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[Mael <>]

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`.

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;
    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`.

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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