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diff --git a/src/stlplus/persistence/persistent_interface.tpp b/src/stlplus/persistence/persistent_interface.tpp
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+++ b/src/stlplus/persistence/persistent_interface.tpp
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+////////////////////////////////////////////////////////////////////////////////
+
+//   Author:    Andy Rushton
+//   Copyright: (c) Southampton University 1999-2004
+//              (c) Andy Rushton           2004-2009
+//   License:   BSD License, see ../docs/license.html
+
+//   Polymorphous classes using the interface approach
+
+//   format: magic [ key data ]
+
+////////////////////////////////////////////////////////////////////////////////
+#include "persistent_int.hpp"
+
+namespace stlplus
+{
+
+  ////////////////////////////////////////////////////////////////////////////////
+
+  template<typename T>
+  void dump_interface(dump_context& context, const T* const data)
+    throw(persistent_dump_failed)
+  {
+    try
+    {
+      // register the address and get the magic key for it
+      std::pair<bool,unsigned> mapping = context.pointer_map(data);
+      dump_unsigned(context,mapping.second);
+      // if the address is null, then that is all that we need to do
+      // however, if it is non-null and this is the first sight of the address, dump the contents
+      if (data && !mapping.first)
+      {
+        // interface method
+        // the lookup just finds the magic key and the type has a dump method
+        // this will throw persistent_illegal_type if the type is not registered
+        unsigned key = context.lookup_interface(typeid(*data));
+        // dump the magic key for the type
+        dump_unsigned(context, key);
+        // now call the dump method defined by the interface
+        data->dump(context);
+      }
+    }
+    catch (const persistent_illegal_type& except)
+    {
+      // convert this to a simpler dump failed exception
+      throw persistent_dump_failed(except.what());
+    }
+  }
+
+  ////////////////////////////////////////////////////////////////////////////////
+
+  template<typename T>
+  void restore_interface(restore_context& context, T*& data)
+    throw(persistent_restore_failed)
+  {
+    try
+    {
+      // first delete any previous object pointed to since the restore creates the object of the right subclass
+      if (data)
+      {
+        delete data;
+        data = 0;
+      }
+      // get the magic key
+      unsigned magic = 0;
+      restore_unsigned(context,magic);
+      // now lookup the magic key to see if this pointer has already been restored
+      // null pointers are always flagged as already restored
+      std::pair<bool,void*> address = context.pointer_map(magic);
+      if (address.first)
+      {
+        // seen before, so simply map it to the existing address
+        data = (T*)address.second;
+      }
+      else
+      {
+        // now restore the magic key that denotes the particular subclass
+        unsigned key = 0;
+        restore_unsigned(context, key);
+        // interface approach
+        // first clone the sample object stored in the map - lookup_interface can throw persistent_illegal_type
+        data = (T*)(context.lookup_interface(key)->clone());
+        // add this pointer to the set of already seen objects
+        // do this before restoring the object so that self-referential structures restore correctly
+        context.pointer_add(magic,data);
+        // now restore the contents using the object's method
+        data->restore(context);
+      }
+    }
+    catch (const persistent_illegal_type& exception)
+    {
+      // convert this to a simpler dump failed exception
+      throw persistent_restore_failed(exception.what());
+    }
+  }
+
+  ////////////////////////////////////////////////////////////////////////////////
+
+} // end namespace stlplus