33 Thread support library [thread]

33.6 Futures [futures]

33.6.7 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, share, or valid on a future object for which valid() == false is undefined.

[ Note

It is valid to move from a future object for which valid() == false.

— end note

]

[ 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() noexcept;

    // 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.

Postconditions: 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:

(8.1)
valid() returns the same value as rhs.valid() prior to the constructor invocation.
(8.2)
rhs.valid() == false.

~future();

Effects:

(9.1)
Releases any shared state ([futures.state]);
(9.2)
destroys *this.

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

Effects:

(10.1)
Releases any shared state ([futures.state]).
(10.2)
move assigns the contents of rhs to *this.

Postconditions:

(11.1)
valid() returns the same value as rhs.valid() prior to the assignment.
(11.2)
rhs.valid() == false.

shared_future<R> share() noexcept;

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

Postconditions: 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.

— end note

]

Effects:

(15.1)
wait()s until the shared state is ready, then retrieves the value stored in the shared state;
(15.2)
releases any shared state ([futures.state]).

Returns:

(16.1)
future::get() returns the value v stored in the object's shared state as std::move(v).
(16.2)
future<R&>::get() returns the reference stored as value in the object's shared state.
(16.3)
future<void>::get() returns nothing.

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

Postconditions: 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:

(22.1)
future_status::deferred if the shared state contains a deferred function.
(22.2)
future_status::ready if the shared state is ready.
(22.3)
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:

(25.1)
future_status::deferred if the shared state contains a deferred function.
(25.2)
future_status::ready if the shared state is ready.
(25.3)
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]).