Copying polymorphic C++ objects using an inherited dup() method

In order to copy polymorphic objects in C++, it can be convenient to equip the base and derived classes with a .dup() method (or .clone() method) that returns a pointer to a copy of the (derived) object. When you have a large amount of different derived classes, overriding the base class's .dup() method for each of them can be a bit of a nuisance. In order to solve this, I sometimes use an "intermediate" class template that can be inherited instead of the base class to provide the .dup() method. This solution is not perfect, because it does not provide the possibility of covariant return types.

The class template Cloneable is defined as follows:

In the following code snippet, the use of the Cloneable is demonstrated:
If someone has a better way to do this, let me know.