Re: hash_value( complex<T> )

From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2021-10-16 23:16:58

Given two values x and y, if x == y, it is convenient for hash(x) == hash(y).

In this situation, given a floating point value r, complex{r) == r, so it makes sense that hash(complex{r}) == hash(r).  This might enable heterogeneous hash table lookup.

Howard

On Oct 16, 2021, at 6:44 PM, Peter Dimov via Boost <boost_at_[hidden]> wrote:
> 
> At the moment, boost::hash_value for complex<T> is specified as
> 
>    When T is a built in type and val.imag() == 0, the result is equal
>    to hash_value(val.real()). Otherwise an unspecified value, except
>    that equal arguments shall yield the same result.
> 
> This is a bit odd; the natural implementation for a two-argument
> point/tuple type is
> 
>        std::size_t seed = 0;
> 
>        boost::hash_combine( seed, v.real() );
>        boost::hash_combine( seed, v.imag() );
> 
>        return seed;
> 
> Can anyone offer an opinion on whether or why the original behavior
> is preferable?
> 
> 
> 
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