Improvement: Step 2 tutorial and get_num_global_elements to mesh.hxx

mesh_handle/constructor_wrappers.hxx

@@ -47,7 +47,8 @@ namespace t8_mesh_handle
  * \param [in] comm          MPI communicator to use.
  * \param [in] do_face_ghost If true, a layer of ghost elements is created.
  * \tparam TMeshClass        The mesh handle class.
- * \return Unique pointer to a uniformly refined mesh handle with coarse mesh \a cmesh and refinement level \a level.
+ * \return                   Unique pointer to a uniformly refined mesh handle with coarse mesh \a cmesh and refinement level \a level, 
+ *                           partitioned across the processes in \a comm.
  */
 template <T8MeshType TMeshClass>
 std::unique_ptr<TMeshClass>

mesh_handle/mesh.hxx

@@ -141,6 +141,16 @@ class mesh: public TMeshCompetencePack::template apply<mesh<TElementCompetencePa
     return t8_forest_get_local_num_leaf_elements (m_forest);
   }
 
+  /**
+   * Getter for the number of global elements in the mesh.
+   * \return Number of global elements in the mesh.
+   */
+  t8_gloidx_t
+  get_num_global_elements () const
+  {
+    return t8_forest_get_global_num_leaf_elements (m_forest);
+  }
+
   /**
    * Getter for the number of ghost elements.
    * \return Number of ghost elements in the mesh.

test/mesh_handle/t8_gtest_compare_handle_to_forest.cxx

@@ -50,6 +50,7 @@ TEST (t8_gtest_compare_handle_to_forest, compare_handle_to_forest)
 
   const t8_mesh_handle::mesh<> mesh = t8_mesh_handle::mesh<> (forest);
   EXPECT_EQ (mesh.get_num_local_elements (), t8_forest_get_local_num_leaf_elements (forest));
+  EXPECT_EQ (mesh.get_num_global_elements (), t8_forest_get_global_num_leaf_elements (forest));
   EXPECT_EQ (mesh.get_num_ghosts (), t8_forest_get_num_ghosts (forest));
   EXPECT_EQ (mesh.get_dimension (), t8_forest_get_dimension (forest));
 

tutorials/CMakeLists.txt

@@ -83,4 +83,5 @@ copy_tutorial_file (features/t8_features_curved_meshes_generate_cmesh_tri.geo)
 
 if( T8CODE_BUILD_MESH_HANDLE )
     add_mesh_handle_tutorial( NAME t8_mesh_element_data SOURCES mesh_handle/t8_mesh_element_data.cxx )
+    add_mesh_handle_tutorial( NAME t8_mesh_step2_uniform_mesh SOURCES mesh_handle/t8_mesh_step2_uniform_mesh.cxx )
 endif()

tutorials/general/t8_step1_coarsemesh.cxx

@@ -110,7 +110,7 @@ main (int argc, char **argv)
   /* Initialize the sc library, has to happen before we initialize t8code. */
   sc_init (sc_MPI_COMM_WORLD, 1, 1, NULL, SC_LP_ESSENTIAL);
   /* Initialize t8code with log level SC_LP_PRODUCTION. See sc.h for more info on the log levels. */
-  t8_init (SC_LP_DEBUG);
+  t8_init (SC_LP_PRODUCTION);
 
   /* Print a message on the root process. */
   t8_global_productionf (" [step1] \n");

tutorials/general/t8_step2_uniform_forest.cxx

@@ -33,7 +33,7 @@
  * forest how elements of each shape (t8_eclass_t) are refined, what their neighbor
  * are etc.
  * The default scheme in t8_schemes/t8_default/t8_default.hxx provides an implementation for
- * all element shapes that t8code supports (with pyramids currently under construction).
+ * all element shapes that t8code supports.
  *
  * How you can experiment here:
  *  - Use Paraview to visualize the output files.
@@ -138,7 +138,7 @@ main (int argc, char **argv)
   /* Initialize the sc library, has to happen before we initialize t8code. */
   sc_init (sc_MPI_COMM_WORLD, 1, 1, NULL, SC_LP_ESSENTIAL);
   /* Initialize t8code with log level SC_LP_PRODUCTION. See sc.h for more info on the log levels. */
-  t8_init (SC_LP_DEBUG);
+  t8_init (SC_LP_PRODUCTION);
 
   /* Print a message on the root process. */
   t8_global_productionf (" [step2] \n");

