Async and Launch Options

• std::async() may start a new thread for the task
• Or it may run the task in the same thread
• Controlled by the "launch flag"
  • Optional argument to std::async()

Launch Flags

• std::launch::async

  • A new thread is started for the task
  • The task is executed as soon as the thread starts

• std::launch::deferred

  • Nothing happens until get() is called on the returned future
  • The task is then executed ("lazy evaluation")

• If both flags are set

  • The implementation decides whether to start a new thread
  • This is the default

Default Launch Policy

• Lack of certainty

  • The task could execute synchronously with the initiating thread
  • The task could execute concurrently with the initiating thread
  • It could execute concurrently with the thread that calls get()
  • If get() is not called, the task may not execute at all

• Thread local storage (TLS)

  • We do not know which thread's data will be used

Launch Policy Recommendations

• Use the async launch option if any of these are true

  • The task must execute in a separate thread
  • The task must start immediately
  • The task will use thread-local storage
  • The task function must be executed, even if get() is not called
  • The thread receiving the future will call wait_for() or wait_until()

• Use the deferred launch option if

  • The task must be run in the thread which calls get()
  • The task must be executed, even if no more threads can be created
  • You want lazy execution of the task

• Otherwise, let the implementation choose

Return Value from wait() and Friends

• wait() Returns nothing

• wait_for()

• wait_until()

  • Return std::future_status::ready if the result is available
  • Return std;;future_status::timeout if the timeout has expired
  • Return std::future_status::deferred if the result is being lazily evaluated

• In lazy evaluation, the task does not run until get() is called