extern(C++) struct - what is it?

[Nicholas Wilson via Digitalmars-d-learn]

On Friday, 16 December 2016 at 12:40:19 UTC, Ilya Yaroshenko 
wrote:
> It was in DMD sources.
> How it can be used?
> Are methods virtual?
> How multiple inheritance works?
> Can this be used in betterC mode?
> What different between classes in C++?
>
> Thanks,
> Ilya

See also https://dlang.org/spec/cpp_interface.html in particular 
https://dlang.org/spec/cpp_interface.html#classes