An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a std::future object to the creator of that asynchronous operation. The creator of the asynchronous operation can then use a variety of methods to query, wait for, or extract a value from the std::future.
The code above might look ugly, but all you have to understand is that the FutureBuilder widget takes two arguments: future and builder, future is just the future you want to use, while builder is a function that takes two parameters and returns a widget. FutureBuilder will run this function before and after the future completes.
C++ includes built-in support for threads, atomic operations, mutual exclusion, condition variables, and futures.
The class template std::packaged_task wraps any Callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects.
If the future is the result of a call to std::async that used lazy evaluation, this function returns immediately without waiting. This function may block for longer than timeout_duration due to scheduling or resource contention delays. The standard recommends that a steady clock is used to measure the duration.
future (const future &) = delete; ~future (); future & operator =(const future &) = delete; future & operator =(future &&) noexcept; shared_future
wait_until waits for a result to become available. It blocks until specified timeout_time has been reached or the result becomes available, whichever comes first. The return value indicates why wait_until returned. If the future is the result of a call to async that used lazy evaluation, this function returns immediately without waiting. The behavior is undefined if valid () is false before ...
The scoped enumeration std::future_errc defines the error codes reported by std::future and related classes in std::future_error exception objects. Only four error codes are required, although the implementation may define additional error codes.
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