Loading triangles from obj file using CGAL

I've found myself redoing this code relatively frequently, so I thought I would post it.  Nothing special, just a snippet that loads the file input.obj, builds a CGAL::Polyhedron_3 from it, and writes it out as dump.off. The code also includes a rudimentary Wavefront OBJ loader.

#include<fstream>
#include<vector>
#include<string>
#include<algorithm>

#include<CGAL/Simple_cartesian.h>
#include<CGAL/Polyhedron_incremental_builder_3.h>
#include<CGAL/Polyhedron_3.h>
#include<CGAL/IO/Polyhedron_iostream.h>

typedef CGAL::Simple_cartesian<double>     Kernel;
typedef CGAL::Polyhedron_3<Kernel>         Polyhedron;
typedef Polyhedron::HalfedgeDS             HalfedgeDS;

// A modifier creating a triangle with the incremental builder.
template<class HDS>
class polyhedron_builder : public CGAL::Modifier_base<HDS> {
public:
 std::vector<double> &coords;
 std::vector<int>    &tris;
    polyhedron_builder( std::vector<double> &_coords, std::vector<int> &_tris ) : coords(_coords), tris(_tris) {}
    void operator()( HDS& hds) {
  typedef typename HDS::Vertex   Vertex;
        typedef typename Vertex::Point Point;

  // create a cgal incremental builder
        CGAL::Polyhedron_incremental_builder_3<HDS> B( hds, true);
        B.begin_surface( coords.size()/3, tris.size()/3 );
  
  // add the polyhedron vertices
  for( int i=0; i<(int)coords.size(); i+=3 ){
   B.add_vertex( Point( coords[i+0], coords[i+1], coords[i+2] ) );
  }
  
  // add the polyhedron triangles
  for( int i=0; i<(int)tris.size(); i+=3 ){
   B.begin_facet();
   B.add_vertex_to_facet( tris[i+0] );
   B.add_vertex_to_facet( tris[i+1] );
   B.add_vertex_to_facet( tris[i+2] );
   B.end_facet();
  }
  
  // finish up the surface
        B.end_surface();
    }
};

// reads the first integer from a string in the form
// "334/455/234" by stripping forward slashes and
// scanning the result
int get_first_integer( const char *v ){
 int ival;
 std::string s( v );
 std::replace( s.begin(), s.end(), '/', ' ' );
 sscanf( s.c_str(), "%d", &ival );
 return ival;
}

// barebones .OFF file reader, throws away texture coordinates, normals, etc.
// stores results in input coords array, packed [x0,y0,z0,x1,y1,z1,...] and
// tris array packed [T0a,T0b,T0c,T1a,T1b,T1c,...]
void load_obj( const char *filename, std::vector<double> &coords, std::vector<int> &tris ){
 double x, y, z;
 char line[1024], v0[1024], v1[1024], v2[1024];
 
 // open the file, return if open fails
 FILE *fp = fopen(filename, "r" );
 if( !fp ) return;
 
 // read lines from the file, if the first character of the
 // line is 'v', we are reading a vertex, otherwise, if the
 // first character is a 'f' we are reading a facet
 while( fgets( line, 1024, fp ) ){
  if( line[0] == 'v' ){
   sscanf( line, "%*s%lf%lf%lf", &x, &y, &z );
   coords.push_back( x );
   coords.push_back( y );
   coords.push_back( z );
  } else if( line[0] == 'f' ){
   sscanf( line, "%*s%s%s%s", v0, v1, v2 );
   tris.push_back( get_first_integer( v0 )-1 );
   tris.push_back( get_first_integer( v1 )-1 );
   tris.push_back( get_first_integer( v2 )-1 );
  }
 }
 fclose(fp); 
}

int main() {
 // two vectors to hold point coordinates and
 // triangle vertex indices
 std::vector<double> coords;
 std::vector<int>    tris;
 
 // load the input file
 load_obj( "input.obj", coords, tris );
 if( coords.size() == 0 )
  return 1;
 
 // build a polyhedron from the loaded arrays
 Polyhedron P;
 polyhedron_builder<HalfedgeDS> builder( coords, tris );
 P.delegate( builder );
 
 // write the polyhedron out as a .OFF file
 std::ofstream os("dump.off");
 os << P;
 os.close();
 
    return 0;
}

From:http://jamesgregson.blogspot.com/2012/05/example-code-for-building.html