STYLE: use 'return nullptr' for empty autoPtr/tmp returns

- both autoPtr and tmp are defined with an implicit construct from
  nullptr (but with explicit construct from a pointer to null).
  Thus is it safe to use 'nullptr' when returning an empty autoPtr or tmp.

applications/solvers/multiphase/compressibleMultiphaseInterFoam/multiphaseMixtureThermo/phaseModel/phaseModel.C

@@ -81,7 +81,7 @@ Foam::phaseModel::phaseModel
 Foam::autoPtr<Foam::phaseModel> Foam::phaseModel::clone() const
 {
     NotImplemented;
-    return autoPtr<phaseModel>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/interCondensatingEvaporatingFoam/temperaturePhaseChangeTwoPhaseMixtures/twoPhaseMixtureEThermo/twoPhaseMixtureEThermo.C

@@ -259,7 +259,7 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::THE
 ) const
 {
     NotImplemented;
-    return tmp<Foam::scalarField>();
+    return nullptr;
 }
 
 
@@ -272,7 +272,7 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::THE
 ) const
 {
     NotImplemented;
-    return tmp<Foam::scalarField>();
+    return nullptr;
 }
 
 
@@ -436,8 +436,8 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::Cpv
 
 Foam::tmp<Foam::volScalarField> Foam::twoPhaseMixtureEThermo::CpByCpv() const
 {
-     NotImplemented;
-     return tmp<Foam::volScalarField>();
+    NotImplemented;
+    return nullptr;
 }
 
 
@@ -448,8 +448,8 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::CpByCpv
     const label patchi
 ) const
 {
-     NotImplemented;
-     return tmp<Foam::scalarField>();
+    NotImplemented;
+    return nullptr;
 }
 
 

applications/solvers/multiphase/multiphaseEulerFoam/multiphaseSystem/phaseModel/phaseModel.C

@@ -205,7 +205,7 @@ Foam::phaseModel::~phaseModel()
 Foam::autoPtr<Foam::phaseModel> Foam::phaseModel::clone() const
 {
     NotImplemented;
-    return autoPtr<phaseModel>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/multiphaseInterFoam/multiphaseMixture/phase/phase.C

@@ -68,7 +68,7 @@ Foam::phase::phase
 Foam::autoPtr<Foam::phase> Foam::phase::clone() const
 {
     NotImplemented;
-    return autoPtr<phase>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/IsothermalPhaseModel/IsothermalPhaseModel.C

@@ -65,7 +65,7 @@ template<class BasePhaseModel>
 Foam::tmp<Foam::fvScalarMatrix>
 Foam::IsothermalPhaseModel<BasePhaseModel>::heEqn()
 {
-    return tmp<fvScalarMatrix>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/MultiComponentPhaseModel/MultiComponentPhaseModel.C

@@ -152,7 +152,7 @@ Foam::MultiComponentPhaseModel<BasePhaseModel>::YiEqn
         )
     )
     {
-        return tmp<fvScalarMatrix>();
+        return nullptr;
     }
 
     const volScalarField& alpha = *this;

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/PurePhaseModel/PurePhaseModel.C

@@ -57,8 +57,7 @@ Foam::PurePhaseModel<BasePhaseModel>::YiEqn
 )
 {
     NotImplemented;
-
-    return tmp<fvScalarMatrix>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/phaseModel/phaseModel.C

@@ -77,7 +77,7 @@ Foam::phaseModel::phaseModel
 Foam::autoPtr<Foam::phaseModel> Foam::phaseModel::clone() const
 {
     NotImplemented;
-    return autoPtr<phaseModel>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/reactingTwoPhaseEulerFoam/twoPhaseSystem/diameterModels/IATE/IATEsources/IATEsource/IATEsource.H

@@ -115,7 +115,7 @@ public:
         autoPtr<IATEsource> clone() const
         {
             NotImplemented;
-            return autoPtr<IATEsource>();
+            return nullptr;
         }
 
 

applications/solvers/multiphase/twoPhaseEulerFoam/twoPhaseSystem/diameterModels/IATE/IATEsources/IATEsource/IATEsource.H

