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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
	Main template
	void set_value( const R& value ); (1) (since C++11)
	void set_value( R&& value );	  (2) (since C++11)
	std::promise<R&> specializations
	void set_value( R& value );	  (3) (since C++11)
	std::promise<void> specialization
	void set_value();		  (4) (since C++11)

	1-3)  Atomically  stores 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.

	Calls to this function do not introduce data races with calls to get_fu-
     ture
	(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 constructor selected to copy an object of
     type R.
	2) Any exception thrown by the constructor selected to move an object of
     type R.

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

     // Run this code

      #include <algorithm>
      #include <cctype>
      #include <chrono>
      #include <future>
      #include <iostream>
      #include <iterator>
      #include <sstream>
      #include <thread>
      #include <vector>

      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());

	  // does nothing if numbers were already printed
	  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

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	 Applied to	  Behavior as published 	     Correct be-
     havior
			    overloads (1,2) could only throw  they can throw the
     exceptions
			    the 			      thrown
	LWG 2098 C++11	    exceptions thrown by the	      by the actual con-
     structor
			    copy/move			      selected
			    constructor  of  R	respectively	 to copy/move an
     object of type
							      R

See also
				 sets the result to specific value while  deliv-
     ering the
	set_value_at_thread_exit notification only at thread exit
				 (public member function)
	set_exception		 sets the result to indicate an exception
				 (public member function)

http://cppreference.com 	   2024.06.10	      std::promise::set_value(3)
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