Std::promise::set_value_at_thread_exit (3) Linux Manual Page

std::promise<R>::set_value_at_thread_exit – std::promise<R>::set_value_at_thread_exit

Synopsis

void set_value_at_thread_exit(const R &value);
(1)(member only of generic promise template)(since C++ 11)

    void set_value_at_thread_exit(R &&value);
(2)(member only of generic promise template)(since C++ 11)

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

    void set_value_at_thread_exit()(4)(member only of promise<void> template specialization)(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.
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 (but 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 category is set to no_state.

* The shared state already stores a value or exception. The error category is set to promise_already_satisfied.

Additionally:
1-2) Any exception thrown by the copy constructor of value
3) Any exception thrown by the move constructor of value

Example

// Run this code

#include <iostream>

#include <future>

#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

std::promise::set_value_at_thread_exit (3) Linux Manual Page