In addition to the basic version of std::async(), there is a second version which allows one to specify the launch policy as its first argument. This is a bitmask value of type std::launch with the following possible values:

* launch::async 
* launch::deferred 

The async flag means that a new thread and execution context for the worker function is created immediately. The deferred flag means that this is postponed until wait() or get() is called on the future object. Specifying both flags causes the function to choose the method automatically depending on the current system situation.

The std::async() version, without explicitly specified bitmask values, defaults to the latter, automatic method.