From e2aacc6bf57f2807e59ee6476c8007d0a89ede4a Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Sun, 24 Oct 2021 19:47:54 -0600
Subject: [PATCH] Make Patch<0,spacedim>::n_subdivisions a 'const' variable.

---
 include/deal.II/base/data_out_base.h |  2 +-
 source/base/data_out_base.cc         | 19 +++++++++++++++++--
 source/particles/data_out.cc         |  5 ++---
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/include/deal.II/base/data_out_base.h b/include/deal.II/base/data_out_base.h
index 6d8e17febd..c517955581 100644
--- a/include/deal.II/base/data_out_base.h
+++ b/include/deal.II/base/data_out_base.h
@@ -433,7 +433,7 @@ namespace DataOutBase
      * this variable is not used but exists only to allow access
      * from general code in a generic fashion.
      */
-    static unsigned int n_subdivisions;
+    static const unsigned int n_subdivisions;
 
     /**
      * Data vectors. The format is as follows: <tt>data(i,.)</tt> denotes the
diff --git a/source/base/data_out_base.cc b/source/base/data_out_base.cc
index 295babfe60..f2a28610de 100644
--- a/source/base/data_out_base.cc
+++ b/source/base/data_out_base.cc
@@ -2070,7 +2070,7 @@ namespace DataOutBase
     Patch<0, spacedim>::no_neighbor};
 
   template <int spacedim>
-  unsigned int Patch<0, spacedim>::n_subdivisions = 1;
+  const unsigned int Patch<0, spacedim>::n_subdivisions = 1;
 
   template <int spacedim>
   const ReferenceCell Patch<0, spacedim>::reference_cell =
@@ -9088,7 +9088,22 @@ namespace DataOutBase
     for (unsigned int i : patch.reference_cell.face_indices())
       in >> patch.neighbors[i];
 
-    in >> patch.patch_index >> patch.n_subdivisions;
+    in >> patch.patch_index;
+
+    // If dim>1, we also need to set the number of subdivisions, whereas
+    // in dim==1, this is a const variable equal to one that can't be changed.
+    unsigned int n_subdivisions;
+    in >> n_subdivisions;
+#ifdef DEAL_II_HAVE_CXX17
+    if constexpr (dim > 1)
+      patch.n_subdivisions = n_subdivisions;
+#else
+    // If we can't use 'if constexpr', work around the fact that we can't
+    // write to a 'const' variable by using a const_cast that is a no-op
+    // whenever the code is actually executed
+    if (dim > 1)
+      const_cast<unsigned int &>(patch.n_subdivisions) = n_subdivisions;
+#endif
 
     in >> patch.points_are_available;
 
diff --git a/source/particles/data_out.cc b/source/particles/data_out.cc
index 901a7ea050..f0c7273b21 100644
--- a/source/particles/data_out.cc
+++ b/source/particles/data_out.cc
@@ -80,9 +80,8 @@ namespace Particles
     auto particle = particles.begin();
     for (unsigned int i = 0; particle != particles.end(); ++particle, ++i)
       {
-        patches[i].vertices[0]    = particle->get_location();
-        patches[i].patch_index    = i;
-        patches[i].n_subdivisions = 1;
+        patches[i].vertices[0] = particle->get_location();
+        patches[i].patch_index = i;
 
         // We have one more data components than dataset_names (the particle id)
         patches[i].data.reinit(n_data_components, 1);
-- 
2.39.5