Empty DynamicCellSet doesn't segfault on certain queries.

Fixes #370

vtkm/cont/DynamicCellSet.h

@@ -161,7 +161,10 @@ public:
   DynamicCellSetBase<CellSetList> NewInstance() const
   {
     DynamicCellSetBase<CellSetList> newCellSet;
-    newCellSet.CellSet = this->CellSet->NewInstance();
+    if (this->CellSet)
+    {
+      newCellSet.CellSet = this->CellSet->NewInstance();
+    }
     return newCellSet;
   }
 
@@ -172,22 +175,44 @@ public:
   const vtkm::cont::CellSet* GetCellSetBase() const { return this->CellSet.get(); }
 
   VTKM_CONT
-  std::string GetName() const { return this->CellSet->GetName(); }
+  std::string GetName() const { return this->CellSet ? this->CellSet->GetName() : std::string{}; }
 
   VTKM_CONT
-  vtkm::Id GetNumberOfCells() const { return this->CellSet->GetNumberOfCells(); }
+  vtkm::Id GetNumberOfCells() const
+  {
+    return this->CellSet ? this->CellSet->GetNumberOfCells() : 0;
+  }
 
   VTKM_CONT
-  vtkm::Id GetNumberOfFaces() const { return this->CellSet->GetNumberOfFaces(); }
+  vtkm::Id GetNumberOfFaces() const
+  {
+    return this->CellSet ? this->CellSet->GetNumberOfFaces() : 0;
+  }
 
   VTKM_CONT
-  vtkm::Id GetNumberOfEdges() const { return this->CellSet->GetNumberOfEdges(); }
+  vtkm::Id GetNumberOfEdges() const
+  {
+    return this->CellSet ? this->CellSet->GetNumberOfEdges() : 0;
+  }
 
   VTKM_CONT
-  vtkm::Id GetNumberOfPoints() const { return this->CellSet->GetNumberOfPoints(); }
+  vtkm::Id GetNumberOfPoints() const
+  {
+    return this->CellSet ? this->CellSet->GetNumberOfPoints() : 0;
+  }
 
   VTKM_CONT
-  void PrintSummary(std::ostream& stream) const { return this->CellSet->PrintSummary(stream); }
+  void PrintSummary(std::ostream& stream) const
+  {
+    if (this->CellSet)
+    {
+      this->CellSet->PrintSummary(stream);
+    }
+    else
+    {
+      stream << " DynamicCellSet = nullptr" << std::endl;
+    }
+  }
 
 private:
   std::shared_ptr<vtkm::cont::CellSet> CellSet;

vtkm/cont/testing/UnitTestDynamicCellSet.cxx

@@ -41,6 +41,43 @@ class DummyCellSet : public vtkm::cont::CellSet
 {
 };
 
+void CheckEmptyDynamicCellSet()
+{
+  vtkm::cont::DynamicCellSet empty;
+
+  VTKM_TEST_ASSERT(empty.GetName() == std::string{}, "DynamicCellSet should have no name");
+  VTKM_TEST_ASSERT(empty.GetNumberOfCells() == 0, "DynamicCellSet should have no cells");
+  VTKM_TEST_ASSERT(empty.GetNumberOfFaces() == 0, "DynamicCellSet should have no faces");
+  VTKM_TEST_ASSERT(empty.GetNumberOfEdges() == 0, "DynamicCellSet should have no edges");
+  VTKM_TEST_ASSERT(empty.GetNumberOfPoints() == 0, "DynamicCellSet should have no points");
+
+  empty.PrintSummary(std::cout);
+
+  using CellSet2D = vtkm::cont::CellSetStructured<2>;
+  using CellSet3D = vtkm::cont::CellSetStructured<3>;
+  VTKM_TEST_ASSERT(!empty.template IsType<CellSet2D>(), "DynamicCellSet reports wrong type.");
+  VTKM_TEST_ASSERT(!empty.template IsType<CellSet3D>(), "DynamicCellSet reports wrong type.");
+  VTKM_TEST_ASSERT(!empty.template IsType<DummyCellSet>(), "DynamicCellSet reports wrong type.");
+
+  CellSet2D instance;
+  VTKM_TEST_ASSERT(!empty.IsSameType(instance), "DynamicCellSet reports wrong type.");
+
+  bool gotException = false;
+  try
+  {
+    instance = empty.Cast<CellSet2D>();
+  }
+  catch (vtkm::cont::ErrorBadType&)
+  {
+    gotException = true;
+  }
+  VTKM_TEST_ASSERT(gotException, "Empty DynamicCellSet should have thrown on casting");
+
+  auto empty2 = empty.NewInstance();
+  VTKM_TEST_ASSERT(empty.GetCellSetBase() == nullptr, "DynamicCellSet should contain a nullptr");
+  VTKM_TEST_ASSERT(empty2.GetCellSetBase() == nullptr, "DynamicCellSet should contain a nullptr");
+}
+
 template <typename CellSetType, typename CellSetList>
 void CheckDynamicCellSet(const CellSetType& cellSet,
                          vtkm::cont::DynamicCellSetBase<CellSetList> dynamicCellSet)
@@ -119,6 +156,9 @@ void TestDynamicCellSet()
   std::cout << "*** Explicit Grid Constant Shape ********" << std::endl;
   TryNonDefaultCellSet(
     vtkm::cont::CellSetExplicit<vtkm::cont::ArrayHandleConstant<vtkm::UInt8>::StorageTag>());
+
+  std::cout << std::endl << "Try empty DynamicCellSet." << std::endl;
+  CheckEmptyDynamicCellSet();
 }
 
 } // anonymous namespace