diff --git a/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.C b/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.C
index c406271fb6..a31166200b 100644
--- a/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.C
+++ b/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.C
@@ -94,4 +94,13 @@ void Foam::thermophysicalFunction::write(Ostream& os, const word& name) const
 }
 
 
+// * * * * * * * * * * * * * * * Ostream Operator  * * * * * * * * * * * * * //
+
+Foam::Ostream& Foam::operator<<(Ostream& os, const thermophysicalFunction& f)
+{
+    f.write(os);
+    return os;
+}
+
+
 // ************************************************************************* //
diff --git a/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.H b/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.H
index b787b9c200..ac0b6452e5 100644
--- a/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.H
+++ b/src/thermophysicalModels/thermophysicalProperties/thermophysicalFunctions/thermophysicalFunction/thermophysicalFunction.H
@@ -44,6 +44,10 @@ Description
 namespace Foam
 {
 
+// Forward declaration of friend functions and operators
+class thermophysicalFunction;
+Ostream& operator<<(Ostream& os, const thermophysicalFunction& l);
+
 /*---------------------------------------------------------------------------*\
                     Class thermophysicalFunction Declaration
 \*---------------------------------------------------------------------------*/
@@ -102,11 +106,11 @@ public:
 
     // Ostream Operator
 
-        friend Ostream& operator<<(Ostream& os, const thermophysicalFunction& f)
-        {
-            f.write(os);
-            return os;
-        }
+        friend Ostream& operator<<
+        (
+            Ostream& os,
+            const thermophysicalFunction& f
+        );
 };