Re: [boost] [hash] Extract module from functional + std::hash_combine

Subject: Re: [boost] [hash] Extract module from functional + std::hash_combine
From: Richard Hodges (hodges.r_at_[hidden])
Date: 2017-12-19 14:12:38

IMHO

boost::hash is perfect, in every respect. Follow that (including all the
missing functionality in std::hash - particularly ADL-friendly hash_value())
and you won't go wrong.

#include <hash> is fine by me. If we ever want any other kind of hash,
surely is could go into #include <md5> or #include <sha256> etc.

until std::hash >= boost::hash, it's not useful.

In addition, I would propose (I am sure I'm not the first!):

namespace std
{
  template<class T = void> struct hash { ... };

  template<>
  struct hash<void>
  {
    template<class T>
    auto operator()(T&& arg) const {
      return hash_value(arg);  // ADL to the rescue
    }
  };
}

and then:

template<class K, class V, class H = std::hash<>, class E = std::equal<>>
struct unordered_map; // std::equal<V1 = void, V2 = void> should of course
compare any 2 comparable things

Which finally makes unordered_map declarations sane.

Whoever decided to downgrade boost hash before solidifying its uselessness
in the c++11 standard?

Anticipating: but but, backward compatibility...

Answer: nostalgia is the death of innovation. Look forward my son...

On 19 December 2017 at 14:31, Daniel James via Boost <boost_at_[hidden]>
wrote:

> Hi,
>
> I'm planning to extract hash from functional into its own module, as
> that will apparently break the circular dependencies that functional
> is part of. Any objections? Also, any opinions on what the module
> should be called? 'hash' might suggest a more general purpose hash
> function, so maybe it should be called something more specific.
>
> Daniel
>
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