Initializing std::atomic_bool?

This is a known issue in Visual Studio 2012 (known as VC11), you should vote on the existing Connect item so that Microsoft knows it affects more folks as they've deferred the fix.

Hi,

Thanks for reporting this bug. I'm Microsoft's maintainer of the STL, and I wanted to let you know that while this bug remains active in our database, it won't be fixed in VC11 RTM (VS 2012 RTM). All bugs are important to us, but some are more severe than others and rise to the top of our priority queue.

I'm copying-and-pasting this response across all of the STL's active Connect bugs, but the following terse comments apply specifically to your bug:

• Yep, we're missing these constructors on atomic_bool, atomic_int, etc. (atomic<bool>, atomic<int>, etc. have them). 29.5 [atomics.types.generic]/7 says "There shall be named types corresponding to the integral specializations of atomic, as specified in Table 145, and a named type atomic_bool corresponding to the specified atomic<bool>. Each named type is a either typedef to the corresponding specialization or a base class of the corresponding specialization. If it is a base class, it shall support the same member functions as the corresponding specialization." which makes me really want to use typedefs (1 type is always simpler than 2 types), but I'll need to see if that would introduce any other issues.

I can't promise when we'll be able to resolve this bug, but we hope to do so as soon as possible (and I'll send another response when that happens) - our first opportunity will be the "out of band" release between VC11 and VC12 that Herb Sutter announced at the GoingNative 2012 conference.

Note: Connect doesn't notify me about comments. If you have any further questions, please E-mail me.

Stephan T. Lavavej Senior Developer - Visual C++ Libraries [email protected]

Basically, you'll need to use std::atomic<T> for now.

Try this:

atomic_bool my_bool = ATOMIC_VAR_INIT(false);

http://en.cppreference.com/w/cpp/atomic/ATOMIC_VAR_INIT

PROBLEM:

You cannot use copy-initialization, because std::atomic_bool is not copy-constructible:

std::atomic_bool World::mStopEvent = false; // ERROR!

In fact, the above is equivalent to:

std::atomic_bool World::mStopEvent = std::atomic_bool(false); // ERROR!

However, you can use direct-initialization:

std::atomic_bool World::mStopEvent(false);

At your wish, you may choose to use braces instead of parentheses:

std::atomic_bool World::mStopEvent{false};

BUG:

While copy-initialization is illegal no matter what compiler you choose, it seems that the implementation of the Standard Library shipped with VC11 has a bug that won't let you perform direct-initialization either.

So how I am supposed to initialize such a variable?

WORKAROUND:

As a possible workaround, you can provide a pair of static getter/setter wrappers that - respectively - set and return the value of the atomic boolean flag, but not before making sure it has been initialized at least once and not more than once to the desired initial value in a thread-safe manner (you can consider this some kind of lazy initialization):

#include <atomic>
#include <mutex>

struct World
{
    static bool is_stop_event_set()
    {
        std::call_once(mStopEventInitFlag, [] () { mStopEvent = false; });
        return mStopEvent;
    }

    static void set_stop_event(bool value)
    {
        std::call_once(mStopEventInitFlag, [value] () { mStopEvent = value; });
        mStopEvent = value;
    }

    static std::atomic_bool mStopEvent;
    static std::once_flag mStopEventInitFlag;
};

std::atomic_bool World::mStopEvent;
std::once_flag World::mStopEventInitFlag;

Now instead of accessing mStopEvent directly, its value shall be read through the is_stop_event_set() function:

#include <iostream>

int main()
{
    std::cout << World::is_stop_event_set(); // Will return false
}