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.
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).
Effects:
releases any shared state ([futures.state]);
destroys *this.
shared_future& operator=(shared_future&& rhs) noexcept;
Effects:
releases any shared state ([futures.state]);
move assigns the contents of rhs to *this.
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.
Returns: true only if *this refers to a shared state.
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.
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.