c/c++:计算可变参数宏 __VA_ARGS__ 的参数个数

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发布于 2019-05-25 21:48:32

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发布于 2019-05-25 21:48:32

文章被收录于专栏：10km的专栏10km的专栏

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》

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