Question

I’m new to move constructor and quite confused with VS2010 behavior.
I design a move constructor (of class A) which in my knowledge is like this:

A(A&& input) {some code}

When I use list’s emplace and put an instance of class A:

mylist.emplace(a);

My move constructor is not called, and a non-const copy constructor is called instead:

A(A& input) {the same code as move constructor}

On the other hand when I do this:

mylist.emplace(A(2));

My move constructor is called like it supposed to. So, my question is:

1. Why is list’s emplace call my non-const copy constructor instead of my move constructor?
2. Is the non-const copy constructor is actually an alternative way to define a move constructor?
3. Is this behavior is correct (for a c++0x compiler) or it just VS2010’s behavior?

Thanks a bunch in advance.

Answer 1

mylist.emplace(a);

Here a is an l-value, hence it’s copied rather than moved. You need to move it explicitely:

mylist.emplace(std::move(a));

Yes, the behavior is correct.