I would like to interface with a C++ library called FLTK. I'm trying to implement the binding for the classes based to what I've read here but it seems that It doesn't work as expected for me. I want to implement the "abstract" class "Fl_Widget" and the class "Fl_Group" which inherits from the "Fl_Widget". I will put a part of the code as there is no reason to put the whole code and make the post unnecessarily big. If in any case however, you need the whole reference: Fl_Widget and Fl_Group. Here is the code:

extern(C++) {
  abstract class Fl_Widget {
    void   _clear_fullscreen();
    void   _set_fullscreen();
  }

  class Fl_Group : Fl_Widget {
    ref Fl_Widget*    _ddfdesign_kludge();
    void              add(ref Fl_Widget);
    void              add(Fl_Widget*o);
  }
}

Normally this should work but the "Fl_Group" class makes in bug and I'm getting the following error message:

Linking...
/usr/bin/ld: .dub/build/application-debug-linux.posix-x86_64-dmd_v2.098.0-A4E26D206D34B9B9CA1A0366B3CE0F2C/dfltk.o:(.data._D4test8Fl_Group6__vtblZ+0x0): undefined reference to `Fl_Widget::_clear_fullscreen()'
/usr/bin/ld: .dub/build/application-debug-linux.posix-x86_64-dmd_v2.098.0-A4E26D206D34B9B9CA1A0366B3CE0F2C/dfltk.o:(.data._D4test8Fl_Group6__vtblZ+0x8): undefined reference to `Fl_Widget::_set_fullscreen()'
/usr/bin/ld: .dub/build/application-debug-linux.posix-x86_64-dmd_v2.098.0-A4E26D206D34B9B9CA1A0366B3CE0F2C/dfltk.o:(.data._D4test8Fl_Group6__vtblZ+0x10): undefined reference to `Fl_Group::_ddfdesign_kludge()'
/usr/bin/ld: .dub/build/application-debug-linux.posix-x86_64-dmd_v2.098.0-A4E26D206D34B9B9CA1A0366B3CE0F2C/dfltk.o:(.data._D4test8Fl_Group6__vtblZ+0x18): undefined reference to `Fl_Group::add(Fl_Widget*&)'
/usr/bin/ld: .dub/build/application-debug-linux.posix-x86_64-dmd_v2.098.0-A4E26D206D34B9B9CA1A0366B3CE0F2C/dfltk.o:(.data._D4test8Fl_Group6__vtblZ+0x20): undefined reference to `Fl_Group::add(Fl_Widget**)'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1

Anyone has an idea?

On Thursday, 9 December 2021 at 19:05:08 UTC, rempas wrote:

The referenced methods like Fl_Widget::_clear_fullscreen are implemented directly in the header, so the D code also needs the implementation, because it is not included in the compiled library.

Methods, which are not virtual in C++, also have to be marked final in D, because C++ and D use a different default.

On Thursday, 9 December 2021 at 21:35:14 UTC, Tim wrote:

What is funny about that is that I looked an the official class reference and copy pasted the code from here but I also looked at the header files and saw what you said but for some reason it completely slipped from my head...

Is this a must or just good practices?

On Friday, 10 December 2021 at 12:46:07 UTC, rempas wrote:

All virtual methods have to match exactly including order. If a virtual method is missing or added, then calling this or a later virtual method could call the wrong method or generate a segmentation fault. Non-virtual methods have to be marked final in D, but can also be removed.

On Friday, 10 December 2021 at 16:42:33 UTC, Tim wrote:

Great info! I'll have everything in mind! Thanks a lot and have an amazing day!!!