C++11 Thread safety of std::atomic<T> copy constructors

Question

I was going through some problems with my atomic container and saw this link.

Is there a reason why std::atomic isn't copy-constructable? The solution seems to be this where they just pass the T value to the non-atomic constructor with the atomic load function (if I'm not mistaken).

So in general, is this copy constructor thread safe?

template<typename T>
struct MobileAtomic
{
    std::atomic<T> atomic;

    explicit MobileAtomic(std::atomic<T> const& a) : atomic(a.load()) {}

};

Answer 1

Is there a reason why std::atomic isn't copy-constructable?

Yes.

When you are asking for a copy constructible atomic, you're asking for the "normal" rules of single-threaded sequential consistency to apply to a variable that doesn't follow those rules.

In essence, there is no generalized solution.

By using the constructor you show in the question, you sacrifice a deterministic outcome in that you have no guarantee that the source and destination objects are equivalent after construction is complete.