std::experimental::when_all(3)

NAME
std::experimental::when_all - std::experimental::when_all

Synopsis
   Defined in header <experimental/future>
   template< class InputIt >
   auto when_all( InputIt first, InputIt last ) (1) (concurrency TS)
       -> future<std::vector<typename
   std::iterator_traits<InputIt>::value_type>>;
   template< class... Futures >
   auto when_all( Futures&&... futures ) (2) (concurrency TS)
       -> future<std::tuple<std::decay_t<Futures>...>>;
   Create a future object that becomes ready when all of the input futures and
   shared_futures 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   After this call, every input future is no longer valid; every input shared_future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 futures and shared_futures 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.