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# std::experimental::when_all

From cppreference.com
< cpp‎ | experimental

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After this call, every input `future` is no longer valid; every input `shared_future` remains valid.

### [edit] 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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