On Fri, 21 May 2021, 07:31 François Dumont via Libstdc++, < libstd...@gcc.gnu.org> wrote:
> On 20/05/21 6:44 pm, Jonathan Wakely wrote: > > On 06/05/21 22:03 +0200, François Dumont via Libstdc++ wrote: > >> Hi > >> > >> Considering your feedback on backtrace in debug mode is going to > >> take me some time so here is another one. > >> > >> Compared to latest submission I've added a _Hash_arg_t partial > >> specialization for std::hash<>. It is not strictly necessary for the > >> moment but when we will eventually remove its nested argument_type it > >> will be. I also wonder if it is not easier to handle for the > >> compiler, not sure about that thought. > > > > The std::hash specializations in libstdc++ define argument_type, but > > I'm already working on one that doesn't (forstd::stacktrace). > > > > And std::hash<acme::ProgramDefinedType> can be specialized by users, > > and is not required to provide argument_type. > > > > So it's already not valid to assume that std::hash<T>::argument_type > > exists. > > Yes, I know that the plan is to get rid of argument_type. But as long as > it is there we can still use it even if I didn't realize that you were > already in the process of removing it so soon. > I'm adding a new specialization for a C++23 type, and not adding the nested types. > > > > >> @@ -850,9 +852,56 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > >> iterator > >> _M_emplace(const_iterator, false_type __uks, _Args&&... __args); > >> > >> + template<typename _Kt, typename _Arg, typename _NodeGenerator> > >> + std::pair<iterator, bool> > >> + _M_insert_unique(_Kt&&, _Arg&&, const _NodeGenerator&); > >> + > >> + // Detect nested argument_type. > >> + template<typename _Kt, typename _Ht, typename = __void_t<>> > >> + struct _Hash_arg_t > >> + { typedef _Kt argument_type; }; > >> + > >> + // std::hash > >> + template<typename _Kt, typename _Arg> > >> + struct _Hash_arg_t<_Kt, std::hash<_Arg>> > >> + { typedef _Arg argument_type; }; > >> + > >> + // Nested argument_type. > >> + template<typename _Kt, typename _Ht> > >> + struct _Hash_arg_t<_Kt, _Ht, > >> + __void_t<typename _Ht::argument_type>> > >> + { typedef typename _Ht::argument_type argument_type; }; > >> + > >> + // Function pointer. > >> + template<typename _Kt, typename _Arg> > >> + struct _Hash_arg_t<_Kt, std::size_t(*)(const _Arg&)> > >> + { typedef _Arg argument_type; }; > >> + > >> + template<typename _Kt, > >> + typename _ArgType > >> + = typename _Hash_arg_t<_Kt, _Hash>::argument_type> > >> + static typename conditional< > >> + __is_nothrow_convertible<_Kt, _ArgType>::value, _Kt&&, > >> key_type>::type > > > > Please use __conditional_t<...> here instead of > > typename conditional<...>::type. > > > > The purpose of the _Hash_arg_t type is to determine whether invoking > > the hash function with _Kt&& can throw, right? > > No, the purpose of _Hash_arg_t is to find out what is the argument type > of the _Hash functor. With this info I can check if invoking that > functor is going to instantiate a temporary using a throwing operation. Right, that's what I mean. > If so, I create a temporary at _Hashtable code level and move it to its > final storage place when needed. > > The tricky part is that _Hash can accept different argument types, for > the moment I just do not create a temporary in this case. > > > > > And if it can throw, you force a conversion early, and if it can't, > > you don't do the conversion. > > > > Can't you use __is_nothrow_invocable<_Hash&, _Kt> for that, instead of > > this fragile approach? > > > I think I already try but I'll check. > > I fear that __is_nothrow_invocable<_Hash&, _Kt> tells if the chosen > operator()(const _Arg&) is noexcept qualified. No. Not if the conversion > from _Kt to _Arg is noexcept. > It does exactly what you need. > >