bool is_ready() const;

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

template <typename F>
see below then(F&& func);

template <typename F>
see below then(launch policy, F&& func);

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

Effects:

Constructs a future object from the shared state referred to by rhs and unwrapping the inner future. The future becomes ready when one of the following occurs:

• Both the outer and the inner futures are ready. The future stores the value or the exception from the inner future.
• The outer future is ready but the inner future is invalid. The future stores an exception of type std::future_error, with an error condition of std::future_errc::broken_promise.

Postconditions:

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

template <typenameclass F>
see below then(F&& func);
template <typename F>
see below then(launch policy, F&& func);

Requires:

INVOKE(DECAY_COPY (std::forward<F>(func)), std::move(*this)) shall be a valid expression.

Notes:

The two functions differ only by input parameters. The first only takes a callable object which accepts a future object as a parameter. The second function takes a launch policy as the first parameter and a callable object as the second parameter. The function takes a callable object which accepts a future object as a parameter.

Effects:

The function creates a shared state that is associated with the returned future object. The further behavior of the function is as follows.

• The continuation INVOKE(DECAY_COPY (std::forward<F>(func))) is called when the object's shared state is ready (has a value or exception stored).
• When the object's shared state is ready, the continuation INVOKE(DECAY_COPY(std::forward<F>(func)), std::move(*this)) is called on an unspecified thread of execution with the call to DECAY_COPY() being evaluated in the thread that called then.
• The continuation launches according to the specified launch policy.
• When the launch policy is not provided the continuation inherits the parent's launch policy.
• Any value returned from the continuation is stored as the result in the shared state of the resulting future. Any exception propagated from the execution of the continuation is stored as the exceptional result in the shared state of the resulting future.
• If the parent was created with std::promise or with a packaged_task (has no associated launch policy), the continuation behaves the same as in the second overload with a policy argument of launch::async | launch::deferred and the same argument for func.
• If the parent has a policy of launch::deferred, then it is filled by calling wait() or get() on the resulting future. [ Example:

  
  auto f1 = async(launch::deferred, [] { return 1; });
  
  auto f2 = f1.then([](future n) { return 2; });
  
  f2.wait(); // execution of f1 starts here, followed by f2
        

  — end example ]

Returns:

The return type of then depends on the return type of the closure func as defined below:

• When result_of_t<decay_t<F>(future<R>)> is future<R2>, the function returns future<R2>.
• Otherwise, the function returns future<result_of_t<decay_t<F>(future<R>)>>.
  [ Note: The first rule above is called implicit unwrapping. Without this rule, the return type of then taking a closure returning a future<R> would have been future<future<R>>. This rule avoids such nested future objects. The type of f2 below is future<int> and not future<future<int>>: [ Example:

  future<int> f1 = g();
  future<int> f2 = f1.then([](future<int> f) {
                      future<int> f3 = h();
                      return f3;
                   });
  

  — end example ] — end note ]

Postconditions:

• The future object is moved to the parameter of the continuation function.
• valid() == false on the original future.
• valid() == true on the future returned from then. object immediately after it returns.
  Notes:
  In case of implicit unwrapping, the validity of the future returned from then cannot be established until after the completion of the continuation. If it is not valid, the resulting future becomes ready with an exception of type std::future_error, with an error condition of std::future_errc::broken_promise.

bool is_ready() const noexcept;

Returns:

true if the shared state is ready, otherwise false.

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

Effects:

Constructs a shared_future object from the shared state referred to by rhs. The shared_future becomes ready when one of the following occurs:

• Both the outer future and the inner shared_future are ready. The shared_future stores the value or the exception from the inner shared_future.
• The outer future is ready but the inner shared_future is invalid. The shared_future stores an exception of type std::future_error, with an error condition of std::future_errc::broken_promise.

Postconditions:

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

template <typenameclass F>
see below then(F&& func) const;

template <class F>
see below then(launch policy, F&& func) const;

Requires:

INVOKE(DECAY_COPY (std::forward<F>(func)), *this) shall be a valid expression.

Notes:

The two functions differ only by input parameters. The first only takes a callable object which accepts a shared_future object as a parameter. The second function takes a launch policy as the first parameter and a callable object as the second parameter. The function takes a callable object which accepts a shared_future object as a parameter.

Effects:

The function creates a shared state that is associated with the returned future object. The further behavior of the function is as follows.

