How to make a class with a member of unique_ptr work with std::move and std::swap?

Question

I have a class. It has a member of unique_ptr.

class A
{
    std::unique_ptr<int> m;
};

I hope it works with the following statements

A a;
A b;
a = std::move(b);
std::swap(a, b);

How to make it?

According to the comments, I have a question. Is this compiler dependent? If I do nothing, it cannot pass compilation in VC++ 2012.

I tried before

struct A
{
    A() {}

    A(A&& a)
    {
        mb = a.mb;
        ma = std::move(a.ma);
    }

    A& operator = (A&& a)
    {
        mb = a.mb;
        ma = std::move(a.ma);
        return *this;
    }

    unique_ptr<int> ma;
    int mb;
};

But not sure if this is the best and simplest way.

You need to do precisely nothing. It should work exactly as defined in your first example, without any user defined constructors or assignment operators. — Benjamin Lindley, Aug 29 '13 at 21:44
You should follow The Rule of Zero and let the compiler generated the default move/copy assignment operators. — David G, Aug 29 '13 at 21:45
You [don't need to define](http://coliru.stacked-crooked.com/view?id=b8e0d2214e) any constructors/assignment operators explicitly to get that to work. Refer to the [Rule of Zero](http://flamingdangerzone.com/cxx11/2012/08/15/rule-of-zero.html) for an explanation. — Praetorian, Aug 29 '13 at 21:49
@BenjaminLindley Is this compiler dependent? If I do nothing, it cannot pass compilation in VC++ 2012. — user1899020, Aug 29 '13 at 22:23
@user1899020: it shouldn't be compiler dependent, but VC++ 2010 and 2012 have a serious bug where it refuses to compile this correctly. At this rate I'm afraid they won't fix it :( Your second block of code is _close_ to the easiest way to do it in VC++2012. Though VC++2013 will allow the simpler standard version. — Mooing Duck, Aug 29 '13 at 22:25

score 2 · Accepted Answer · answered Sep 17 '13 at 09:20

Your first example is absolutely correct in C++11. But VC++ 2012 currently doesn't implement N3053. As a result, the compiler does not implicitly generate move constructor or assignment operator for you. So if you stuck with VC++ 2012 you need to implement them by yourself.

How to make a class with a member of unique_ptr work with std::move and std::swap?

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