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std::shared_future(3)	      C++ Standard Libary	 std::shared_future(3)

NAME
       std::shared_future - std::shared_future

Synopsis
	  Defined in header <future>
	  template< class T > class shared_future;     (1) (since C++11)
	  template< class T > class shared_future<T&>; (2) (since C++11)
	  template<> class shared_future<void>;	       (3) (since C++11)

	  The class template std::shared_future	provides a mechanism to	access
       the result of
	  asynchronous	operations, similar to std::future, except that	multi-
       ple threads are
	  allowed to wait for the same shared state. Unlike std::future, which
       is only
	  moveable (so only one	instance can refer to any particular asynchro-
       nous result),
	  std::shared_future is	copyable and multiple  shared  future  objects
       may refer to the
	  same shared state.

	  Access  to  the  same	 shared	state from multiple threads is safe if
       each thread does	it
	  through its own copy of a shared_future object.

Member functions
	  constructor	constructs the future object
			(public	member function)
	  destructor	destructs the future object
			(public	member function)
	  operator=	assigns	the contents
			(public	member function)

Getting	the result
	  get		returns	the result
			(public	member function)

State
	  valid		checks if the future has a shared state
			(public	member function)
	  wait		waits for the result to	become available
			(public	member function)
			waits for the result, returns if it is	not  available
       for the specified
	  wait_for	timeout	duration
			(public	member function)
			waits  for  the	result,	returns	if it is not available
       until specified
	  wait_until	time point has been reached
			(public	member function)

Example
	  A shared_future may be used to signal	 multiple  threads  simultane-
       ously, similar to
	  std::condition_variable::notify_all()

       // Run this code

	#include <iostream>
	#include <future>
	#include <chrono>

	int main()
	{
	    std::promise<void>	       ready_promise,	     t1_ready_promise,
       t2_ready_promise;
	    std::shared_future<void> ready_future(ready_promise.get_future());

	    std::chrono::time_point<std::chrono::high_resolution_clock>	start;

	    auto fun1 =	[&, ready_future]()  ->	 std::chrono::duration<double,
       std::milli>
	    {
		t1_ready_promise.set_value();
		ready_future.wait(); //	waits for the signal from main()
		return std::chrono::high_resolution_clock::now() - start;
	    };

	    auto  fun2	= [&, ready_future]() -> std::chrono::duration<double,
       std::milli>
	    {
		t2_ready_promise.set_value();
		ready_future.wait(); //	waits for the signal from main()
		return std::chrono::high_resolution_clock::now() - start;
	    };

	    auto fut1 =	t1_ready_promise.get_future();
	    auto fut2 =	t2_ready_promise.get_future();

	    auto result1 = std::async(std::launch::async, fun1);
	    auto result2 = std::async(std::launch::async, fun2);

	    // wait for	the threads to become ready
	    fut1.wait();
	    fut2.wait();

	    // the threads are ready, start the	clock
	    start = std::chrono::high_resolution_clock::now();

	    // signal the threads to go
	    ready_promise.set_value();

	    std::cout << "Thread 1 received the	signal "
		      << result1.get().count() << " ms after start\n"
		      << "Thread 2 received the	signal "
		      << result2.get().count() << " ms after start\n";
	}

Possible output:
	Thread 1 received the signal 0.072 ms after start
	Thread 2 received the signal 0.041 ms after start

See also
	  async	  runs a function asynchronously (potentially in a new thread)
       and returns a
	  (C++11) std::future that will	hold the result
		  (function template)
	  future  waits	for a value that is set	asynchronously
	  (C++11) (class template)

http://cppreference.com		  2022.07.31		 std::shared_future(3)

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