std::async is included in the C++11 standard. The number of threads that async will use is implementation dependent, I bet on Windows with VS2012 you will see a

std::async is included in the C++11 standard. The number of threads that async will use is implementation dependent, I bet on Windows with VS2012 you will see a different number of threads. You could force async to run asynchronously or in a lazy evaluation mode using launch policies.