Fl_End
#include <FL/Fl_Group.H>
class MyClass {
  Fl_Group group;
  Fl_Button button_in_group;
  Fl_End end;
  Fl_Button button_outside_group;
  MyClass();
};
MyClass::MyClass() :
  group(10,10,100,100),
  button_in_group(20,20,60,30),
  end(),
  button_outside_group(10,120,60,30)
{}