Templates and overloadering operator << [duplicate]

Answer 1

You don't need to make your friend a template:

friend std::ostream& operator<<(std::ostream& os, const Derived& dt);

This needs to be changed to:

friend std::ostream& operator<< <>(std::ostream& os, const Derived& dt);

The operator<< function will need to be visible before the friend declaration for this form to work.

You should prefer this form to making the friend a template because you don't need all of the operator's instantiations to be friends, just one of them.