Question

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).

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^*: 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.

Answer 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.