Re: [Boost-users] Boost 1.68.0 - boost hashing changed ?

Subject: Re: [Boost-users] Boost 1.68.0 - boost hashing changed ?
From: degski (degski_at_[hidden])
Date: 2018-10-23 08:19:39

On Tue, 23 Oct 2018 at 08:45, Miguel Ojeda via Boost-users <
boost-users_at_[hidden]> wrote:

> On Mon, Oct 22, 2018 at 7:57 PM Shailja Prasad via Boost-users
> <boost-users_at_[hidden]> wrote:
> >
> > I was trying to upgrade boost 1.53.0 to boost 1.68.0. But, it looks like
> hashing code generation has changed, since the following line gives two
> different hashcode for same string input.
>
> Hm... why would you expect the hash to be always the same between
> releases, compilers, etc.?

Well, uhm, because that seems to be quite handy. All NIST implementations
do exactly this.

I cannot find it with a quick look at
> Boost.Hash's docs anything regarding a guarantee of that. If it is
> like std::hash, then it is only guaranteed to remain equal for the
> duration of the program.

Sort of: "Hash functions are only required to produce the same result for
the same input within a single execution of a program". The standard states
a minimum requirement [with an intended [narrow] use case in mind,
std::ordered_map's]. I'm not sure that is a great one and by the time we
might [would like to] have constexpr std::ordered_map maybe not even
tenable.

> In other words, you cannot rely on saving it
> nor comparing them to other hashes from other vendors, platforms,
> architectures, compiler releases, etc.
>

In my view this is an omission, the option to have exactly that should
[have been] available.

degski
-- 
*“If something cannot go on forever, it will stop" - Herbert Stein*