30 Thread support library [thread]

30.6 Futures [futures]

30.6.1 Overview [futures.overview]

[futures] describes components that a C++ program can use to retrieve in one thread the result (value or exception) from a function that has run in the same thread or another thread. [ Note: These components are not restricted to multi-threaded programs but can be useful in single-threaded programs as well.  — end note ]

Header <future> synopsis

namespace std {
  enum class future_errc {

  enum class launch : unspecified {
    async = unspecified,
    deferred = unspecified,

  enum class future_status {

  template <> struct is_error_code_enum<future_errc> : public true_type { };
  error_code make_error_code(future_errc e) noexcept;
  error_condition make_error_condition(future_errc e) noexcept;

  const error_category& future_category() noexcept;

  class future_error;

  template <class R> class promise;
  template <class R> class promise<R&>;
  template <> class promise<void>;

  template <class R>
    void swap(promise<R>& x, promise<R>& y) noexcept;

  template <class R, class Alloc>
    struct uses_allocator<promise<R>, Alloc>;

  template <class R> class future;
  template <class R> class future<R&>;
  template <> class future<void>;

  template <class R> class shared_future;
  template <class R> class shared_future<R&>;
  template <> class shared_future<void>;

  template <class> class packaged_task;   // undefined
  template <class R, class... ArgTypes>
    class packaged_task<R(ArgTypes...)>;

  template <class R>
    void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&) noexcept;

  template <class R, class Alloc>
    struct uses_allocator<packaged_task<R>, Alloc>;

  template <class F, class... Args>
    future<typename result_of<F(Args...)>::type>
    async(F&& f, Args&&... args);
  template <class F, class... Args>
    future<typename result_of<F(Args...)>::type>
    async(launch policy, F&& f, Args&&... args);

The enum type launch is an implementation-defined bitmask type ([bitmask.types]) with launch::async and launch::deferred denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched by async() applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload of async() by adding their extensions to the launch policy under the “as if” rule.  — end note ]