add a new overloaded Assign method

src/core/callback.h

@@ -331,6 +331,16 @@ public:
     const Callback<R, T1,T2,T3,T4,T5> *goodType = static_cast<const Callback<R,T1,T2,T3,T4,T5> *> (&other);
     *this = *goodType;
   }
+  void Assign (Ptr<CallbackImplBase> other) {
+    CallbackImpl<R,T1,T2,T3,T4,T5> *impl = dynamic_cast<CallbackImpl<R,T1,T2,T3,T4,T5> *> (PeekPointer (other));
+    if (other == 0)
+      {
+        NS_FATAL_ERROR ("Incompatible types. (feed to \"c++filt -t\")"
+                        " got=" << typeid (other).name () << 
+                        ", expected=" << typeid (*impl).name ());
+      }
+    *this = Callback<R,T1,T2,T3,T4,T5> (impl);
+  }
 private:
   virtual CallbackImpl<R,T1,T2,T3,T4,T5> *PeekImpl (void) const {
     return PeekPointer (m_impl);