Re: [hash2][review] Early review (due to holidays)

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2024-12-06 14:17:01

On 12/6/24 16:47, Peter Dimov via Boost wrote:
> Andrey Semashev wrote:
>> I also had compile-time dependencies in mind when I said "lightweight".
>> To take the example from the docs:
>>
>> https://pdimov.github.io/hash2/doc/html/hash2.html#hashing_objects_user_defined_types
>>
>>   class X
>>   {
>>   private:
>>     std::string a;
>>     int b;
>>
>>   // ...
>>   public:
>>     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);
>>     }
>>   };
>>
>> This requires one to depend on Boost.Hash2 for hash_append_tag and
>> hash_append. I would prefer if there was a way (properly documented and
>> supported) to avoid this dependency entirely.
> 
> Once things stabilize, you will be allowed to copy hash_append_fwd.hpp
> locally (or just declare the functions yourself.)
> 
> This has always been the intent, but GCC bugs and BOOST_CXX14_CONSTEXPR
> complicated matters.
> 
> As I mentioned, we briefly considered requiring C++14 so that the latter isn't
> an issue, but the feedback on Slack was negative. People love C++11 for some
> reason or other. Either way, this still wouldn't have solved the GCC issue.
> 
> https://github.com/pdimov/hash2/blob/develop/include/boost/hash2/hash_append_fwd.hpp

Forward declarations are kind of fragile and they limit future evolution
of the library. And they are tedious to add, considering that they would
be needed everywhere where hashing support is needed. I would rather
prefer the approach that doesn't need forward declarations.