Converting std::unique_ptr<Derived> to std::unique_ptr<Base>

Question

Using C++11, let's say I have factory functions dealing with base and derived classes:

#include <memory>

using namespace std;

struct B { virtual ~B() {} };
struct D : B {};

unique_ptr<B> MakeB()
{
    auto b = unique_ptr<B>( new B() );
    return b; // Ok!
}

unique_ptr<B> MakeD()
{
    auto d = unique_ptr<D>( new D() );
    return d; // Doh!
}

On the last line above, I need move(d) in order to make it work, otherwise I get "Error: invalid conversion from std::unique_ptr<D> to std::unique_ptr<D>&&." My intuition said that in this context, the compiler should know that it could implicitly make d an rvalue and move it into the base pointer, but it doesn't.

Is this a non-conformancy in my compilers (gcc 4.8.1 and VS2012)? The intended design of unique_ptr? A defect in the standard?

---

Update: C++14 fixes this. Newer compilers such as GCC 9 accept the original code even with -std=c++11.

Answer 1

The compiler's behaviour is correct. There is only an implicit move when the types are the same, because implicit move is specified in terms of the compiler failing to perform copy elision in cases where it is actually allowed (see 12.8/31 and 12.8/32).

12.8/31 (copy elision):

in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type...

12.8/32 (implicit move):

When the criteria for elision of a copy operation are met, [...], overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.

Answer 2

With the addition of the std::move calls in the return statements, this code works for me on Visual Studio 2013:

#include <memory>

using namespace std;

struct B { virtual ~B() {} };
struct D : B {};

unique_ptr<B> MakeB()
{
  auto b = unique_ptr<B>( new B() );
  return std::move( b );  // *** std::move() added here! ***
}

unique_ptr<B> MakeD()
{
  auto d = unique_ptr<D>( new D() );
  return std::move( d );  // *** std::move() added here! ***
}

The following also works

unique_ptr<B> MakeB()
{
  return unique_ptr<B>( new B() );  // *** Returning a temporary! ***
}

unique_ptr<B> MakeD()
{
  return unique_ptr<D>( new D() );  // *** Returning a temporary! ***
}

Of course, if you're returning std::unique_ptr<B> anyway, why not shove the D instance into a std::unique_ptr<B> in the first place? Then there's no conversion necessary at all!