Re: Possible to declare move constructor of basic_symbol as noexcept?

[Adrian]

I meant potentially throwing destructor* (is a bad thing to do)

On Thu, Jan 7, 2021 at 4:40 AM Adrian <withoutpointk@gmail.com> wrote:
>
> Hi Akim,
>
> Yes, that was what I was referring to. Since
> basic_symbol(basic_symbol&&) calls semantic_type::move<T>, a concern
> is if that might throw. But I checked semantic_type::move<T> calls
> semantic_type::emplace<T, U...> which is what calls the constructor,
> semantic_type::as<T> which is already declared noexcept, and
> semantic_type::destroy<T> which calls the destructor of T, which I
> think could be throwing, but pretty sure having a potentially throwing
> constructor is a Very Bad Thing To Do.
>
> Another concern is the need for the <type_traits> header for
> std::is_nothrow_move_constructible, which is/can be implemented in
> terms of std::is_nothrow_constructible, which I don't think can be
> implemented without intrinsics? Since Bison currently doesn't depend
> on (as far as I could find) <type_traits>, introducing this dependency
> for such a niche case is probably undesirable. Maybe it could only be
> enabled with a Bison declaration, but then that's extra maintenance
> baggage. So I wasn't sure if this would be something you would be open
> to
>
> Thanks for your time!
>
> Kind regards,
> Adrian
>
> On Thu, Jan 7, 2021 at 12:30 AM Akim Demaille <akim@lrde.epita.fr> wrote:
> >
> > Hi Adrian,
> >
> > > Le 6 janv. 2021 à 20:43, Adrian <withoutpointk@gmail.com> a écrit :
> > >
> > > Hello,
> > >
> > > Is there any possibility we can declare the move constructor of
> > > basic_symbol as noexcept (if the semantic_types are noexcept
> > > moveable)?
> > >
> > > I have a semantic value that is non-copyable (moveable only) and I
> > > want an std::vector<symbol_type>. std::vector only calls the move
> > > constructor when resizing if the move constructor is declared noexcept
> > >
> > > It seems to me that the issue might be that you would require all of
> > > the semantic types to be noexcept moveable. You would need a
> > > (potentially huge) noexcept predicate, but since it's generated code
> > > maybe it's ok
> >
> > My C++ is rusting.  What exactly are you asking for?  From
> >
> > > #if 201103L <= YY_CPLUSPLUS
> > >       /// Move constructor.
> > >       basic_symbol (basic_symbol&& that)
> > >         : Base (std::move (that))
> > >         , value ()
> > >       {
> > >         switch (this->kind ())
> > >     {
> > >       case symbol_kind::S_NUMBER: // NUMBER
> > >         value.move< int > (std::move (that.value));
> > >         break;
> > >
> > >       case symbol_kind::S_TEXT: // TEXT
> > >       case symbol_kind::S_item: // item
> > >         value.move< std::string > (std::move (that.value));
> > >         break;
> > >
> > >       case symbol_kind::S_list: // list
> > >         value.move< std::vector<std::string> > (std::move (that.value));
> > >         break;
> > >
> > >       default:
> > >         break;
> > >     }
> > >
> > >       }
> > > #endif
> >
> > to go to
> >
> > > #if 201103L <= YY_CPLUSPLUS
> > >       /// Move constructor.
> > >       basic_symbol (basic_symbol&& that)
> > >         noexcept (
> > >           std::is_nothrow_move_constructible< int >::value
> > >           && std::is_nothrow_move_constructible< std::string >::value
> > >           && std::is_nothrow_move_constructible< std::vector<std::string> 
> > > >::value
> > >
> > >         )
> > >         : Base (std::move (that))
> > >         , value ()
> >
> > right?