std::promise<R>::set_value
From cppreference.com
| Main template |
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| void set_value( const R& value ); |
(1) | (since C++11) |
| void set_value( R&& value ); |
(2) | (since C++11) |
| std::promise<R&> specializations |
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| void set_value( R& value ); |
(3) | (since C++11) |
| std::promise<void> specialization |
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| 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).
Contents |
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 |
|---|---|---|---|
| LWG 2098 | C++11 | overloads (1,2) could only throw the exceptions thrown by the copy/move constructor of R respectively
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they can throw the exceptions thrown by the actual constructor selected to copy/move an object of type R
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See also
| sets the result to specific value while delivering the notification only at thread exit (public member function) | |
| sets the result to indicate an exception (public member function) |
