using qsort on a member function in c++

Question

I'm trying to use qsort in a member function, but I get the following error

I don't know what's wrong with the following code. so I tried to put the address of operator but still it didn't work.

 error C2664: 'void qsort(void *,std::size_t,std::size_t,int (__cdecl *)(const void *,const void *))': cannot convert argument 4 from 'int (__thiscall Rasterizer::* )(facet **,facet **)' to 'int (__cdecl *)(const void *,const void *)'

here are the two functions:

  int Rasterizer:: Poly_Compare(facet_ptr arg1, facet_ptr arg2)
{
    // this function comapares the average z's of two polygons and is used by the
    // depth sort surface ordering algorithm

    float z1, z2;


    // dereference the poly pointers



    // compute z average of each polygon

    if (arg1->num_points == 3)
    {
        // compute average of 3 point polygon

        z1 = (float)0.33333*(arg1->vertex_list[0].z +
            arg1->vertex_list[1].z +
            arg1->vertex_list[2].z);
    }
    else
    {
        // compute average of 4 point polygon

        z1 = (float)0.25*(arg1->vertex_list[0].z +
            arg1->vertex_list[1].z +
            arg1->vertex_list[2].z +
            arg1->vertex_list[3].z);

    } // end else




      // now polygon 2

    if (arg2->num_points == 3)
    {
        // compute average of 3 point polygon

        z2 = (float)0.33333*(arg2->vertex_list[0].z +
            arg2->vertex_list[1].z +
            arg2->vertex_list[2].z);
    }
    else
    {
        // compute average of 4 point polygon

        z2 = (float)0.25*(arg2->vertex_list[0].z +
            arg2->vertex_list[1].z +
            arg2->vertex_list[2].z +
            arg2->vertex_list[3].z);

    } // end else

      // compare z1 and z2, such that polys' will be sorted in descending Z order

    if (z1 > z2)
        return(-1);
    else
        if (z1 < z2)
            return(1);
        else
            return(0);

} // end Poly_Compare


  //////////////////////////////////////////////////////////////////////////////

void Rasterizer::Sort_Poly_List(void)
{
    // this function does a simple z sort on the poly list to order surfaces
    // the list is sorted in descending order, i.e. farther polygons first

    std::sort(world_polys, num_polys_frame + world_polys, Rasterizer::Poly_Compare);

} 

typedef struct facet_typ
{

    int num_points;  // number of vertices

    int color;       // color of polygon

    int shade;       // the final shade of color after lighting

    int shading;     // type of shading to use

    int two_sided;   // is the facet two sided

    int visible;     // is the facet transparent

    int clipped;     // has this poly been clipped

    int active;      // used to turn faces on and off

    point_3d vertex_list[MAX_POINTS_PER_POLY]; // the points that make
    // up the polygon facet

    float normal_length;  // holds pre-computed length of normal

} facet, *facet_ptr;