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) |
Atomically stores the value into the shared state and 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.
| value | - | value to store in the shared state |
(none).
std::future_error on the following conditions:
*this has no shared state. The error category is set to no_state. promise_already_satisfied. Additionally:
value
value
This example shows how promise<void> can be used as signals between threads.
#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
| sets the result to indicate an exception (public member function) |
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