c/c++:计算可变参数宏 __VA_ARGS__ 的参数个数

墨蓝 2022-05-22 09:46 287阅读 0赞

如何计算可变参数`__VA_ARGS__`中的参数个数?  
google上找了一下，网上答案不少，仔细看看原理都是一样的，比如这篇stackoverflow上的问题，给出的答案真不少，  
\[《C++ preprocessor `__VA_ARGS__` number of arguments》\]\[1\]

但是排名第一的最佳答案有bug,在参数为空的时候计算错误。经测试找到正确计算方式，自己写了一个，代码如下，gcc上测试通过(MSV不支持)

// 计算 __VA_ARGS__ 参数个数,最大支持64个参数
    #define FL_ARG_COUNT(...) FL_INTERNAL_ARG_COUNT_PRIVATE(0, ##__VA_ARGS__,\
    	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 FL_INTERNAL_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, N, ...) N
    	
    // 测试代码 
    FL_ARG_COUNT()					->0
    FL_ARG_COUNT(1)					->1
    FL_ARG_COUNT(1,2)				->2
    FL_ARG_COUNT(1,2,3)				->3
    FL_ARG_COUNT(1,2,3,4)			->4
    FL_ARG_COUNT(1,2,3,4,5)			->5
    FL_ARG_COUNT(1,2,3,4,5,6)		->6
    FL_ARG_COUNT(1,2,3,4,5,6,7)		->7
    FL_ARG_COUNT(1,2,3,4,5,6,7,8)	->8
    FL_ARG_COUNT(1,2,3,4,5,6,7,8,9)	->9
    FL_ARG_COUNT(1,2,3,4,5,6,7,8,9,10)	->10
    FL_ARG_COUNT(\
    		1,2,3,4,5,6,7,8,9,10,\
    		1,2,3,4,5,6,7,8,9,10,\
    		1,2,3,4,5,6,7,8,9,10,\
    		1,2,3,4,5,6,7,8,9,10,\
    		1,2,3,4,5,6,7,8,9,10,\
    		1,2,3,4,5,6,7,8,9,10,\
    		1,2,3,4\
    		)                      ->64

# 参考资料 #

\[《C++ preprocessor `__VA_ARGS__` number of arguments》\]\[1\]  
\[《Macros with a Variable Number of Arguments.》\]\[2\]  
\[1\]:[https://stackoverflow.com/questions/2124339/c-preprocessor-va-args-number-of-arguments][https_stackoverflow.com_questions_2124339_c-preprocessor-va-args-number-of-arguments]  
\[2\]:[https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/Variadic-Macros.html\#Variadic-Macros][https_gcc.gnu.org_onlinedocs_gcc-5.2.0_gcc_Variadic-Macros.html_Variadic-Macros]

[https_stackoverflow.com_questions_2124339_c-preprocessor-va-args-number-of-arguments]: https://stackoverflow.com/questions/2124339/c-preprocessor-va-args-number-of-arguments
[https_gcc.gnu.org_onlinedocs_gcc-5.2.0_gcc_Variadic-Macros.html_Variadic-Macros]: https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/Variadic-Macros.html#Variadic-Macros