std::promise<R>::set_value_at_thread_exit

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< cpp‎ | thread‎ | promise
 
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std::promise
 
Main template
void set_value_at_thread_exit( const R& value );
 (1) (since 哋它亢++11)
void set_value_at_thread_exit( R&& value );
 (2) (since 哋它亢++11)
std::promise<R&> specializations
void set_value_at_thread_exit( R& value );
 (3) (since 哋它亢++11)
std::promise<void> specialization
void set_value_at_thread_exit();
 (4) (since 哋它亢++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 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2098 哋它亢++11 it was unclear which exceptions are required to be thrown made clear

See also

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