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std::promise::set_value(3)    C++ Standard Libary   std::promise::set_value(3)

NAME
       std::promise::set_value - std::promise::set_value

Synopsis
	  void	set_value(  const  R&  value  );  (1)  (member only of generic
       promise template)
						(since C++11)
	  void set_value( R&&  value  );       (2)  (member  only  of  generic
       promise template)
						(since C++11)
						(member	 only  of  promise<R&>
       template
	  void set_value( R& value );	    (3)	specialization)
						(since C++11)
						(member	only of	 promise<void>
       template
	  void set_value();		    (4)	specialization)
						(since C++11)

	  1-3) Atomically stores the value into	the shared state and makes the
       state ready.
	  4) Makes the state ready

	  The	operation   behaves   as   though   set_value,	set_exception,
       set_value_at_thread_exit,
	  and set_exception_at_thread_exit acquire a single  mutex  associated
       with the	promise
	  object while updating	the promise object.

	  An  exception	 is  thrown  if	there is no shared state or the	shared
       state already
	  stores a value or exception.

	  Calls	to this	function do not	introduce data	races  with  calls  to
       get_future
	  (therefore they need not synchronize with each other).

Parameters
	  value	- value	to store in the	shared state

Return value
	  (none)

Exceptions
	  std::future_error on the following conditions:

	    * *this has	no shared state. The error code	is set to no_state.

	    *  The shared state	already	stores a value or exception. The error
       code is set to
	      promise_already_satisfied.

	  Additionally:

	  1) Any exception thrown by the copy constructor of value
	  2) Any exception thrown by the move constructor of value

Example
	  This example shows how std::promise<void> can	be used	as signals be-
       tween threads.

       // Run this code

	#include <thread>
	#include <future>
	#include <cctype>
	#include <vector>
	#include <algorithm>
	#include <iterator>
	#include <iostream>
	#include <sstream>
	#include <chrono>
	using namespace	std::chrono_literals;

	int main()
	{
	    std::istringstream iss_numbers{"3 4	1 42 23	-23 93 2 -289 93"};
	    std::istringstream iss_letters{" a 23 b,e a2 k k?a;si,ksa c"};

	    std::vector<int> numbers;
	    std::vector<char> letters;
	    std::promise<void> numbers_promise,	letters_promise;

	    auto numbers_ready = numbers_promise.get_future();
	    auto letter_ready =	letters_promise.get_future();

	    std::thread	value_reader([&]
	    {
		// I/O operations.
		std::copy(std::istream_iterator<int>{iss_numbers},
			  std::istream_iterator<int>{},
			  std::back_inserter(numbers));

		// Notify for numbers.
		numbers_promise.set_value();

		std::copy_if(std::istreambuf_iterator<char>{iss_letters},
			     std::istreambuf_iterator<char>{},
			     std::back_inserter(letters),
			     ::isalpha);

		// Notify for letters.
		letters_promise.set_value();
	    });

	    numbers_ready.wait();

	    std::sort(numbers.begin(), numbers.end());

	    if (letter_ready.wait_for(1s) == std::future_status::timeout)
	    {
		// Output the numbers while letters are	being obtained.
		for (int num : numbers)	std::cout << num << ' ';
		numbers.clear(); // Numbers were already printed.
	    }

	    letter_ready.wait();
	    std::sort(letters.begin(), letters.end());

	    // If numbers were already printed,	it does	nothing.
	    for	(int num : numbers) std::cout << num <<	' ';
	    std::cout << '\n';

	    for	(char let : letters) std::cout << let << ' ';
	    std::cout << '\n';

	    value_reader.join();
	}

Output:
	-289 -23 1 2 3 4 23 42 93 93
	a a a a	b c e i	k k k s	s

See also
	  set_exception	sets the result	to indicate an exception
			(public	member function)

http://cppreference.com		  2022.07.31	    std::promise::set_value(3)

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