On 27 Jan 2016 7:10 am, "Elie Morisse via Digitalmars-d" <digitalmars-d@puremagic.com> wrote:
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> On Tuesday, 26 January 2016 at 15:53:39 UTC, Manu wrote:
>>
>> So, you're saying that if you have an 'I' pointer, which may be implemented by a C++ class, it still needs to be able to cast to Object, and therefore fails? I wonder if a C++ class could somehow be promoted to a D class when being derived in D, by having an Object placed somewhere.
>
>
> It helps that Object is a just a handful of virtual methods.
>
>
> On Tuesday, 26 January 2016 at 16:25:35 UTC, Benjamin Thaut wrote:
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>> For a D class the first entry in the vtable is the classinfo. Thus the problem if you derive a D class from a extern(C++) base class. I don't see any way to actually fix this, adjusting the this pointer won't help. Once you derive a D class from a extern(C++) base class it is no longer a fully functional D class. For example monitor (e.g. synchronized methods) won't work.
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> Calypso has "hybrid" D classes inheriting from C++, with this layout:
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>  - D vptr
>  - D monitor
>   { start of C++ class }
>  - C++ vptr
>  - C++ fields...
>  - might be other vptr and fields if the C++ class has more than one base
>   { end of C++ class }
>  - D fields...
>
> and when downcasted to the C++ base class the "this" pointer gets adjusted to point towards the start of the C++ class.

This is exactly what I had in mind.
An alternative would be to place Object at the start of 'D fields', same fixup logic.