tutorials/mesh_handle/t8_mesh_step2_uniform_mesh.cxx

@@ -0,0 +1,147 @@
+/*
+  This file is part of t8code.
+  t8code is a C library to manage a collection (a forest) of multiple
+  connected adaptive space-trees of general element types in parallel.
+
+  Copyright (C) 2026 the developers
+
+  t8code is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  t8code is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with t8code; if not, write to the Free Software Foundation, Inc.,
+  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+*/
+
+/** \file t8_mesh_step2_uniform_mesh.cxx
+ * This is step 2 of the t8code mesh handle tutorials.
+ * Therefore, this is the same as general/t8_step2_uniform_forest.cxx but using the mesh handle interface instead of the forest
+ * interface.
+ * After we learned how to create a cmesh in step1, we will
+ * now build our first partitioned mesh, get its local and global
+ * element count, and output it into .vtu files.
+ *
+ * When we create a mesh from a coarse mesh using the mesh handle interface, the mesh will always be
+ * uniform (every element has the same refinement level) and can then be adapted
+ * later (see the following steps).
+ * Together with the cmesh, we also need a refinement scheme. This scheme tells the
+ * mesh how elements of each shape (t8_eclass_t) are refined, what their neighbors
+ * are etc.
+ * The default scheme in t8_schemes/t8_default/t8_default.hxx provides an implementation for
+ * all element shapes that t8code supports.
+*/
+#include <t8.h>                                 /** General t8code header, always include this. */
+#include <mesh_handle/mesh.hxx>                 /** General Mesh Header, always needed for mesh_handle code. */
+#include <t8_cmesh/t8_cmesh.h>                  /** cmesh definition and basic interface. */
+#include <mesh_handle/constructor_wrappers.hxx> /** Wrapper for basic Cmesh to mesh_handle conversions. */
+#include <mesh_handle/mesh_io.hxx>              /** Used to export mesh to vtk files. */
+#include <t8_schemes/t8_default/t8_default.hxx> /** default refinement scheme. */
+#include <memory>
+
+/** Builds cmesh of 2 prisms that build up a unit cube.
+ * See step1 for a detailed description.
+ * \param [in] comm   MPI Communicator to use.
+ * \return            The coarse mesh.
+ */
+static t8_cmesh_t
+t8_step2_build_prismcube_coarse_mesh (sc_MPI_Comm comm)
+{
+  t8_cmesh_t cmesh;
+
+  /* Build a coarse mesh of 2 prisms that form a cube. */
+  t8_cmesh_init (&cmesh);
+  t8_cmesh_new_hypercube (&cmesh, T8_ECLASS_PRISM, comm, 0, 0, 0);
+  t8_global_productionf (" [tutorial] Constructed coarse mesh with 2 prisms.\n");
+
+  return cmesh;
+}
+
+/** Build a uniform mesh on a cmesh using the default refinement scheme. 
+ * \param [in] comm   MPI Communicator to use.
+ * \param [in] cmesh  The coarse mesh to build the uniform mesh on.
+ * \param [in] level  The initial uniform refinement level.
+ * \return            A uniform mesh with the given refinement level that is
+ *                    partitioned across the processes in \a comm.
+ */
+template <typename MeshType>
+static std::unique_ptr<MeshType>
+t8_step2_build_uniform_mesh (sc_MPI_Comm comm, t8_cmesh_t cmesh, int level)
+{
+  const t8_scheme *scheme = t8_scheme_new_default (); /** Default refinement scheme. */
+
+  /* Build the uniform mesh, it is automatically partitioned among the processes. */
+  std::unique_ptr<MeshType> mesh = t8_mesh_handle::handle_new_uniform<MeshType> (cmesh, scheme, level, comm);
+
+  t8_global_productionf (" [t8_step2] Constructed uniform mesh with refinement level %d.\n", level);
+
+  return mesh;
+}
+
+int
+main (int argc, char **argv)
+{
+  /** File prefix for our vtk files. */
+  const char *prefix = "t8_step2_uniform_mesh";
+  /** Uniform refinement level of the mesh. */
+  const int level = 3;
+
+  /** Initialize MPI. This has to happen before we initialize sc or t8code. */
+  int mpiret = sc_MPI_Init (&argc, &argv);
+  /** Error check the MPI return value. */
+  SC_CHECK_MPI (mpiret);
+
+  /** Initialize the sc library, has to happen before we initialize t8code. */
+  sc_init (sc_MPI_COMM_WORLD, 1, 1, NULL, SC_LP_ESSENTIAL);
+  /** Initialize t8code with log level SC_LP_PRODUCTION. See sc.h for more info on the log levels. */
+  t8_init (SC_LP_PRODUCTION);
+
+  /** Print a message on the root process. */
+  t8_global_productionf (" [t8_step2] \n");
+  t8_global_productionf (" [t8_step2] Hello, this is the step2 example of t8code using the mesh handle.\n");
+  t8_global_productionf (" [t8_step2] In this example we build our first uniform mesh and output it to vtu files.\n");
+  t8_global_productionf (" [t8_step2] \n");
+
+  /** We will use MPI_COMM_WORLD as a communicator. */
+  sc_MPI_Comm comm = sc_MPI_COMM_WORLD;
+
+  /** Create the cmesh. */
+  t8_cmesh_t cmesh = t8_step2_build_prismcube_coarse_mesh (comm);
+  /**
+   * We will put the Mesh in a separate scope here, 
+   * because it will destroy itself completely on its own when reaching the end of this scope.
+  */
+  {
+    /** Build the uniform mesh. */
+    auto mesh = t8_step2_build_uniform_mesh<t8_mesh_handle::mesh<>> (comm, cmesh, level);
+    /** Get the number of local elements. */
+    const t8_locidx_t local_num_elements = mesh->get_num_local_elements ();
+    /** Get the number of global elements. */
+    const t8_gloidx_t global_num_elements = mesh->get_num_global_elements ();
+
+    /** Print information on the mesh. */
+    t8_global_productionf (" [t8_step2] Created uniform mesh.\n");
+    t8_global_productionf (" [t8_step2] Refinement level:\t\t\t%i\n", level);
+    t8_global_productionf (" [t8_step2] Local number of elements:\t\t%i\n", local_num_elements);
+    t8_global_productionf (" [t8_step2] Global number of elements:\t%" T8_GLOIDX_FORMAT "\n", global_num_elements);
+
+    /** Write mesh to vtu files. */
+    t8_mesh_handle::write_mesh_to_vtk (*mesh, prefix);
+    t8_global_productionf (" [t8_step2] Wrote mesh to vtu files:\t%s*\n", prefix);
+
+  } /** End of Mesh scope. */
+  t8_global_productionf (" [t8_step2] Mesh scope ended.\n");
+
+  sc_finalize ();
+
+  mpiret = sc_MPI_Finalize ();
+  SC_CHECK_MPI (mpiret);
+
+  return 0;
+}