@@ -114,7 +114,7 @@ public:
         autoPtr<IATEsource> clone() const
         {
             NotImplemented;
-            return autoPtr<IATEsource>();
+            return nullptr;
         }
 
 

applications/utilities/mesh/generation/foamyMesh/conformalVoronoiMesh/DelaunayMesh/DistributedDelaunayMesh.C

@@ -794,14 +794,12 @@ Foam::DistributedDelaunayMesh<Triangulation>::distribute
 {
     if (!Pstream::parRun())
     {
-        return autoPtr<mapDistribute>();
+        return nullptr;
     }
 
     distributeBoundBoxes(decomposition.procBounds());
 
-    autoPtr<mapDistribute> mapDist = decomposition.distributePoints(points);
-
-    return mapDist;
+    return decomposition.distributePoints(points);
 }
 
 

applications/utilities/mesh/generation/foamyMesh/conformalVoronoiMesh/searchableSurfaceFeatures/searchableSurfaceFeatures.H

@@ -101,7 +101,7 @@ public:
         virtual autoPtr<searchableSurfaceFeatures> clone() const
         {
             NotImplemented;
-            return autoPtr<searchableSurfaceFeatures>();
+            return nullptr;
         }
 
 
@@ -139,7 +139,7 @@ public:
         //- Return an extendedFeatureEdgeMesh containing the features
         virtual autoPtr<extendedFeatureEdgeMesh> features() const
         {
-            return autoPtr<extendedFeatureEdgeMesh>();
+            return nullptr;
         }
 };
 

applications/utilities/mesh/manipulation/createBaffles/faceSelection/faceSelection.H

@@ -109,7 +109,7 @@ public:
         autoPtr<faceSelection> clone() const
         {
             NotImplemented;
-            return autoPtr<faceSelection>();
+            return nullptr;
         }
 
 

applications/utilities/mesh/manipulation/createBaffles/faceSelection/faceZoneSelection.H

@@ -86,7 +86,7 @@ public:
         autoPtr<faceSelection> clone() const
         {
             NotImplemented;
-            return autoPtr<faceSelection>();
+            return nullptr;
         }
 
 

applications/utilities/mesh/manipulation/createBaffles/faceSelection/searchableSurfaceSelection.H

@@ -79,7 +79,7 @@ public:
         autoPtr<faceSelection> clone() const
         {
             NotImplemented;
-            return autoPtr<faceSelection>();
+            return nullptr;
         }
 
 

applications/utilities/parallelProcessing/reconstructParMesh/reconstructParMesh.C

@@ -251,7 +251,7 @@ autoPtr<mapPolyMesh> mergeSharedPoints
 
     if (returnReduce(pointToMaster.size(), sumOp<label>()) == 0)
     {
-        return autoPtr<mapPolyMesh>();
+        return nullptr;
     }
 
     polyTopoChange meshMod(mesh);

applications/utilities/postProcessing/lumped/lumpedPointForces/lumpedPointForces.C

@@ -71,7 +71,7 @@ autoPtr<GeoFieldType> loadField
         );
     }
 
-    return autoPtr<GeoFieldType>();
+    return nullptr;
 }
 
 

applications/utilities/surface/surfacePatch/searchableSurfaceModifier/autoPatch.H

@@ -76,7 +76,7 @@ public:
         autoPtr<searchableSurfaceModifier> clone() const
         {
             NotImplemented;
-            return autoPtr<searchableSurfaceModifier>();
+            return nullptr;
         }
 
 

applications/utilities/surface/surfacePatch/searchableSurfaceModifier/cut.H

@@ -100,7 +100,7 @@ public:
         autoPtr<searchableSurfaceModifier> clone() const
         {
             NotImplemented;
-            return autoPtr<searchableSurfaceModifier>();
+            return nullptr;
         }
 
 

applications/utilities/surface/surfacePatch/searchableSurfaceModifier/searchableSurfaceModifier.H

@@ -96,7 +96,7 @@ public:
         autoPtr<searchableSurfaceModifier> clone() const
         {
             NotImplemented;
-            return autoPtr<searchableSurfaceModifier>();
+            return nullptr;
         }