[BlockVectorWindow] Demonstrate behaviour of copying blocks

dune/istl/test/vbvectortest.cc

@@ -3,6 +3,8 @@
 #include "config.h"
 #include <dune/istl/vbvector.hh>
 #include <dune/common/fvector.hh>
+#include <dune/common/typetraits.hh>
+#include <dune/common/test/testsuite.hh>
 
 #include <dune/istl/test/vectortest.hh>
 
@@ -10,6 +12,8 @@ using namespace Dune;
 
 int main()
 {
+  TestSuite suite;
+
   VariableBlockVector<FieldVector<double,1> > v1;
   VariableBlockVector<FieldVector<double,1> > v2 = v1;
   VariableBlockVector<FieldVector<double,1> > v3(10);
@@ -25,11 +29,30 @@ int main()
 
   v3 = 1.0;
 
+  /* Copy-ing specific blocks with `auto` from a VariableBlockVector is tricky, because
+   * the returned object will be a reference:
+   */
+  auto block0_copy_reference = v3[0];
+  block0_copy_reference[0] = 4.2; // change first entry in the copy which has reference semantics. This also changes v3!
+  suite.check(v3[0][0] != 1.0, "Show auto x = v3[0] has reference semantics")
+    << "Unexpected behaviour: v3[0][0] is " << v3[0][0];
+
+  v3[0][0]=1.0; // reset v3
+
+  // For an actual copy, use the Dune::autoCopy() mechanism
+  // This will give a BlockVector with the contents of v3[0].
+  auto block0_autoCopy = Dune::autoCopy(v3[0]);
+  block0_autoCopy[0] = 4.2;
+  suite.check(v3[0][0] == 1.0, "Show that v3 was not modified when copying via autoCopy")
+    << "Unexpected behaviour: v3[0][0] is " << v3[0][0];
+
+
+  // Perform more general vector tests:
   testHomogeneousRandomAccessContainer(v3);
   Dune::testConstructibility<VariableBlockVector<FieldVector<double,1> > >();
   testNorms(v3);
   testVectorSpaceOperations(v3);
   testScalarProduct(v3);
 
-  return 0;
+  return suite.exit();
 }