map (vs) unordered_map with pair as key

Question

This produces compiler error

unordered_map<pair<int,int>,int> umap;

I understand that a hash function must be provided for it to work.

But

map<pair<int,int>,int> omap;

works perfectly without any hash function.

Why is that so?

Answer 1

std::map does not use a hash function. Rather, its "keys are sorted by using the comparison function..." where "search, removal, and insertion operations have logarithmic complexity.". Lookup is done by traversing a search tree rather than via hashing.