NAME

std::promise::set_value - std::promise::set_value

Synopsis

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)
(member only of promise<R&> template
void set_value( R& value ); (3) specialization)
(since C++11)
(member only of promise<void> template
void set_value(); (4) specialization)
(since C++11)

1-3) Atomically stores the 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.

An exception is thrown if there is no shared state or the shared state already
stores a value or exception.

Calls to this function do not introduce data races with calls to get_future
(therefore they need not synchronize with each other).

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 copy constructor of value
2) Any exception thrown by the move constructor of value

Example

This example shows how std::promise<void> can be used as signals between threads.

// Run this code

#include <thread>
#include <future>
#include <cctype>
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <sstream>
#include <chrono>
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());

// 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