std::promise<R>::set_value

#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();
}

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