Tagging/Encoding Pointers

Question

I need a way to tag a pointer as being either part of set x or part of set y (ie: the tag has only 2 \'states\'), I\'m that means one can assume untagged = x and tagged = y.

Answer 1

If performance is not a big issue, two std::set's can be used.

If it's important to get this information quickly, and it's acceptable to use only 2-byte aligned pointers, the lowest bit can be used to store this information. But having "hacked" pointers may appear to be quite error-prone...