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[Maxime Gimeno <>]

Le jeu. 13 août 2020 à 06:36, Ben Laurie <> a écrit :

It turns out, BTW, that you can do this:

    typedef typename Mesh::Point Point;
    typedef typename CGAL::Kernel_traits<Point>::Kernel Kernel;

You can do that if you know your mesh is a Surface_mesh. If it can be another type, then you can't, all types don't define the Point type.

On Thu, 13 Aug 2020 at 05:23, Ben Laurie <> wrote:

On Sun, 9 Aug 2020 at 22:07, Mael <> wrote:

Hi,

If you look at the documentation of Kernel_traits (https://doc.cgal.org/latest/Kernel_23/structCGAL_1_1Kernel__traits.html), you'll see that this traits class searches for a `R` typedef, which is the kernel. This `R` typedef exists for CGAL kernel objects (e.g. CGAL::Point_2, Point_3, etc.) but not for mesh types such as `Surface_mesh`.

Any particular reason why not?

 

Because a mesh is not a Kernel object. It is not expected to work with that function. 

What you would usually do here is to extract the point type from the mesh vertex->point property map of your mesh and call `Kernel_traits` on that point type, so something like:

    typedef typename property_map_value<Mesh, CGAL::vertex_point_t>::type Point;

If I try that, I get:

error: expected a qualified name after 'typename'

 The right code would be 

typedef boost::property_traits< boost::property_map< Mesh, vertex_point_t>::type >::value_type Point;

But again, with a Surface_mesh, you can just use the typename Surface_mesh<K>::Point.

If I take out the typename, I get:

error: no template named 'property_map_value'

Not sure what header defines it, but if I include this header, which also uses property_map_value, I get the same error...

#include <CGAL/Polygon_mesh_processing/measure.h>

 

    typedef typename CGAL::Kernel_traits<Point>::Kernel Kernel;

Note that this still assumes that your mesh is compatible with the BGL interface, which is valid for all meshes in CGAL and so more from other libraries, see https://doc.cgal.org/latest/BGL/index.html#title0. Furthermore, your `Point` type must be compatible with `Kernel_traits` (i.e. it defines a `R` typedef), otherwise you will need a partial specialization of `Kernel_traits`.

I am using CGAL::Surface_mesh.

 

Best,
Mael

On 2020-08-09 14:00, Ben Laurie wrote:

I want to write a template function that does something to a mesh, but without assuming anything about how the mesh is defined (i.e. I don't know what the kernel is).

I thought I could do something like this:

template<class Mesh>
void someFn(const Mesh &m)
    {
    typedef typename CGAL::Kernel_traits<Mesh>::Kernel Kernel;

    typedef typename Kernel::Point_3 Point;

   ...

but this gives me errors like:

      'CGAL::internal_kernel_traits::Dummy_kernel<CGAL::Surface_mesh<CGAL::Point_3<CGAL::Epeck>
      > >'
    typedef typename CGAL::Kernel_traits<Mesh>::Kernel::Point_3 Point;

What am I doing wrong?

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