extern(C++) template problem

[Manu]

On 23 May 2018 at 03:14, Nicholas Wilson via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> On Wednesday, 23 May 2018 at 07:09:40 UTC, Manu wrote:
>>
>> C++:
>> ---------------------------------------
>> class C {};
>>
>> template <typename T>
>> void create(T** x);
>> ---------------------------------------
>>
>> D:
>> ---------------------------------------
>> extern(C++) class C {}
>>
>> void create(T)(T* x);
>> ---------------------------------------
>>
>> Trouble is; in CPP, the template mangles as a C, but in D, the class
>> mangles as C*... so the mangling doesn't match.
>>
>> `template<typename Class>` is an unbelievably common occurrence in C++...
>> but how do we express a signature that mangles correctly in D? D always
>> mangles `Class` as `Class*`... which changes the signature.
>>
>>> _<
>
>
> I believe you are looking for
>
>> extern(C++,class) struct C {}
>>
>> void create(T)(T** x);
>
>
> or
>
>> extern(C++,struct) class C {}
>>
>> void create(T)(T** x);
>
>
> I can't remember which is the one you want here (still half asleep from jet
> lag).
>
> in the case of e.g.  extern(C++,struct) class C {}
> this tells the D compiler that is should mangle it as though it is a struct
> (as it is declared in C++ as a struct) but it actually has a vtable and
> behaves like a class from the D school of thought. Vice versa for
> extern(C++,class) struct C {}

Sadly, neither of these are correct.
The type is a class (has vtable, etc), so it is declared in D as a
class... It is also a class in C++, so it must mangle like a class.
It's also the case that it's passed by pointer, in C++ and in D. It's
a class that definitely behaves like a class.
The trouble is getting the class name into the function signature as
T, where the D compiler really wants to put Class* because that's the
type it sees a D class to be.