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

Question

I'm trying to use a unique_ptr to derived class in a function that takes a unique_ptr to a base class. Something like:

class Base {};  class Derived : public Base {};  void f(unique_ptr<Base> const &base) {}  …  unique_ptr<Derived> derived = unique_ptr<Derived>(new Derived); f(derived); 

If I understand this answer correctly, this code should work, but it causes the following compile errors:

error C2664: 'f' : cannot convert parameter 1 from 'std::unique_ptr<_Ty>' to 'const std::unique_ptr<_Ty> &'

IntelliSense: no suitable user-defined conversion from "std::unique_ptr<Derived, std::default_delete<Derived>>" to "const std::unique_ptr<Base, std::default_delete<Base>>" exists

If I change f to take unique_ptr<Derived> const &derived, it works fine, but that's not what I want.

Am I doing something wrong? What can I do to work around this?

I'm using Visual Studio 2012.

Answer 1

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);