std::promise<R>::set_value
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
See also
sets the result to indicate an exception (public member function) |