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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