30 Thread support library [thread]

30.6 Futures [futures]

30.6.7 Class template shared_future [futures.shared_future]

The class template shared_future defines a type for asynchronous return objects which may share their shared state with other asynchronous return objects. A default-constructed shared_future object has no shared state. A shared_future object with shared state can be created by conversion from a future object and shares its shared state with the original asynchronous provider ([futures.state]) of the shared state. The result (value or exception) of a shared_future object can be set by calling a respective function on an object that shares the same shared state.

Note: Member functions of shared_future do not synchronize with themselves, but they synchronize with the shared shared state.  — end note ]

The effect of calling any member function other than the destructor, the move-assignment operator, or valid() on a shared_future object for which valid() == false is undefined. [ Note: Implementations are encouraged to detect this case and throw an object of type future_error with an error condition of future_errc::no_state.  — end note ]

namespace std {
  template <class R>
  class shared_future {
  public:
    shared_future() noexcept;
    shared_future(const shared_future& rhs);
    shared_future(future<R>&&) noexcept;
    shared_future(shared_future&& rhs) noexcept;
    ~shared_future();
    shared_future& operator=(const shared_future& rhs);
    shared_future& operator=(shared_future&& rhs) noexcept;

    // retrieving the value
    see below get() const;

    // functions to check state
    bool valid() const noexcept;

    void wait() const;
    template <class Rep, class Period>
      future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
    template <class Clock, class Duration>
      future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
  };
}

The implementation shall provide the template shared_future and two specializations, shared_future<R&> and shared_future<void>. These differ only in the return type and return value of the member function get, as set out in its description, below.

shared_future() noexcept;

Effects: constructs an empty shared_future object that does not refer to a shared state.

Postcondition: valid() == false.

shared_future(const shared_future& rhs);

Effects: constructs a shared_future object that refers to the same shared state as rhs (if any).

Postcondition: valid() returns the same value as rhs.valid().

shared_future(future<R>&& rhs) noexcept; shared_future(shared_future&& rhs) noexcept;

Effects: move constructs a shared_future object that refers to the shared state that was originally referred to by rhs (if any).

Postconditions:

  • valid() returns the same value as rhs.valid() returned prior to the constructor invocation.

  • rhs.valid() == false.

~shared_future();

Effects:

shared_future& operator=(shared_future&& rhs) noexcept;

Effects:

Postconditions:

  • valid() returns the same value as rhs.valid() returned prior to the assignment.

  • rhs.valid() == false.

shared_future& operator=(const shared_future& rhs);

Effects:

  • releases any shared state ([futures.state]);

  • assigns the contents of rhs to *this. [ Note: As a result, *this refers to the same shared state as rhs (if any).  — end note ]

Postconditions: valid() == rhs.valid().

const R& shared_future::get() const; R& shared_future<R&>::get() const; void shared_future<void>::get() const;

Note: as described above, the template and its two required specializations differ only in the return type and return value of the member function get.

Note: access to a value object stored in the shared state is unsynchronized, so programmers should apply only those operations on R that do not introduce a data race ([intro.multithread]).

Effects: wait()s until the shared state is ready, then retrieves the value stored in the shared state.

Returns:

  • shared_future::get() returns a const reference to the value stored in the object's shared state. [ Note: Access through that reference after the shared state has been destroyed produces undefined behavior; this can be avoided by not storing the reference in any storage with a greater lifetime than the shared_future object that returned the reference.  — end note ]

  • shared_future<R&>::get() returns the reference stored as value in the object's shared state.

  • shared_future<void>::get() returns nothing.

Throws: the stored exception, if an exception was stored in the shared state.

bool valid() const noexcept;

Returns: true only if *this refers to a shared state.

void wait() const;

Effects: blocks until the shared state is ready.

template <class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

Effects: none if the shared state contains a deferred function ([futures.async]), otherwise blocks until the shared state is ready or until the relative timeout ([thread.req.timing]) specified by rel_time has expired.

Returns:

  • future_status::deferred if the shared state contains a deferred function.

  • future_status::ready if the shared state is ready.

  • future_status::timeout if the function is returning because the relative timeout ([thread.req.timing]) specified by rel_time has expired.

Throws: timeout-related exceptions ([thread.req.timing]).

template <class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

Effects: none if the shared state contains a deferred function ([futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout ([thread.req.timing]) specified by abs_time has expired.

Returns:

  • future_status::deferred if the shared state contains a deferred function.

  • future_status::ready if the shared state is ready.

  • future_status::timeout if the function is returning because the absolute timeout ([thread.req.timing]) specified by abs_time has expired.

Throws: timeout-related exceptions ([thread.req.timing]).