Passing by reference to std::thread c++0x in VS2012

Question

void ThreadFn(int& i)
{
    cout<<"Hi from thread "<<i<<endl;
}
int x = 0;
void CreateThreads(vector<thread>& workers)
{
    for(int i = 0; i< 10; i++)
    {
        workers.push_back(thread(&ThreadFn, x));
    }
}

I was expecting a compilation error in the thread creation (workers.push_back(thread(&ThreadFn, x));) since x should be passed by ref. I though the right syntax should've been:

workers.push_back(thread(&ThreadFn, std::ref(x)));

Of course, the code compiles fine and also behaves properly. I am using VC11. Any idea why this isn't being flagged?

score 3 · Answer 1 · answered Jul 25 '13 at 13:20

This is a VC11 bug, the thread object makes internal copies of the arguments (as it should) but then it doesn't forward them to the ThreadFn function correctly, so what happens is the reference binds to the thread object's internal int member.

GCC's std::thread used to have a similar bug because we used std::bind to implement it, but I replaced the use of std::bind with a different implementation detail that forwards the captured arguments to the function by value.

Passing by reference to std::thread c++0x in VS2012

1 Answers1