字符数字['0'...'9']需要具有连续的数字值吗? [英] Are the character digits ['0'..'9'] required to have contiguous numeric values?
问题描述
必须使用C ++实现将chars'0' - '9'设置为具有连续的数值,即:
Must a C++ implementation set the chars '0'-'9' to have contiguous numeric values, i.e. so that:
'0' -> 0+n
'1' -> 1+n
m -> m+n
'9' -> 9+n
我找不到在 isdigit
([classification](22.3.3.1字符分类)) * ,
也不能在locale文档中找到它(但也许我看起来不够强)。
I cannot find it mentioned in the documentation of isdigit
([classification] (22.3.3.1 Character classification)) *,
nor can I find it in the locale documentation (but maybe I did not look hard enough).
在2.3个字符集中,我们发现
In 2.3 Character sets, we find that
基本源字符集由96个字符组成:空格字符,表示
水平制表符,垂直制表符,换页符和换行符的控制字符,以及以下91个图形字符
The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters
但它没有提到任何顺序(但也许我没有看起来很难)。
But it doesn't mention any ordering (but maybe I did not look hard enough).
* :有趣的脚注:
在循环中使用时,缓存ctype<> facet并直接使用它(而不是isdigit()等,end comment],或使用ctype<> :: is的向量形式。
When used in a loop, it is faster to cache the ctype<> facet and use it directly [instead of isdigit() et al, end comment], or use the vector form of ctype<>::is.
推荐答案
确实没有看起来很难:在2.3。字符集,项目3:
Indeed not looked hard enough: In 2.3. Character sets, item 3:
在源和执行基本字符集中,在十进制数字列表之上的
应为 上一个的值一个。 b $ b
In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.
这是上面的小数位数列表:
And this is above list of decimal digits:
0 1 2 3 4 5 6 7 8 9
因此,实现必须使用字符集十进制数字具有连续的表示。因此,您依赖此属性的优化是安全的;然而,您依赖于其他数字(例如'a'..'z')的一致性的优化是不是便携式w.r.t.到标准(另见标题< cctype>
)。如果这样做,请确保声明该属性。
Therefore, an implementation must use a character set where the decimal digits have a contiguous representation. Thus, optimizations where you rely on this property are safe; however, optimizations where you rely on the coniguity of other digits (e.g. 'a'..'z') are not portable w.r.t. to the standard (see also header <cctype>
). If you do this, make sure to assert that property.
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