std::async doesn't parallelize tasks

Question

Using C++11 std::async in this snippet:

int foo()
{
    ::sleep(2);
    return 123;
}

int main()
{
    future<int> r1(async(foo));
    int r2 = foo();
    cout << r1.get() + r2 << endl;
    return 0;
}

It produces the right result, but runs both foo's serially (whole app runs 4 seconds). Compiled as: g++ -std=gnu++11 -O2 foo.cc -lpthread (Ubuntu 12.10 64bit, gcc 4.7.2)

Answer 1

You might need to add a launch policy of std::launch::async:

std::async(std::launch::async, foo);