NAME¶

std::promise::set_value_at_thread_exit - std::promise::set_value_at_thread_exit

Synopsis¶

Main template
void set_value_at_thread_exit( const R& value ); (1) (since C++11)
void set_value_at_thread_exit( R&& value ); (2) (since C++11)
std::promise<R&> specializations
void set_value_at_thread_exit( R& value ); (3) (since C++11)
std::promise<void> specialization
void set_value_at_thread_exit(); (4) (since C++11)

Stores the value into the shared state without making the state ready immediately.
The state is made ready when the current thread exits, after all variables with
thread-local storage duration have been destroyed.

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¶

// Run this code

#include <future>
#include <iostream>
#include <thread>

int main()
{
using namespace std::chrono_literals;
std::promise<int> p;
std::future<int> f = p.get_future();
std::thread([&p]
{
std::this_thread::sleep_for(1s);
p.set_value_at_thread_exit(9);
}).detach();

std::cout << "Waiting... " << std::flush;
f.wait();
std::cout << "Done!\nResult is: " << f.get() << '\n';
}

Output:¶

Waiting... Done!
Result is: 9

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 it was unclear which exceptions are required to made clear
be thrown

See also¶

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