NAME¶

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

Synopsis¶

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).

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 constructor selected to copy an object of type R.
2) Any exception thrown by the constructor selected to move an object of type R.

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
overloads (1,2) could only throw they can throw the exceptions
the thrown
LWG 2098 C++11 exceptions thrown by the by the actual constructor
copy/move selected
constructor of R respectively to copy/move an object of type
R

See also¶

sets the result to specific value while delivering the
set_value_at_thread_exit notification only at thread exit
(public member function)
set_exception sets the result to indicate an exception
(public member function)