tmp: Limit the number of references to a temporary object to 2

which reduces the number of potential problems with the reuse of
temporary objects.

In order to avoid unnecessary creation of tmp's referring to temporary
objects the assignment operator now transfers ownership of the object
and resets the argument.

src/OpenFOAM/memory/tmp/tmp.H

@@ -70,6 +70,11 @@ class tmp
         mutable T* ptr_;
 
 
+    // Private member operators
+
+        inline void operator++();
+
+
 public:
 
     // Constructors

src/OpenFOAM/memory/tmp/tmpI.H

@@ -26,6 +26,23 @@ License
 #include "error.H"
 #include <typeinfo>
 
+// * * * * * * * * * * * * * Private Member Operators  * * * * * * * * * * * //
+
+template<class T>
+inline void Foam::tmp<T>::operator++()
+{
+    ptr_->operator++();
+
+    if (ptr_->count() > 1)
+    {
+        FatalErrorInFunction
+            << "Attempt to create more than 2 tmp's referring to"
+               " the same object of type " << typeName()
+            << abort(FatalError);
+    }
+}
+
+
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 template<class T>
@@ -62,7 +79,7 @@ inline Foam::tmp<T>::tmp(const tmp<T>& t)
     {
         if (ptr_)
         {
-            ptr_->operator++();
+            operator++();
         }
         else
         {
@@ -90,7 +107,7 @@ inline Foam::tmp<T>::tmp(const tmp<T>& t, bool allowTransfer)
             }
             else
             {
-                ptr_->operator++();
+                operator++();
             }
         }
         else
@@ -342,8 +359,7 @@ inline void Foam::tmp<T>::operator=(const tmp<T>& t)
         }
 
         ptr_ = t.ptr_;
-
-        ptr_->operator++();
+        t.ptr_ = 0;
     }
     else
     {