Why is this code trying to call the copy constructor?

Question

I just spent an inordinate amount of time fiddling with a complilation error in Visual Studio. I have distilled the code into the small compilable example below and tried it on IdeOne and got the same error which you can see here.

I am wondering why the following code tries to call B(const B&) instead of B(B&&):

#include <iostream>

using namespace std;

class A {
public:
    A() : data(53) { }
    A(A&& dying) : data(dying.data) { dying.data = 0; }

    int data;

private:
    // not implemented, this is a noncopyable class
    A(const A&);
    A& operator=(const A&);
};

class B : public A { };

int main() {
    B binst;

    char* buf = new char[sizeof(B)];

    B* bptr = new (buf) B(std::move(binst));

    cout << bptr->data << endl;

    delete[] buf;
}

I didn't explicitly define any constructors, so B(std::move(binst)) should call the compiler generated B(B&&), no?

When I change B to

class B : public A {
public:
    B() { }
    B(B&&) { }
};

It compiles fine. Why is this?

It will be extremely inconvenient if this can't be fixed from the base class because I have a template class which uses placement new and move constructors like the example, and it will require every class that is not copyable (which is not and definitely should not be a requirement for use with my template class) to have an explicitly defined move constructor.

Answer 1

If you are using Visual Studio 2010 or 2012, be advised: the compiler does not automatically generate move constructors for you. That wasn't implemented. So you need to write them yourself.