CopyData

diff --git a/include/deal.II/meshworker/copy_data.h b/include/deal.II/meshworker/copy_data.h
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+++ b/include/deal.II/meshworker/copy_data.h
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+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2019 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, redistribute
+// it, and/or modify it under the terms of the GNU Lesser General
+// Public License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+
+#ifndef dealii_meshworker_copy_data_h
+#define dealii_meshworker_copy_data_h
+
+#include <deal.II/base/config.h>
+
+#include <deal.II/base/types.h>
+
+#include <deal.II/lac/full_matrix.h>
+#include <deal.II/lac/vector.h>
+
+#include <vector>
+
+DEAL_II_NAMESPACE_OPEN
+
+namespace MeshWorker
+{
+  /**
+   * Helper copy data struct.
+   *
+   * This class is a good default drop in CopyData object for the
+   * WorkStream::run() and MeshWorker::mesh_loop() functions.
+   *
+   * It arrays of (local) full matrices, vectors, and local degrees of freedom
+   * index vectors, with size determined by the corresponding template argument.
+   *
+   * In particular, you can specify the following template arguments
+   *
+   * - @p n_matrices: Size of the array of matrices
+   * - @p n_vectors: size of the array of vectors
+   * - @p n_dof_indices: size of the array of local dof indices
+   *
+   * @author Luca Heltai, 2019.
+   */
+  template <int n_matrices    = 1,
+            int n_vectors     = n_matrices,
+            int n_dof_indices = n_matrices>
+  struct CopyData
+  {
+    /**
+     * Initialize everything with the same @p size. This is usually the number
+     * of local degrees of freedom.
+     */
+    CopyData(const unsigned int size);
+
+    /**
+     * For every object, specify the size they should have.
+     */
+    CopyData(
+      const std::array<std::array<unsigned int, 2>, n_matrices> &matrix_sizes,
+      const std::array<unsigned int, n_vectors> &                vector_sizes,
+      const std::array<unsigned int, n_dof_indices> &dof_indices_sizes);
+
+    /**
+     * Deep copy constructor.
+     */
+    CopyData(const CopyData<n_matrices, n_vectors, n_dof_indices> &other) =
+      default;
+
+    /**
+     * Allow resetting of all elements of the struct to zero, by simply
+     * calling `(*this) = 0;`
+     *
+     * Notice that the only allowed number here is really `0`. Calling this
+     * function with any other number will trigger an assertion.
+     *
+     * The elements of the arrays of local degrees of freedom indices are
+     * all set to numbers::invalid_dof_index.
+     */
+    void
+    operator=(const double &number);
+
+    /**
+     * An array of local matrices.
+     */
+    std::array<FullMatrix<double>, n_matrices> matrices;
+
+    /**
+     * An array of local vectors.
+     */
+    std::array<Vector<double>, n_vectors> vectors;
+
+    /**
+     * An array of local degrees of freedom indices.
+     */
+    std::array<std::vector<types::global_dof_index>, n_dof_indices>
+      local_dof_indices;
+  };
+
+
+#ifndef DOXYGEN
+  //
+  // Template definitions
+  //
+  template <int n_matrices, int n_vectors, int n_dof_indices>
+  CopyData<n_matrices, n_vectors, n_dof_indices>::CopyData(
+    const unsigned int size)
+  {
+    for (auto &m : matrices)
+      m.reinit({size, size});
+    for (auto &v : vectors)
+      v.reinit(size);
+    for (auto &d : local_dof_indices)
+      d.resize(size);
+  }
+
+
+
+  template <int n_matrices, int n_vectors, int n_dof_indices>
+  CopyData<n_matrices, n_vectors, n_dof_indices>::CopyData(
+    const std::array<std::array<unsigned int, 2>, n_matrices> &matrix_sizes,
+    const std::array<unsigned int, n_vectors> &                vector_sizes,
+    const std::array<unsigned int, n_dof_indices> &dof_indices_sizes)
+  {
+    for (unsigned int i = 0; i < n_matrices; ++i)
+      matrices[i].reinit(matrix_sizes[i++]);
+
+    for (unsigned int i = 0; i < n_vectors; ++i)
+      vectors[i].reinit(vector_sizes[i++]);
+
+    for (unsigned int i = 0; i < n_dof_indices; ++i)
+      local_dof_indices[i].resize(dof_indices_sizes[i++]);
+  }
+
+
+
+  template <int n_matrices, int n_vectors, int n_dof_indices>
+  void
+  CopyData<n_matrices, n_vectors, n_dof_indices>::
+  operator=(const double &number)
+  {
+    Assert(number == 0.0,
+           ExcMessage("You should only call this method with "
+                      "argument 0.0"));
+
+    for (auto &m : matrices)
+      m = number;
+    for (auto &v : vectors)
+      v = number;
+    for (auto &d : local_dof_indices)
+      for (auto &val : d)
+        val = numbers::invalid_dof_index;
+  }
+
+#endif // DOXYGEN
+} // namespace MeshWorker
+
+DEAL_II_NAMESPACE_CLOSE
+
+#endif