Re: Formal review request for Hash2

From: Ivan Matek (libbooze_at_[hidden])
Date: 2024-12-02 21:35:04

On Mon, Dec 2, 2024 at 9:59 PM Ivan Matek <libbooze_at_[hidden]> wrote:

>
> Read documentation for 20 minutes so apologies in advance is some of
> questions are too obvious.
>
> 1) did you consider using something like std::span here? I presume answer
> is yes, but you do not want to limit library to C++20 onwards.
>
> HashAlgorithm( unsigned char const* seed, std::size_t n ); void update(
> void const* data, std::size_t n );
>
> 3) I see there is nice trait is_contiguously_hashable trait(padding was
> one of first things I wondered when reading docs), and also a documentation
> on how to deal with other types, including when user provides his own hash
> customization.
> One thing I would ask regarding design is the following example:
> std::string a; int b; // not part of the salient state void const* c;
> friend bool operator==( X const& x1, X const& x2 ) { return x1.a == x2.a
> && x1.b == x2.b; } template<class Hash, class Flavor> friend void
> tag_invoke( boost::hash2::hash_append_tag const&, Hash& h, Flavor const& f
> , X const& v ) { boost::hash2::hash_append(h, f, v.a); boost::hash2::
> hash_append(h, f, v.b); }
> Here both tag_invoke and operator ==  need to remember to use same
> fields.
> Would it be possible to have support so that both hash and operator ==
> can use member function helper that returns tuple of references( to salient
> fields)?
> e.g. assume this is my current code and I want to add hash support for it.
>
> struct X {
> int a;
> short b;
> short cached_something;
> constexpr auto repr() const {
> return std::tie(a,b);
> }
> constexpr bool operator == (const X& other) const {
> return repr() == other.repr();
> }
> };
>
> I know Mp11 has tuple_for_each, but I wonder if it would be nice to have
> something that makes this idiom easier to use?
>
>
>>
To answer my own questions:
1)  this is never directly called by users so no need for safer API
3) there is overload of hash_append that works for tuples, nice

Sorry for noise.

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