CGAL users discussion list

[Philipp Moeller <>]

"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