Launch policies

Apart from specifying the function or callable object as an argument when using the std::async function, we can also specify the launch policy. Launch policies control how std::async schedules the execution of asynchronous task. These are defined in the <future> library.

The launch policy must be specified as the first argument when calling std::async. This argument is of the type std::launch, a bitmask value where its bits control the allowed methods of execution, which can be one or more of the following enumeration constants:

• std::launch::async: The task is executed in a separate thread.
• std::launch::deferred: Enables lazy evaluation by executing the task in the calling thread the first time its result is requested via the future get() or wait() method. All further accesses to the same std::future will return the result immediately. That means that the task will only be executed when the result is explicitly requested, which can lead to unexpected...