CGAL users discussion list

["Tomislav Maric" <>]

Thank you very much for your advice! :)

I have some further questions, regarding the functors. 

[1] If I will later on run geometrical algorithms on top of the Polyhedron_3 
that uses some other Point class, I will need to provide the functors that 
are required by the geometrical algorithms, right (e.g. convex hull of a 
polyhedron with my own Point type)? I have read about this for the 
triangulation class in the attached tutorial, page 41 (pdf), but 
triangulation provides a basic template for which the needed functors are 
defined, and as soon as I plug in my Point type there, everything fits. This 
is probably not possible to generalize to Polyhedron_3, because there are a 
lot of geometrical algorithms in CGAL runing on polyhedra.... 

[2] I'm kind of getting lost with concepts and refinements, because they are 
intertwining (seem to be to me, I'm a complete newb), how can I best find out 
what requirements need to be satisfied for certain operations with types? 
Should I implement the requirements provided by the concept closest to what I 
need? There seem to be multiple solutions to the same type problems.... and I 
keep getting lost... 

Thanks again! 

Tomislav

> ----- Original Message -----
> From: Philipp Moeller
> Sent: 04/24/12 10:47 AM
> To: 
> 
> Subject: Re: [cgal-discuss] using a Point class from another library
> 
> "Tomislav Maric" 
> <>
>  writes:
> 
> > I have managed to make Polyhedron work with my Point class, but when I 
> > want to read it from an .off file, 
> >
> > this function:
> >
> > template < class Point> inline
> > Point&
> > file_scan_vertex( File_scanner_OFF& scanner, Point& p) {
> >    typedef typename Point::R R;
> >    typedef typename R::RT    RT;
> >    double x, y, z, w;
> >    scanner.scan_vertex( x, y, z, w);
> >    if ( w == 1)
> >        p = Point( RT(x), RT(y), RT(z));
> >    else
> >        p = Point( RT(x), RT(y), RT(z), RT(w));
> >    return p;
> > }
> >
> > from the file "File_scanner_OFF.h" expects my point to have a constructor 
> > with 4 arguments. For a simple 
> > example Point class that I wrote, I have added this without a problem, 
> > but if a Point class is coming from another 
> > library, it is a problem. I cannot change the library Point class. What 
> > should I do? 
> >
> > Option 1) Inherit from the Point class of the other library and add a 
> > dummy constructor, counting on 
> >          implicit casting every time i call a method on the Point to cast 
> > it up? 
> >       2) Change the .off File_scanner and adapt it somehow? 
> >
> >       ?) ?
> 
> The problem is a more complicated. Technically, Polyhedron shouldn't
> make any assumptions about the concrete type of Point and use a
> Construct_point_3 functor to construct them. This additional functor
> would go into the traits as well.
> 
> As a workaround, I wouldn't change file_scan_vertex but add an overload
> to the CGAL namespace, e.g.
> 
> namespace CGAL {
> inline
> MyPoint&
> file_scan_vertex( File_scanner_OFF& scanner, MyPoint& p) {
>  // do my thing here
> }
> }
> 
> to change the behavior. This should work as far as I have looked at
> things. If not, I'd prefer method 1.
> 
> >
> > I don't need homogeneous coordinate representation of points, so this 
> > kind of constructor is not necessary.. 
> >
> > Working code is attached. 
> >
> > Thank you! 
> >
> > Tomislav
> 
> HTH,
> Philipp Moeller
> GeometryFactory
> 
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