instanceOf now uses RTTI and checks its answer.

Next I'll just remove it, but this commit lets me
have something in history such that I can fall back to
check for bugs that might get introduced.

lout/identity.cc

@@ -99,7 +99,7 @@ void IdentifiableObject::registerName (const char *className, const std::type_in
  * \brief Returns, whether this class is an instance of the class, given by
  *    \em otherClassId, or of a sub class of this class.
  */
-bool IdentifiableObject::instanceOf (std::intptr_t otherClassId)
+bool IdentifiableObject::instanceOf_impl (std::intptr_t otherClassId)
 {
    if (otherClassId == -1)
       // Other class has not been registered yet, while it should have been,

lout/identity.hh

@@ -3,6 +3,8 @@
 
 #include <memory>
 #include <typeindex>
+#include <exception>
+#include <stdexcept>
 #include <vector>
 #include <map>
 
@@ -143,7 +145,19 @@ public:
     */
    const char *getClassName() { return classesById.at( classId )->className; }
 
-   bool instanceOf (std::intptr_t otherClassId);
+   template< typename T >
+   [[deprecated]]
+   inline bool
+   instanceOf()
+   {
+      const bool expected= dynamic_cast< T * >( this );
+      const bool witness= instanceOf_impl( T::CLASS_ID );
+      if( witness != expected ) throw std::logic_error{ "RTTI based `instanceOf` disagrees with Dillo's lout:: impl." };
+	  return expected;
+   }
+
+  private:
+   bool instanceOf_impl (std::intptr_t otherClassId);
 };
 
 } // namespace identity