std::stop_callback(3)

NAME
std::stop_callback - std::stop_callback

Synopsis
   Defined in header <stop_token>
   template< class Callback > (since C++20)
   class stop_callback;
   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
   constructor constructs new stop_callback object
                 (public member function)
   destructor destructs the stop_callback object
                 (public member function)
   operator= stop_callback is not assignable
   [deleted] (public member function)
  Deduction guides

Example
// Run this code
 #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;
 public:
         ~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] {
                 if(worker.request_stop())
                        Writer() << "Stop request executed by thread: "
                        << std::this_thread::get_id() << '\n';
                 else
                        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);
         stopper1.join();
         stopper2.join();
         // 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