std::experimental:: when_all

From cppreference.com
C++
Experimental
Extensions for concurrency
Defined in header <experimental/future>
template < class InputIt >

auto when_all ( InputIt first, InputIt last )

- > future < std:: vector < typename std:: iterator_traits < InputIt > :: value_type >> ;
(1) (concurrency TS)
template < class ... Futures >

auto when_all ( Futures && ... futures )

- > future < std:: tuple < std:: decay_t < Futures > ... >> ;
(2) (concurrency TS)

Create a future object that becomes ready when all of the input future s and shared_future s become ready. The behavior is undefined if any input future or shared_future is invalid.

In particular, let Sequence be a std:: vector < typename std:: iterator_traits < InputIt > :: value_type > for (1) and std:: tuple < std:: decay_t < Futures > ... > for (2) . This function template creates a shared state containing Sequence and returns a future referring to the shared state. Every input future is moved into the corresponding object in the Sequence in the shared state, and every input shared_future is copied to the corresponding object in the Sequence in the shared state. The order of the objects in the Sequence matches the order of arguments.

1) This function does not participate in overload resolution unless InputIt 's value type (i.e., typename std:: iterator_traits < InputIt > :: value_type ) is a std::experimental::future or std::experimental::shared_future .
2) This function does not participate in overload resolution unless every argument is either a (possibly cv-qualified) std::experimental::shared_future or a cv-unqualified std::experimental::future . (Formally, for every type Fn in Futures , either std:: remove_reference_t < Fn > is std:: experimental :: future < Rn > , or std:: decay_t < Fn > is std:: experimental :: shared_future < Rn > .)

After this call, every input future is no longer valid; every input shared_future remains valid.

Return value

A future referring to the shared state created by the call. The future is always valid ( ) , and it becomes ready when all of the input future s and shared_future s the call are ready.

1) If the range is empty (i.e., first == last ), the returned future contains an empty vector and is ready immediately.
2) If there are no arguments supplied, a future<std::tuple<>> is returned and is immediately ready.
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