So we have a use-case: you want to create a container of both Qt and Wx
widgets. A safe solution that you can use today:

```C++

std::vector<std::variant<QWindow, wxWidget>> g_all_windows;

for (auto& p : g_all_windows) {
  if(auto* p = std::get_if<QWindow>(&p); p != nullptr) DoSomething();
  else if(auto* p = std::get_if<wxWidget>(&p); p != nullptr)
DoSomethingElse();
}
```

// Robin

On Tue, Jun 10, 2025, 11:30 Frederick Virchanza Gotham via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Tue, Jun 10, 2025 at 12:07 AM Oliver Hunt <oliver_at_[hidden]> wrote:
> >
> > Not sure of the MS implementation, but in stdlibc++ and libc++
> > __dynamic_cast assumes that the source type_info and the object has
> > already been adjusted to support the path to the target type.
>
>
> If needed, I'm happy to take the implementation of '__dynamic_cast'
> from those libraries and tweak it if needed. But it might work as is,
> I'll test and see.
>
>
> > The fundamental problem that you seem unwilling to understand that the
> > c++ type system does not have a polymorphic bottom type, and what you’re
> > doing is essentially trying to create one by hacking around assumptions
> in
> > the type system.
>
>
> I don't know if you're a Star Trek fan, but in Picard's words:
>
> "Space: the final frontier. These are the voyages of the
> starship Enterprise. Its continuing mission: to explore strange
> new worlds; to seek out new life and new civilizations; to
> boldly go where no man has gone before!"
>
> On the day that I discovered on the GNU g++ compiler that the vtable
> pointer is always at address [base + 0x00] in any polymorphic object,
> I started to explore. And I started to realise that a lot more is
> possible with polymorphic objects than the C++ Standard is letting on.
> I'm happy to learn little by little here, and to add more and more to
> 'polyhandle'. It was actually only yesterday that I started thinking
> that maybe I could perform a 'dynamic_cast' from a ' void * '. (Note
> that that previous sentence says 'from' instead of 'to').
>
> As you said, C++ hasn't got a polymorphic bottom type. So let's say
> that I'm writing a program that uses wxWidgets for some of the
> windows, and Qt for some of the other windows. Let's say I have a
> global container of all the windows in my program:
>
> std::vector< polyhandle > g_all_windows;
>
> It would be good if I could, at runtime, check if it's a QWindow or a
> wxWindow:
>
> for ( polyhandle p : g_all_windows )
> {
> if ( p.dynamicCast<QWindow> ) DoSomething();
> else if ( p.dynamicCast<wxWidget> ) DoSomethingElse();
> }
>
> I'm taking a look at two ABI's here: The Itanium ABI and the Microsoft
> ABI, to see what can be made possible in both.
> --
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>