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.
@@ -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.
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. Not if the conversion
from _Kt to _Arg is noexcept.
