__VA_ARGS__)returns 1 andPP_HASCOMMA(PP_COMMA __VA_ARGS__ ())returns 1.
The PP_NARG_HELPERx macros are just needed to resolve these cases.
Edit:
In order to fix the func(0, ) problem, we need to test whether we have supplied zero
or more arguments. The PP_ISZERO macro comes into play here.
#define PP_ISZERO(x) PP_HASCOMMA(PP_ISZERO_HELPER_ ## x)
#define PP_ISZERO_HELPER_0 ,
Now let's define another macro which prepends the number of arguments to an argument list:
#define PP_PREPEND_NARG(...) \
PP_PREPEND_NARG_HELPER1(PP_NARG(__VA_ARGS__), __VA_ARGS__)
#define PP_PREPEND_NARG_HELPER1(N, ...) \
PP_PREPEND_NARG_HELPER2(PP_ISZERO(N), N, __VA_ARGS__)
#define PP_PREPEND_NARG_HELPER2(z, N, ...) \
PP_PREPEND_NARG_HELPER3(z, N, __VA_ARGS__)
#define PP_PREPEND_NARG_HELPER3(z, N, ...) \
PP_PREPEND_NARG_HELPER4_ ## z (N, __VA_ARGS__)
#define PP_PREPEND_NARG_HELPER4_1(N, ...) 0
#define PP_PREPEND_NARG_HELPER4_0(N, ...) N, __VA_ARGS__
The many helpers are again needed to expand the macros to numeric values. Finally test it:
#define my_func(...) func(PP_PREPEND_NARG(__VA_ARGS__))
my_func() // expands to func(0)
my_func(x) // expands to func(1, x)
my_func(x, y) // expands to func(2, x, y)
my_func(x, y, z) // expands to func(3, x, y, z)
Online example:
http://coliru.stacked-crooked.com/a/73b4b6d75d45a1c8
See also:
Please have also a look at the P99 project, which has much more advanced preprocessor solutions, like these.
回答2:
I came up with the following workaround for PP_NARG:
#define PP_NARG(...) (sizeof(#__VA_ARGS__) - 1 ? \
PP_NARG_(__VA_ARGS__, PP_RSEQ_N()) : 0)
It stringifies __VA_ARGS__, so if it's empty its length equals 1 (because #__VA_ARGS__ == '\0').
It works with -std=c99 -pedantic.
I still have problems with wrapping the variadic function though. When __VA_ARGS__ is empty, my_func expands to func(0, ) which triggers a compilation error.
回答3:
It is possible to do in GCC using the ##VA_ARGS extension:
#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
/* Note 63 is removed */
#define PP_RSEQ_N() \
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 PP_NARG_(...) PP_ARG_N(__VA_ARGS__)
/* Note dummy first argument _ and ##__VA_ARGS__ instead of __VA_ARGS__ */
#define PP_NARG(...) PP_NARG_(_, ##__VA_ARGS__, PP_RSEQ_N())
#define my_func(...) func(PP_NARG(__VA_ARGS__), __VA_ARGS__)
Now PP_NARG(a, b, c) gives 3 and PP_NARG() gives 0.
Unfortunately I don't see a way to make it work in general.
回答4:
The full example on Mehrwolf's answer unfortunately doesn't compile on VS2010 nor it does on VS2015 (i tried it also on this online VS compiler). A different approach on this problem that allows to produce cross-platform code that compiles on a wide range of compilers is using non-standard extensions that are also available in MS compilers as in my answer to a similar question. Also, differently form the cited answer, the result of PP_NARG((a, b, c)) using the extensions (in both MS and gcc/clang compilers) is 1 and not 0.
来源：https://stackoverflow.com/questions/11317474/macro-to-count-number-of-arguments