C++ binding issues with C++ function returning a simple POD struct.

[Jerry via Digitalmars-d-learn]

On Sunday, 21 May 2017 at 19:33:06 UTC, ParticlePeter wrote:
> I am statically linking to ImGui [1] on Win 10 x64, quite 
> successfully till this issue came up. The noticed error so far 
> comes when an ImGui function returns an ImVec2, a simple POD 
> struct of two float members. I can use this struct as argument 
> to functions but when it is returned from a function I get a 
> 0xC0000005: Access violation reading location 
> 0xFFFFFFFFFFFFFFFF. I can even debug the process with Visual 
> Studion, mixed d and c++ sources. The functions I tested return 
> data from some internal global ImGui data, which I can fully 
> examine, the crash happens on the return statement. Moreover, 
> some functions have variations which return only one component 
> from that ImVec2 POD, which do work as expected, e.g.:
>
>     ImVec2          GetCursorPos();   // crash
>     float           GetCursorPosX();  // works
>     float           GetCursorPosY();  // works
>
> The latter do basically the same as the first one, but return 
> ImVec.x or .y respectively.
>
> How could I further debug this?
> If somebody would be willing to look at the source, the binding 
> is here [2].
>
>
> [1] https://github.com/ocornut/imgui
> [2] https://github.com/ParticlePeter/imgui_lib

IIRC the problem is that it isn't a POD type. ImVec2 has its own 
default constructor. The problem now is that because it no longer 
is POD, Window's ABI handles it different and doesn't put the 
value in a register. Now with D is that you aren't allowed to 
specify your own default constructor, so there's no equivalent 
way for it to know that it isn't a POD. A way around this is to 
specify your own destructor or copy constructor in the D ImVec2. 
I forget what the rules are for it, but I think that should do it.