[Vxl-maintainers] Re: dynamic_cast in vsol, vtol

[Matt L. <ml...@le...>]

> I just think of one unfortunate side effect of using dynamic_cast<*> 
> instead of cast_to_*():
> 
> While one can write
>  	void f(vsol_curve_2d_sptr c) {
>  	  vsol_line_2d_sptr l = c->cast_to_line();
>  	}
> one cannot write
>  	void f(vsol_curve_2d_sptr c) {
>  	  vsol_line_2d_sptr l = dynamic_cast<vsol_line_2d*>(c);
>  	}
> and it is necessary to write
>  	void f(vsol_curve_2d_sptr c) {
>  	  vsol_line_2d_sptr l = dynamic_cast<vsol_line_2d*>(c.ptr());
>  	}
> which is not so nice since using .ptr() on smart pointers actually
> circumvents the smartness of the pointer, which could be dangerous
> (not in this context, but once "ptr()" starts to be used often enough,
> people will start using it in other situatons as well :-{
> 
> 
> --	Peter.

I agree that this is a problem, but it's a problem related to the smart
pointer and goes beyond the use of dynamic_cast.  The same problem
occurs when upcasting smart pointers.  For example:

one cannot write
         void f(vsol_line_2d_sptr l) {
           vsol_curve_2d_sptr c = l;
         }
and it is necessary to write
         void f(vsol_line_2d_sptr l) {
           vsol_curve_2d_sptr c = l.ptr();
         }

This problem is somewhat handled in vgui for tableau.  Tableau classes
create smart pointers that are a subclass of the smart pointers of their
parent class.  The result is a parallel inheritance hierarchy of smart
pointers that mimics that of the classes that they point to. 
Unfortunately, this requires the use of special vgui_tableau_new classes
to construct these smart pointers.  This approach is not widely used
throughout the rest of VXL, and admittedly wouldn't help with down
casting.

The point is we already have this unfortunate need for .ptr() even
without the dynamic_cast :(

-Matt Leotta