unique_ptr to a derived class as an argument to a function that takes a unique_ptr to a base class

You have three options:

1. Give up ownership. This will leave your local variable without access to the dynamic object after the function call; the object has been transferred to the callee:

   f(std::move(derived));
   

2. Change the signature of f:

   void f(std::unique_ptr<Derived> const &);
   

3. Change the type of your variable:

   std::unique_ptr<base> derived = std::unique_ptr<Derived>(new Derived);
   

   Or of course just:

   std::unique_ptr<base> derived(new Derived);
   

   Or even:

   std::unique_ptr<base> derived = std::make_unique<Derived>();
   

4. Update: Or, as recommended in the comments, don’t transfer ownership at all:

   void f(Base & b);
   
   f(*derived);