std::promise<R>::set_value (3) - Linux Manuals

std::promise<R>::set_value: std::promise<R>::set_value

NAME

std::promise<R>::set_value - std::promise<R>::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)
void set_value( R& value ); (3) (member only of promise<R&> template specialization)
                                      (since C++11)
void set_value(); (4) (member only of promise<void> template 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 (but 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 category is set to no_state.

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

Additionally:
1, 3) Any exception thrown by the copy constructor of value
2) Any exception thrown by the move constructor of value

Example

This example shows how promise<void> can be used as signals between threads.
// Run this code

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

  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(std::chrono::seconds(1)) ==
              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