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 |
[edit] Parameters
| value | - | value to store in the shared state |
[edit] Return value
(none)
[edit] 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.[edit] 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
[edit] 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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[edit] 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) |