• The continuation INVOKE(DECAY_COPY (std::forward<F>(func)), *this) is called when the object's shared state is ready (has a value or exception stored).
• When the object's shared state is ready, the continuation INVOKE(DECAY_COPY(std::forward<F>(func)), *this) is called on an unspecified thread of execution with the call to DECAY_COPY() being evaluated in the thread that called then.
• The continuation launches according to the specified policy.
• When the launch policy is not provided the continuation inherits the parent's launch policy.
• Any value returned from the continuation is stored as the result in the shared state of the resulting future. Any exception propagated from the execution of the continuation is stored as the exceptional result in the shared state of the resulting future.
• If the parent was created with std::promise (has no associated launch policy), the continuation behaves the same as in the second function with a policy argument of launch::async | launch::deferred and the same argument for func.
• If the parent has a policy of launch::deferred, then it is filled by calling wait() or get() on the resulting shared_future. [ Note: This is similar to future. See example in 3.3. — end note ]

Returns:

The return type of then depends on the return type of the closure func as defined below:

• When result_of_t<decay_t<F>(shared_future<R>)> is future<R2>, the function returns future<R2>.
• Otherwise, the function returns future<result_of_t<decay_t<F>(shared_future<R>)>>.

  [ Note: This is analogous to future. See the notes on future::then return type in 3.3. — end note ]

Postconditions:

• The shared_future passed to the continuation function is a copy of the original shared_future.
• valid() == true on the original shared_future object. valid() == true on the shared_future returned from then.
  Notes:
  In case of implicit unwrapping, the validity of the future returned from then cannot be established until after the completion of the continuation. In such case, the resulting future becomes ready with an exception of type std::future_error, with an error condition of std::future_errc::broken_promise.

bool is_ready() const noexcept;

Returns:

true if the shared state is ready, otherwise false.

template <class InputIterator>
see below
future<vector<typename iterator_traits<InputIterator>::value_type>>
when_all(InputIterator first, InputIterator last);

template <typenameclass... TFutures>
see below
future<tuple<decay_t<Futures>...>> when_all(TFutures&&... futures);

Requires:

• For the first overload, iterator_traits<InputIterator>::value_type must be future<R> or shared_future<R>, for some type R.
• T is of type future<R> or shared_future<R> All futures and shared_futures passed into when_all must be in a valid state (i.e. valid() == true).

Notes:

• There are two variations of when_all. The first version takes a pair of InputIterators. The second takes any arbitrary number of future<R0> and shared_future<R1> objects, where R0 and R1 need not be the same type.
• Calling the first signatureoverload of when_all where first == last, returns a future with an empty vector that is immediately ready.
• Calling the second signatureoverload of when_allwhen_any with no arguments returns a future<tuple<>> that is immediately ready.

Remarks:

For the second overload, let Ui be decay_t<Fi> for each Fi in Futures. This function shall not participate in overload resolution unless each Ui is either future<Ri> or shared_future<Ri>.

Effects:

• Each future and shared_future is waited upon and then copied into the collection of the output (returned) future, maintaining the order of the futures in the input collection. A new shared state containing a Sequence is created, where Sequence is either tuple or a vector based on the overload, as specified above. A new future object that refers to that shared state is created and returned from when_all.
• Once all the futures and shared_futures supplied to the call to when_all are ready, the futures are moved, and the shared_futures are copied, into, correspondingly, futures or shared_futures of the futures member of Sequence in the shared state.
• The order of the objects in the shared state matches the order of the arguments supplied to when_all.
• The future returned by when_all will not throw an exception, but the futures and shared_futures held in the shared state may.

Postconditions:

• valid() == true.
• For all input futures, valid() == false.
• For all inputoutput shared_futures, valid() == true.

Returns:

• A future object that becomes ready when all the input futures/shared_futures are ready.
• future<tuple<>> if when_all is called with zero arguments.
• future<vector<future<R>>> if the input cardinality is unknown at compile and the iterator pair yields future<R>. R may be void. The order of the future in the output vector will be the same as given by the input iterator.
• future<vector<shared_future<R>>> if the input cardinality is unknown at compile time and the iterator pair yields shared_future<R>. R may be void. The order of the future in the output vector will be the same as given by the input iterator.
• future<tuple<future<R0>, future<R1>, future<R2>...>> if inputs are fixed in number. The inputs can be any arbitrary number of future and shared_future objects. The type of the element at each position of the tuple corresponds to the type of the argument at the same position. Any of R0, R1, R2, etc. maybe void.

template <class InputIterator>
see below
future<when_any_result<vector<typename iterator_traits<InputIterator>::value_type>>>
when_any(InputIterator first, InputIterator last);

template <typenameclass... TFutures>
see below
future<when_any_result<tuple<decay_t<Futures>...>>> when_any(Futures&&... futures);

Requires:

• For the first overload, iterator_traits<InputIterator>::value_type must be future<R> or shared_future<R>, for some type R.
• T is of type future<R> or shared_future<R> All futures and shared_futures passed into when_any must be in a valid state (i.e. valid() == true).

