Bug on C/C++ library to D, parameters suddenly becomming null or invalid

[Hipreme]

I found some really strange problem happening when trying to 
create a binding between
cimgui (https://github.com/cimgui/cimgui), which is a 
automatically generated binding from the main C++ project 
(https://github.com/ocornut/imgui)

While trying to create a binding, everything worked until I 
wanted multiviewports.

The problem was caused by functions pointers members from a 
struct.

Inside the cpp code, I saw that the calling didn't change the 
viewport instance, but when arriving at the D code, the instance 
would be transformed in invalid.


The reference code I'm saying is:

Take a look onto

https://github.com/ocornut/imgui/blob/682249396f02b8c21e5ff333ab4a1969c89387ad/imgui.cpp#L11120

This line executes just correctly the viewport I just created on 
newviewport call from my D code.


While that line:

https://github.com/ocornut/imgui/blob/682249396f02b8c21e5ff333ab4a1969c89387ad/imgui.cpp#L11134

References some of the same viewports, but it strangely became 
invalid memory, while the line from getminimized are all working.

Things got even stranger when I compiled a DLL for not having to 
use my SDL implementation, only the OpenGL which I can't change 
because the OpenGL loader is the bindbc one.

This is a image from my debugger, where the program had break:

https://user-images.githubusercontent.com/10136262/96338382-79a9f700-1042-11eb-8f0f-f0d8155cd82e.png


As you can see, viewport is 0x000000 while void* param has some 
value.
While the c++ code did pass a viewport to the first parameter, as 
seem in:

https://github.com/ocornut/imgui/blob/682249396f02b8c21e5ff333ab4a1969c89387ad/imgui.cpp#L11634

This bug was mentioned in the cimgui repo, take a look as the 
reference:
https://github.com/cimgui/cimgui/issues/156

I solved this bug by casting void* param to ImGuiViewport, so, 
the first parameter was actually null while the second would be 
actually the first parameter.

The func prototype is correctly declared, although is a struct 
with a member callback which is assigned inside some D function.
It is a lot to read, but this bug is really strange for me and I 
don't know what to do else