Re: [hash2] Comments

From: Vinnie Falco (vinnie.falco_at_[hidden])
Date: 2024-12-07 01:03:50

On Fri, Dec 6, 2024 at 4:21 PM Peter Dimov via Boost <boost_at_[hidden]>
wrote:

> A std::hash specialization isn't a good way to do it because you can't pass
> an initial seed, which is a poor practice in 2025.
>

std::hash only requires DefaultConstructible, CopyConstructible, and
Destructible. And it doesn't say anything about not also being
constructible with additional parameters. Couldn't this be made to work:

    struct std::hash<MyT> {
      hash( std::size_t seed );
      ...

All std containers allow construction from an instance of a suitable hash
function, and they take ownership by copy:

https://en.cppreference.com/w/cpp/container/unordered_map/unordered_map

It does look like specializations of std::hash for user-defined types can
construct from seeds. Disclaimer: I did not actually read the text of the
standard, and only looked at cppreference.com.

Thanks