diff --git a/apps/openmw/mwworld/containerstore.cpp b/apps/openmw/mwworld/containerstore.cpp
index 4b4c51547..6a0921013 100644
--- a/apps/openmw/mwworld/containerstore.cpp
+++ b/apps/openmw/mwworld/containerstore.cpp
@@ -176,7 +176,7 @@ MWWorld::ContainerStoreIterator MWWorld::ContainerStore::restack(const MWWorld::
     return retval;
 }
 
-bool MWWorld::ContainerStore::stacks(const Ptr& ptr1, const Ptr& ptr2)
+bool MWWorld::ContainerStore::stacks(const ConstPtr& ptr1, const ConstPtr& ptr2)
 {
     const MWWorld::Class& cls1 = ptr1.getClass();
     const MWWorld::Class& cls2 = ptr2.getClass();
diff --git a/apps/openmw/mwworld/containerstore.hpp b/apps/openmw/mwworld/containerstore.hpp
index e103e16a1..b01a79366 100644
--- a/apps/openmw/mwworld/containerstore.hpp
+++ b/apps/openmw/mwworld/containerstore.hpp
@@ -149,7 +149,7 @@ namespace MWWorld
 
         public:
 
-            virtual bool stacks (const Ptr& ptr1, const Ptr& ptr2);
+            virtual bool stacks (const ConstPtr& ptr1, const ConstPtr& ptr2);
             ///< @return true if the two specified objects can stack with each other
 
             void fill (const ESM::InventoryList& items, const std::string& owner);
diff --git a/apps/openmw/mwworld/inventorystore.cpp b/apps/openmw/mwworld/inventorystore.cpp
index 8041be1ea..0f9b2380c 100644
--- a/apps/openmw/mwworld/inventorystore.cpp
+++ b/apps/openmw/mwworld/inventorystore.cpp
@@ -450,7 +450,7 @@ void MWWorld::InventoryStore::flagAsModified()
     mRechargingItemsUpToDate = false;
 }
 
-bool MWWorld::InventoryStore::stacks(const Ptr& ptr1, const Ptr& ptr2)
+bool MWWorld::InventoryStore::stacks(const ConstPtr& ptr1, const ConstPtr& ptr2)
 {
     bool canStack = MWWorld::ContainerStore::stacks(ptr1, ptr2);
     if (!canStack)
diff --git a/apps/openmw/mwworld/inventorystore.hpp b/apps/openmw/mwworld/inventorystore.hpp
index 2c1be9b03..1e03746ff 100644
--- a/apps/openmw/mwworld/inventorystore.hpp
+++ b/apps/openmw/mwworld/inventorystore.hpp
@@ -167,7 +167,7 @@ namespace MWWorld
             ///< \attention This function is internal to the world model and should not be called from
             /// outside.
 
-            virtual bool stacks (const Ptr& ptr1, const Ptr& ptr2);
+            virtual bool stacks (const ConstPtr& ptr1, const ConstPtr& ptr2);
             ///< @return true if the two specified objects can stack with each other
 
             virtual int remove(const Ptr& item, int count, const Ptr& actor);
diff --git a/apps/openmw/mwworld/ptr.hpp b/apps/openmw/mwworld/ptr.hpp
index 9d370c4cb..d34f516a7 100644
--- a/apps/openmw/mwworld/ptr.hpp
+++ b/apps/openmw/mwworld/ptr.hpp
@@ -195,6 +195,36 @@ namespace MWWorld
     {
         return !(left>right);
     }
+
+    inline bool operator== (const ConstPtr& left, const ConstPtr& right)
+    {
+        return left.mRef==right.mRef;
+    }
+
+    inline bool operator!= (const ConstPtr& left, const ConstPtr& right)
+    {
+        return !(left==right);
+    }
+
+    inline bool operator< (const ConstPtr& left, const ConstPtr& right)
+    {
+        return left.mRef<right.mRef;
+    }
+
+    inline bool operator>= (const ConstPtr& left, const ConstPtr& right)
+    {
+        return !(left<right);
+    }
+
+    inline bool operator> (const ConstPtr& left, const ConstPtr& right)
+    {
+        return right<left;
+    }
+
+    inline bool operator<= (const ConstPtr& left, const ConstPtr& right)
+    {
+        return !(left>right);
+    }
 }
 
 #endif