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Subject: Re: [boost] [hash] regular behaviour of hash function for double values
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-02-01 10:11:27

on Tue Jan 31 2012, Daniel James <> wrote:

> On 31 January 2012 17:57, Andrey Semashev <andrey.semashev_at_[hidden]> wrote:
>> On Tuesday, January 31, 2012 18:33:25 Thomas Klimpel wrote:
>>> > but my understanding is that it's the hash function's job.
>>> What do you mean by "my understanding"? Your understanding of the standard,
>>> or your understanding of the best solution, or your understanding of that
>>> conversation? You also advocated your "understanding" during that
>>> conversation.
>> The standard doesn't favor or preclude either approach, so either way the
>> implementation would be conforming. So, you might say it's my understanding of
>> both the standard and the "best" solution. I realize that my opponent had a
>> different opinion on this and I didn't convince him.
> The requirements for the hash function are:
> For two different values
> t1 and t2, the probability that h(t1) and h(t2) compare
> equal should be very small, approaching 1.0 / numeric_-
> limits<size_t>::max().
> There's no requirement about how h(t1) and h(t2) are distributed so it
> must be the container's responsibility, since the container must work
> well with any hash function that meets these requirements. How else
> can you interpret it?

You can interpret it as a QOI issue, and we should aim to provide the
highest QOI.

Dave Abrahams
BoostPro Computing

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