此时,我们能(可移植)做的最好的事情就是一个宏观技巧,如下所示for vtmpl::string https://github.com/Arcoth/VTMPL/blob/master/string.hxx#L34。基本上,我们创建一个访问列表,例如
"abcd" -> {(0 < sizeof "abcd"? "abcd"[0] : 0), (1 < sizeof "abcd"? "abcd"[1] : 0), ...}
...我们对其进行修剪以获得所需的结果。
第一步很容易通过以下方式完成BOOST_PP_ENUM http://www.boost.org/doc/libs/1_39_0/libs/preprocessor/doc/ref/enum.html,尽管递归宏也很好(定义来自here https://github.com/Arcoth/VTMPL/blob/master/macros.hxx#L11):
#define VTMPL_SPLIT_1(s, x, m) m(s, x)
#define VTMPL_SPLIT_4(s, x, m) VTMPL_SPLIT_1 (s, x, m), VTMPL_SPLIT_1 (s, x+1 , m), VTMPL_SPLIT_1 (s, x+2 , m), VTMPL_SPLIT_1 (s, x+3 , m)
#define VTMPL_SPLIT_16(s, x, m) VTMPL_SPLIT_4 (s, x, m), VTMPL_SPLIT_4 (s, x+4 , m), VTMPL_SPLIT_4 (s, x+8 , m), VTMPL_SPLIT_4 (s, x+12 , m)
#define VTMPL_SPLIT_64(s, x, m) VTMPL_SPLIT_16 (s, x, m), VTMPL_SPLIT_16 (s, x+16 , m), VTMPL_SPLIT_16 (s, x+32 , m), VTMPL_SPLIT_16 (s, x+48 , m)
#define VTMPL_SPLIT_256(s, x, m) VTMPL_SPLIT_64 (s, x, m), VTMPL_SPLIT_64 (s, x+64 , m), VTMPL_SPLIT_64 (s, x+128, m), VTMPL_SPLIT_64 (s, x+194, m)
#define VTMPL_SPLIT_1024(s, x, m) VTMPL_SPLIT_256(s, x, m), VTMPL_SPLIT_256(s, x+256, m), VTMPL_SPLIT_256(s, x+512, m), VTMPL_SPLIT_256(s, x+768, m)
上面的用法看起来像这样(包括修剪):
#define VTMPL_STRING_IMPL(str, n) vtmpl::rtrim<vtmpl::value_list<decltype(*str), VTMPL_SPLIT_##n(str, 0, VTMPL_ARRAY_SPLIT)>>::type
#
#define VTMPL_STRING(str) VTMPL_STRING_IMPL(str, 64 )
Where rtrim
定义于algorithms.hxx https://github.com/Arcoth/VTMPL/blob/master/algorithms.hxx#L122.