Add comment explaining the use if default messaget instance in goto_program_dereferencet

Author: Natasha Yogananda Jeppu

src/pointer-analysis/goto_program_dereference.h

@@ -28,15 +28,24 @@ class goto_program_dereferencet:protected dereference_callbackt
   // for the final argument to value_set_dereferencet.
   // This means that language-inappropriate values such as
   // (struct A*)some_integer_value in Java, may be returned.
+  // Note: value_set_dereferencet requires a messaget instance
+  // as on of its arguments to display the points-to set
+  // during symex. Display is not done during goto-program
+  // conversion. To ensure this the display_points_to_sets
+  // parameter in value_set_dereferencet::dereference()
+  // is set to false by default and is not changed by the
+  // goto program conversion modules. Similarly, here we set
+  // _log to be a default messaget instance.
   goto_program_dereferencet(
     const namespacet &_ns,
     symbol_tablet &_new_symbol_table,
     const optionst &_options,
-    value_setst &_value_sets)
+    value_setst &_value_sets,
+    const messaget &_log = messaget())
     : options(_options),
       ns(_ns),
       value_sets(_value_sets),
-      dereference(_ns, _new_symbol_table, *this, ID_nil, false, messaget())
+      dereference(_ns, _new_symbol_table, *this, ID_nil, false, _log)
   {
   }