Why unordered_set doesn't allow vector as key?

Question

For example:

vector<int> v = {1, 2, 3};
unordered_set<vector<int>> s;
s.insert(v);

will gives error

no match for call to '(const std::hash >) (const std::vector&)' noexcept(declval()(declval()))>

A vector can contain lots of types, so it's hard to generalize a hash function for it. And there's not much call for it. — Mark Ransom, Sep 27 '18 at 03:08
You can supply your own hash function, but depending on your data it's likely not trivial to do so. — Retired Ninja, Sep 27 '18 at 03:09

score 1 · Answer 1 · answered Sep 27 '18 at 03:10

Because most of the C++ containers don't? vector<bool> appears to have an explicit specialization, but that's the unusual case; typically, they didn't provide std::hash overloads for aggregate or collection types (possibly because they didn't want to lock in a hash combining algorithm applicable to arbitrary templated types).

Why unordered_set doesn't allow vector as key?

1 Answers1