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# std::promise::set_value_at_thread_exit

From cppreference.com
< cpp‎ | thread‎ | promise

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.

### [edit] Parameters

value | - | value to store in the shared state

### [edit] Return value

(none)

### [edit] 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`

### [edit] 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

### [edit] See also

set_value | sets the result to specific value (public member function) [edit]

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