“Use of undefined type” with unique_ptr to forward declared class and defaulted move constructor/assignment

10
votes

In the following code, is the only way to avoid a compile error and to include B.h implementing the move constructor / assignment manually in A.cpp?

// A.h
#include <memory>
class B; // implementation somewhere in B.h/B.cpp
class A
{
public:
    A() = default;
    ~A() = default;
    A(const A&) = delete;
    A& operator=(const A&) = delete;
    A(A&&) = default;
    A& operator=(A&&) = default;

private:
    std::unique_ptr<B> m_b;
};

Visual Studio 2015 gives "error C2027: use of undefined type", since the move constructor/assignment operator of std::unique_ptr calls the deleter on m_b (trying to invoke B's destructor), which is obviously not known at this point.

1
c++c++11
related/dup: stackoverflow.com/questions/13414652/… – NathanOliver
I actually didn't know that you could = default in .cpp, too. I thought it was required on declaration. So the proper way of handling my specific case would simply be to declare the move constructor as A(A&&) noexcept; in A.h and define it as A::A(A&&) noexcept = default; in A.cpp. Thanks! – Robert
That sounds like a good idea. – NathanOliver
This is a better solution than the accepted answer imo. – chrisvarnz

1 Answers

16
votes

Yes, you need to have access to the full definition of B from wherever you instantiate std::unique_ptr<B>::~unique_ptr, because it needs to call B's destructor.

In your case, that means that A::~A's definition must be moved to a separate A.cpp file, which includes B.h.