> the compiler won't delete it and complain if bar is not defined

    extern void bar();

will define it to the satisfaction of the compiler. The linker won't complain if the compiler removes the call to it.

And then you have the issue of not having types. consider this

  void my_bar_class_type::bar(my_bar_type<my_bar_type_2>::my_bar_inside_type paramater);

and you are calling something like

   myObject->bar(mySecondObject.getWhatever());

You have to mock up like everything

Or go up an abstraction layer so the type details are not needed.