[futures.task.general]

When the packaged_task object is invoked, its stored task is invoked and the result (whether normal or exceptional) stored in the shared state.

Any futures that share the shared state will then be able to access the stored result.

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namespace std { template<class> class packaged_task; / not defined template<class R, class. ArgTypes> class packaged_task<R(ArgTypes..)> { public: / construction and destruction packaged_task() noexcept; template<class F> explicit packaged_task(F&& f); template<class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f); ~packaged_task(); / no copy packaged_task(const packaged_task&) = delete; packaged_task& operator=(const packaged_task&) = delete; / move support packaged_task(packaged_task&& rhs) noexcept; packaged_task& operator=(packaged_task&& rhs) noexcept; void swap(packaged_task& other) noexcept; bool valid() const noexcept; / result retrieval future<R> get_future(); / execution void operator()(ArgTypes.. ); void make_ready_at_thread_exit(ArgTypes..); void reset(); }; template<class R, class. ArgTypes> packaged_task(R (*)(ArgTypes..) -> packaged_task<R(ArgTypes..)>; template<class F> packaged_task(F) -> packaged_task<see below>; }