C++ covariant templates

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SmartPtr<Base> and SmartPtr<Derived> are two distinct instantiations of a the SmartPtr template. These new classes do not share the inheritance that Base and Derived do. Hence, your problem.

what is the best way to perform this assignment operation?

 SmartPtr<Base> b = d;

Does not invoke assignment operator. This invokes the copy-ctor (the copy is elided in most cases) and is exactly as if you wrote:

 SmartPtr<Base> b(d);

Provide for a copy-ctor that takes a SmartPtr<OtherType> and implement it. Same goes for the assignment operator. You will have to write out the copy-ctor and op= keeping in mind the semantics of SmartPtr.

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