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_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 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 retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       overloads (1,2) could only throw they can throw the exceptions
                       the thrown
   LWG 2098 C++11 exceptions thrown by the by the actual constructor
                       copy/move selected
                       constructor of R respectively to copy/move an object of type
                        R

See also
                        sets the result to specific value while delivering 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)