How does inheritance and vtables work wrt. C++ and interop with D? Fns w/ Multiple-inheritance args impossible to bind to?

[evilrat]

On Tuesday, 25 May 2021 at 17:52:14 UTC, Gavin Ray wrote:
>
> ```d
> void main()
> {
>   Derived dlangDerived = new Derived(123);
>   printf("[D] Derived.Base1::getSomething() = %d \n", 
> dlangDerived.getSomething());
>   printf("[D] Derived.Base2::getOtherThing() = %d \n", 
> dlangDerived.getOtherThing());
>   printf("[D] Derived.Base1::inheritedFunctionWithArgs(5, 10) = 
> %d \n",
>       dlangDerived.inheritedFunctionWithArgs(5, 10));
>   printf("[D] Calling C++ takesADerived() with D Derived* \n");
>   takesADerived(dlangDerived);
> }
> ```
> ![output](https://i.imgur.com/Plbtlow.png)


That last one with someInt is what I warned about. D ctor messed 
up class layout, in this simple case where class data isn't used 
it almost works, but will be practically unusable in real 
scenarios.