I would like to interface with a C++ library called
[FLTK](https://www.fltk.org/). I'm trying to implement the
binding for the classes based to what I've read
[here](https://dlang.org/spec/cpp_interface.html#classes) 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](https://fltk.gitlab.io/fltk/classFl__Widget.html) and
[Fl_Group](https://fltk.gitlab.io/fltk/classFl__Group.html). 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?
