Defined in header <stop_token>
template< class Callback >
class stop_callback;
(since C++20)

The stop_callback class template provides an RAII object type that registers a callback function for an associated std::stop_token object, such that the callback function will be invoked when the std::stop_token's associated std::stop_source is requested to stop.

Callback functions registered via stop_callback's constructor are invoked either in the same thread that successfully invokes request_stop() for a std::stop_source of the stop_callback's associated std::stop_token; or if stop has already been requested prior to the constructor's registration, then the callback is invoked in the thread constructing the stop_callback.

More than one stop_callback can be created for the same std::stop_token, from the same or different threads concurrently. No guarantee is provided for the order in which they will be executed, but they will be invoked synchronously; except for stop_callback(s) constructed after stop has already been requested for the std::stop_token, as described previously.

If an invocation of a callback exits via an exception then std::terminate is called.

std::stop_callback is not CopyConstructible, CopyAssignable, MoveConstructible, nor MoveAssignable.

The template param Callback type must be both invocable and destructible. Any return value is ignored.

Member types

Type Definition
callback_type Callback

Member functions

constructs new stop_callback object
(public member function)
destructs the stop_callback object
(public member function)
stop_callback is not assignable
(public member function)

Deduction guides


#include <chrono>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <sstream>
#include <thread>
using namespace std::chrono_literals;
// Use a helper class for atomic std::cout streaming
class Writer
        std::ostringstream buffer;
        ~Writer() {
                std::cout << buffer.str();
        Writer& operator<<(auto input) {
                buffer << input;
                return *this;
int main()
        // A worker thread
        // It will wait until it is requested to stop.
        std::jthread worker([] (std::stop_token stoken) {
                Writer() << "Worker thread's id: " << std::this_thread::get_id() << '\n';
                std::mutex mutex;
                std::unique_lock lock(mutex);
                std::condition_variable_any().wait(lock, stoken,
                        [&stoken] { return stoken.stop_requested(); });
        // Register a stop callback on the worker thread.
        std::stop_callback callback(worker.get_stop_token(), [] {
                Writer() << "Stop callback executed by thread: "
                        << std::this_thread::get_id() << '\n';
        // stop_callback objects can be destroyed prematurely to prevent execution
                std::stop_callback scoped_callback(worker.get_stop_token(), [] {
                        // This will not be executed.
                        Writer() << "Scoped stop callback executed by thread: "
                                << std::this_thread::get_id() << '\n';
        // Demonstrate which thread executes the stop_callback and when.
        // Define a stopper function
        auto stopper_func = [&worker] {
                        Writer() << "Stop request executed by thread: "
                                << std::this_thread::get_id() << '\n';
                        Writer() << "Stop request not executed by thread: "
                                << std::this_thread::get_id() << '\n';
        // Let multiple threads compete for stopping the worker thread
        std::jthread stopper1(stopper_func);
        std::jthread stopper2(stopper_func);
        // After a stop has already been requested,
        // a new stop_callback executes immediately.
        Writer() << "Main thread: " << std::this_thread::get_id() << '\n';
        std::stop_callback callback_after_stop(worker.get_stop_token(), [] {
                Writer() << "Stop callback executed by thread: "
                        << std::this_thread::get_id() << '\n';

Possible output:

Worker thread's id: 140460265039616
Stop callback executed by thread: 140460256646912
Stop request executed by thread: 140460256646912
Stop request not executed by thread: 140460248254208
Main thread: 140460265043776
Stop callback executed by thread: 140460265043776

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