Why can't I replace std::map with std::unordered_map

Question

This question might be a bit sketchy because I do not have the code available at home, but I know this thing otherwise will bug me the whole weekend.

When I tried to

Answer 1

I guess that because std::unordered_map needs to rehash, and therefore copy elements, the types need to be complete, whereas a map, only ever working with pointers to elements, will not exhibit that problem.

The solution here is to have an unordered map to a pointer:

std::unordered_map >.