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Or, as a more genreal purpose solution, the ability to provide
an additional void* associated with each callback function, so
we can typcast it and invoke the class member-function in
our own static handler.

maybe just have something like this:

class MyHandler
{
protected:
GLUI* _glui;
public:

 virtual void checkbox_changed(int id)
 { .... }

 static void control_cb(int id)
 {
      MyHandler* obj = (MyHandler*)glui->get_user_data() ;
      obj->checkbox_changed(id);
 }

  ...

};

....

MyHandler data;
glui->add_checkbox( "Wireframe", &wireframe, 1,
MyClass::control_cb, (void*)&handler);