[cgal]; Simple Example

I've been studying Triangulation Examples from Cgal guide. I have a big doubt in one exact example, simple_example.cpp. The code is the following:

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Periodic_3_Delaunay_triangulation_traits_3.h>
#include <CGAL/Periodic_3_Delaunay_triangulation_3.h>
#include <CGAL/periodic_3_triangulation_3_io.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <list>
#include <vector>
typedef CGAL::Exact_predicates_inexact_constructions_kernel       K;
typedef CGAL::Periodic_3_Delaunay_triangulation_traits_3<K>       Gt;
typedef CGAL::Periodic_3_Delaunay_triangulation_3<Gt>             P3DT3;
typedef P3DT3::Point             Point;
typedef P3DT3::Iso_cuboid        Iso_cuboid;
typedef P3DT3::Vertex_handle     Vertex_handle;
typedef P3DT3::Cell_handle       Cell_handle;
typedef P3DT3::Locate_type       Locate_type;
int main(int, char**)
  Iso_cuboid domain(-1,-1,-1,2,2,2);  // the fundamental domain
  // construction from a list of points :
  std::list<Point> L;
  P3DT3 T(L.begin(), L.end(), domain); // put the domain with the constructor
  P3DT3::size_type n = T.number_of_vertices();
  // insertion from a vector :
  std::vector<Point> V(3);
  V[0] = Point(0,0,1);
  V[1] = Point(1,1,1);
  V[2] = Point(-1,-1,-1);
  n = n + T.insert(V.begin(), V.end());
  assert( n == 6 );       // 6 points have been inserted
  assert( T.is_valid() ); // checking validity of T
  Locate_type lt;
  int li, lj;
  Point p(0,0,0);
  Cell_handle c = T.locate(p, lt, li, lj);
  // p is the vertex of c of index li :
  assert( lt == P3DT3::VERTEX );
  assert( c->vertex(li)->point() == p );
  Vertex_handle v = c->vertex( (li+1)&3 );
  // v is another vertex of c
  Cell_handle nc = c->neighbor(li);
  // nc = neighbor of c opposite to the vertex associated with p
  // nc must have vertex v :
  int nli;
  assert( nc->has_vertex( v, nli ) );
  // nli is the index of v in nc
  // writing file output
  std::ofstream oFileT("output.tri", std::ios::out); // as a .tri file
  oFileT << T;
  std::ofstream to_off("output_regular.off"); // as a .off file
  CGAL::write_triangulation_to_off(to_off, T);
  std::ofstream d_to_off("output_dual.off");
  draw_dual_to_off(d_to_off, T);
  // reading file output
  P3DT3 T1;
  std::ifstream iFileT("output.tri",std::ios::in);
  iFileT >> T1;
  assert( T1.is_valid() );
  assert( T1.number_of_vertices() == T.number_of_vertices() );
  assert( T1.number_of_cells() == T.number_of_cells() );
  return 0;

My doubt is that I'm not sure what's the output of P3DT3 T. The output files have several rows and each row has different columns, but I'm not sure what does each column mean. I've been searching in the internet and cgal guide, but didn't find anything interesting. Any help?

Read more here: https://stackoverflow.com/questions/68466351/cgal-simple-example

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