问题
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
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).
In 2.3 Character sets, we find that
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
But it doesn't mention any ordering (but maybe I did not look hard enough).
*: Interesting footnote there:
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.
回答1:
Indeed not looked hard enough: In 2.3. Character sets, item 3:
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.
And this is above list of decimal digits:
0 1 2 3 4 5 6 7 8 9
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.
来源：https://stackoverflow.com/questions/9416926/are-the-character-digits-0-9-required-to-have-contiguous-numeric-values