std:: stop_callback

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std::stop_callback
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)
operator=
[deleted]
stop_callback is not assignable
(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
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