30 Thread support library [thread]

30.6 Futures [futures]

30.6.6 Class template future [futures.unique_future]

The class template future defines a type for asynchronous return objects which do not share their shared state with other asynchronous return objects. A default-constructed future object has no shared state. A future object with shared state can be created by functions on asynchronous providers ([futures.state]) or by the move constructor and shares its shared state with the original asynchronous provider. The result (value or exception) of a future object can be set by calling a respective function on an object that shares the same shared state.

Note: Member functions of future do not synchronize with themselves or with member functions of shared_future.  — end note ]

The effect of calling any member function other than the destructor, the move-assignment operator, or valid on a 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 future {
  public:
    future() noexcept;
    future(future &&) noexcept;
    future(const future& rhs) = delete;
    ~future();
    future& operator=(const future& rhs) = delete;
    future& operator=(future&&) noexcept;
    shared_future<R> share();

    // retrieving the value
    see below get();

    // 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 future and two specializations, future<R&> and future<void>. These differ only in the return type and return value of the member function get, as set out in its description, below.

future() noexcept;

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

Postcondition: valid() == false.

future(future&& rhs) noexcept;

Effects: move constructs a 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() prior to the constructor invocation.

  • rhs.valid() == false.

~future();

Effects:

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

Effects:

Postconditions:

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

  • rhs.valid() == false.

shared_future<R> share();

Returns: shared_future<R>(std::move(*this)).

Postcondition: valid() == false.

R future::get(); R& future<R&>::get(); void future<void>::get();

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.

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

Returns:

  • future::get() returns the value v stored in the object's shared state as std::move(v).

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

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

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

Postcondition: valid() == false.

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]).