CGAL users discussion list

Dear CGAL developers and users,

As I understand it, some classes in CGAL derive from a (linear,circular) 
kernel in order to add extra functionality, while still retaining the 
functionality of the base kernel. For instance:
* the circular kernel derives from a linear kernel
* arrangement traits classes often derive from a linear or circular kernel

One would suppose that these derived classes still implement the model of the 
class that they derive from. So it would be legal to do the following:
* define a linear kernel
* derive a circular kernel from it (extending the linear kernel)
* create an arrangement with CGAL::Arr_segment_traits_2<CircularKernel>
  instead of the usual CGAL::Arr_segment_traits_2<LinearKernel>

This doesn't work as the circular kernel hides some functors from the linear 
kernel. For instance, it defines CircularKernel::HasOn_2, which overrides 
LinearKernel::HasOn_2. Both functors answer the same question for different 
argument types. However, the CircularKernel::HasOn_2 cannot answer queries on 
linear kernel objects. This yields errors when trying to use the circular 
kernel where a linear kernel is expected (as in the example above).

Is it possible to extend functors of derived classes to implement the 
functionality of the base class as well (e.g., by deriving from the functor 
of the base class)? This way the concept of the base class would not be 
destroyed as it is now.

Other examples: the functors Compare_y_at_x_2, Equal_2, Intersect_2, 
Compare_y_at_x_2 in the Arr_segment_traits_2 class. These functors (and maybe 
others that I've missed) also exist in the linear kernel that is being 
derived from.

Kind regards,

Michiel De Wilde