std::experimental::when_any
<experimental/future>
struct when_any_result {
std::size_t index;
Sequence futures;
auto when_any( InputIt first, InputIt last )
auto when_any( Futures&&... futures )
Create a future object that becomes ready when at least one 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 when_any_result<Sequence> and returns a future referring to the shared state. Every input future is moved into the corresponding object in the futures member of the when_any_result<Sequence> in the shared state, and every input shared_future is copied to the corresponding object in the futures member of the when_any_result<Sequence> in the shared state. The order of the objects in the Sequence matches the order of arguments.
InputIt's value type (i.e., typename std::iterator_traits <InputIt>::value_type) is a std::experimental::future or std::experimental::shared_future.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.)After this call, every input future is no longer valid; every input shared_future remains valid.
[edit] Return value
A future referring to the shared state created by the call. The future is always valid(), and it becomes ready when at least one of the input futures and shared_futures the call are ready. The index member of the when_any_result contains the position of the ready future or shared_future in the futures member.
future is ready immediately; the futures field of the when_any_result is an empty vector, and the index field is size_t(-1).future is ready immediately; the futures field of the when_any_result is an empty tuple, and the index field is size_t(-1).