Creating New Futures

​C++20 gets four new functions for creating special futures: std::make_ready_future, std::make_execptional_future, std::when_all, and std::when_any. ​First, let’s look at the functions std::make_ready_future, and std::make_exceptional_future.

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std::make_ready_future and std::make_exceptional_future

Both functions create a future that is immediately ready. In the first case, the future has a value; in the second case, an exception. Therefore, what seems to be strange at first actually makes a lot of sense. In C++11, the creation of a ready future requires a promise. This is necessary even if the shared state is immediately available.