std::promise<R>::set_value
From cppreference.com
Main template | ||
void set_value(const R& value ); | (1) | (since C++11) |
void set_value( R&& value ); | (2) | (since C++11) |
std::promise<R&> specializations | ||
void set_value( R& value ); | (3) | (since C++11) |
std::promise<void> specialization | ||
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> usingnamespace 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 operationsstd::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 obtainedfor(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 printedfor(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 | they can throw the exceptions thrown by the actual constructor selected to copy/move an object of type R |
[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) |
