Why is std::hash not guaranteed to be deterministic?

Question

Hereafter, we use N4140 (C++14 Standard).

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According to § 17.6.3.4 Hash requirements,

The value returned shall depend only on t

Answer 1

There is no need for the hash function to be deterministic between runs, but you can still provide your own hash, e.g. for unordered containers if it's a behavior you rely on.

As for why, cppreference says:

Hash functions are only required to produce the same result for the same input within a single execution of a program; this allows salted hashes that prevent collision denial-of-service attacks.

If the Hash requirements tells it to be deterministic, then you wouldn't be able to provide a salted hash without breaking the requirement.

Here is the actual explanation why