class Fl_Box : public Fl_Widget
This widget simply draws its box, and possibly it's label. Putting it
before some other widgets and making it big enough to surround them
will let you draw a frame around them.
Fl_Box::Fl_Box(int x,int y,int w,int h, const char * = 0);
The constructor sets box() to FL_NO_BOX, which means it is invisible.
However such widgets are useful as placeholders or Fl_Group::resizable() values. To change
the box to something visible, use box(n).
Fl_Box::Fl_Box(Fl_Boxtype, int x,int y,int w,int h, const char *);
For convienence there is a constructor that initializes box() to a
passed value.
See here for a list of boxtypes.