Namespaces
Variants
Views
Actions

std::promise<R>::set_value_at_thread_exit

From cppreference.com
< cpp‎ | thread‎ | promise
 
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++20)
this_thread namespace
(C++11)
(C++11)
(C++11)
Mutual exclusion
(C++11)
(C++11)  
Generic lock management
(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Condition variables
(C++11)
Semaphores
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
Safe Reclamation
(C++26)
(C++26)
Hazard Pointers

Atomic types
(C++11)
(C++20)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
 
 
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).

Contents

[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 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.

[edit] Example

#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

[edit] 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 be thrown made clear

[edit] See also

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