ENH: Add null constructor to DiagonalMatrix

src/OpenFOAM/matrices/DiagonalMatrix/DiagonalMatrix.C

@@ -27,6 +27,13 @@ License
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
+template<class Type>
+inline Foam::DiagonalMatrix<Type>::DiagonalMatrix()
+:
+    List<Type>()
+{}
+
+
 template<class Type>
 template<class Form>
 Foam::DiagonalMatrix<Type>::DiagonalMatrix(const Matrix<Form, Type>& a)

src/OpenFOAM/matrices/DiagonalMatrix/DiagonalMatrix.H

@@ -60,6 +60,9 @@ public:
 
     // Constructors
 
+        //- Null constructor.
+        DiagonalMatrix<Type>();
+
         //- Construct from diagonal component of a Matrix
         template<class Form>
         DiagonalMatrix<Type>(const Matrix<Form, Type>&);