Yet another answer on this overly-answered question, which I'm just including for completeness. Between all of the answers here you should find something that works in your application.
So another option is a lookup table:
// On initialization:
bool isAcceptable[256] = { false };
isAcceptable[(unsigned char)'A'] = true;
isAcceptable[(unsigned char)'B'] = true;
isAcceptable[(unsigned char)'C'] = true;
// When you want to check:
char c = ...;
if (isAcceptable[(unsigned char)c]) {
// it's 'A', 'B', or 'C'.
}
Scoff at the C-style static casts if you must, but they do get the job done. I suppose you could use an std::vector if arrays keep you up at night. You can also use types besides bool. But you get the idea.
Obviously this becomes cumbersome with e.g. wchar_t, and virtually unusable with multibyte encodings. But for your char example, or for anything that lends itself to a lookup table, it'll do. YMMV.
