You always know the actual dynamic type at construction time, how would you otherwise construct it? > For example shared-ptr to base can correctly invoke the c

You always know the actual dynamic type at construction time, how would you otherwise construct it? > For example shared-ptr to base can correctly invoke the correct derived type Invoke what exactly? Im sorry I don't understand what you're trying to say here. I guess you can force all derivied types to implement a clone() function, such that box<T> can do the deep copy, but Id consider that a fairly big inconvenience for such a simple pointer type.