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| Member functions | ||||
| Getting the result | ||||
| Setting the result | ||||
promise::set_value | ||||
| Non-member Functions | ||||
| Helper Classes | ||||
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) |
The operation behaves as thoughset_value,set_exception,set_value_at_thread_exit, andset_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 toget_future (therefore they need not synchronize with each other).
Contents |
| value | - | value to store in the shared state |
(none)
std::future_error on the following conditions:
Additionally:
R.R.This example shows howstd::promise<void> can be used as signals between threads.
#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
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 |
| sets the result to specific value while delivering the notification only at thread exit (public member function)[edit] | |
| sets the result to indicate an exception (public member function)[edit] |
