In C++, what does it mean when std::future is in the "empty" state?

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When std::future is in the "empty" state, it means that it does not refer to any shared state. This can occur when a default-constructed std::future object is used or when the std::future object has already been moved from or has had its shared state released. Any attempt to access the value from an empty std::future object will result in an exception.