How to pass a class by (const) reference to C++

[Jan]

On Monday, 13 December 2021 at 07:48:34 UTC, evilrat wrote:
> On Sunday, 12 December 2021 at 21:24:39 UTC, Jan wrote:
>> In D I have an extern(C++) class:
>>
>> ```cpp
>> extern(C++) class A
>> {
>>     ~this();
>>
>>     // other stuff
>> }
>> ```
>>
>> An a function that takes A by const reference:
>>
>> ```cpp
>> void CppFunc(const A& arg);
>> ```
>>
>> But how do I bind this in D ?
>>
>> ```cpp
>> extern(C++) void CppFunc(A arg); // tries to pass as 'A*'
>> extern(C++) void CppFunc(ref const(A) arg); // tries to pass 
>> as 'A const * const &'
>> ```
>>
>> I have solved similar problems with other classes by declaring 
>> them as struct in D, but that only works for classes that have 
>> no virtual functions. I now have a class where I do need to 
>> use a class on the D side, and now I have problems passing 
>> these objects to C++.
>
> You can tell compiler to mangle it as struct/class using 
> extern(C++, struct).
>
> ```d
> extern (C++, struct) // will use struct mangling even though 
> it's a class
> class SomeDClass
> {
>  ...
> }
> ```

I tried this, but it doesn't work, because it seems D decides how 
to pass the object by whether it is a class or struct in D, not 
in C++. So even with the change as you suggested it, it still 
tries to pass the object as a pointer to begin with.