Notes:

• There are two variations of when_any. The first version takes a pair of InputIterators. The second takes any arbitrary number of future<R> and shared_future<R> objects, where R need not be the same type.
• Calling the first signature of when_any where InputIterator first equals last, returns a future with an empty vector that is immediately ready.
• Calling the second signature of when_any with no arguments returns a future<tuple<>> that is immediately ready.
• Calling the first overload of when_any where first == last, returns a future that is immediately ready. The value of the index field of the when_any_result is unspecified. The futures field is an empty vector.
• Calling the second overload of when_any with no arguments returns a future that is immediately ready. The value of the index field of the when_any_result is unspecified. The futures field is tuple<>.

Remarks:

For the second overload, let Ui be decay_t<Fi> for each Fi in Futures. This function shall not participate in overload resolution unless each Ui is either future<Ri> or shared_future<Ri>.

Effects:

• Each future and shared_future is waited upon. When at least one is ready, all the futures are copied into the collection of the output (returned) future, maintaining the order of the futures in the input collection.
• A new shared state containing when_any_result<Sequence> is created, where Sequence is a vector for the first overload and a tuple for the second overload. A new future object that refers to that shared state is created and returned from when_any.
• Once at least one of the futures or shared_futures supplied to the call to when_any is ready, the futures are moved, and the shared_futures are copied into, correspondingly, futures or shared_futures of the futures member of Sequence in the shared state.
• The order of the objects in the shared state matches the order of the arguments supplied to when_any.
• The future returned by when_any will not throw an exception, but the futures and shared_futures held in the shared state may.

Postconditions:

• valid() == true.
• For all input futures, valid() == false.
• For all inputoutput shared_futures, valid() == true.

Returns:

• A future object that becomes ready when any of the input futures/shared_futures are ready.
• future<tuple<>> if when_any is called with zero arguments.
• future<vector<future<R>>> if the input cardinality is unknown at compile time and the iterator pair yields future<R>. R may be void. The order of the future in the output vector will be the same as given by the input iterator.
• future<vector<shared_future<R>>> if the input cardinality is unknown at compile time and the iterator pair yields shared_future<R>. R may be void. The order of the future in the output vector will be the same as given by the input iterator.
• future<tuple<future<R0>, future<R1>, future<R2>...>> if inputs are fixed in number. The inputs can be any arbitrary number of future and shared_future objects. The type of the element at each position of the tuple corresponds to the type of the argument at the same position. Any of R0, R1, R2, etc. maybe void.

template <class InputIterator>
see below
when_any_back(InputIterator first, InputIterator last);

Effects:

• Each future and shared_future is waited upon. When at least one is ready, all the future are copied into the collection of the output (returned) future.
• The future returned by when_any_back will not throw an exception, but the futures and shared_futures held in the shared state may.
• After the copy, the future or shared_future that was first detected as being ready swaps its position with that of the last element of the result collection, so that the ready future or shared_future may be identified in constant time. Only one future or shared_future is thus moved.

Postconditions:

• valid() == true.
• All input future<T>s valid() == false.
• All input shared_future<T> valid() == true.

Returns:

• future<vector<future<R>>> if the input cardinality is unknown at compile time and the iterator pair yields future<R>. R may be void.
• future<vector<shared_future<R>>> if the input cardinality is unknown at compile time and the iterator pair yields shared_future<R>. R may be void.

template <class T>
future<V> make_ready_future(T&& value);

future<void> make_ready_future();
  

Let U be decay_t<T>. Then V is X& if U equals reference_wrapper<X>, otherwise V is U.

Effects:

• For the first overload, the value that is passed in to the function is moved to the shared state of the returned future if it is an rvalue. Otherwise the value is copied to the shared state of the returned future.
• The second overload creates a shared state for the returned future.

Returns:

• future<V>, if function is given a value of type T.
• future<void>, if the function is not given any inputs.

Postconditions:

• Returned future, valid() == true.
• Returned future, is_ready() == true.

template <class T>
future<T> make_exceptional_future(exception_ptr ex);

Effects:

Equivalent to

promise<T> p;
p.set_exception(ex);
return p.get_future();

template <class T, class E>
future<T> make_exceptional_future(E ex);

Effects:

Equivalent to

promise<T> p;
p.set_exception(make_exception_ptr(ex));
return p.get_future();