Please x 20 do NOT rely on ASCII. Don't mention it. Don't assume it is used. Don't even think about it.
The C++ standard does not guarantee what the "underlying character set" is. It could be ASCII, but it could also be something entirely different.
Furthermore, ASCII is an old american-only system. It needs to go away. Unicode is the new future as it is now.
So again, don't assume ASCII, and throw away your ASCII table.