Is this an acceptable use of "ASCII arithmetic"?

Question

I've got a string value of the form 10123X123456 where 10 is the year, 123 is the day number within the year, and the rest is unique system-generated stuff. Under certain circumstances, I need to add 400 to the day number, so that the number above, for example, would become 10523X123456.

My first idea was to substring those three characters, convert them to an integer, add 400 to it, convert them back to a string and then call replace on the original string. That works.

But then it occurred to me that the only character I actually need to change is the third one, and that the original value would always be 0-3, so there would never be any "carrying" problems. It further occurred to me that the ASCII code points for the numbers are consecutive, so adding the number 4 to the character "0", for example, would result in "4", and so forth. So that's what I ended up doing.

My question is, is there any reason that won't always work? I generally avoid "ASCII arithmetic" on the grounds that it's not cross-platform or internationalization friendly. But it seems reasonable to assume that the code points for numbers will always be sequential, i.e., "4" will always be 1 more than "3". Anybody see any problem with this reasoning?

Here's the code.

string input = "10123X123456";
input[2] += 4;
//Output should be 10523X123456

Answer 1

From the C++ standard, section 2.2.3:

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.

So yes, if you're guaranteed to never need a carry, you're good to go.