# std::stop_token::stop_possible

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< cpp‎ | thread‎ | stop token

bool stop_possible() const noexcept; |  | (since C++20)

Checks if the `stop_token` object has associated stop-state, and that state either has already had a stop requested or it has associated std::stop_source object(s).

A default constructed stop_token has no associated stop-state, and thus cannot be stopped; the associated stop-state for which no std::stop_source object(s) exist can also not be stopped if such a request has not already been made.

### Parameters

(none)

### Return value

false if the `stop_token` object has no associated stop-state, or it did not yet receive a stop request and there are no associated std::stop_source object(s); true otherwise.

### Notes

If the `stop_token` object has associated stop-state and a stop request has already been made, this function still returns true.

If the `stop_token` object has associated stop-state from a std::jthread—for example, the `stop_token` was retrieved by invoking get_stop_token() on a std::jthread object—then this function always returns true. A std::jthread always has an internal std::stop_source object, even if the thread's invoking function does not check it.

### Example

Run this code

    #include <chrono>
    #include <condition_variable>
    #include <format>
    #include <iostream>
    #include <mutex>
    #include <string_view>
    #include <thread>
    using namespace std::chrono_literals;

    int main()
    {
        std::cout << std::boolalpha;
        auto print = [](std::string_view name, const std::stop_token& token)
        {
            std::cout << std::format("{}: stop_possible = {:s}, stop_requested = {:s}\n",
                name, token.stop_possible(), token.stop_requested()
            );
        };

        // A worker thread that will listen to stop requests
        auto stop_worker = std::jthread([](std::stop_token stoken)
        {
            for (int i = 10; i; --i)
            {
                std::this_thread::sleep_for(300ms);
                if (stoken.stop_requested())
                {
                    std::cout << "  Sleepy worker is requested to stop\n";
                    return;
                }
                std::cout << "  Sleepy worker goes back to sleep\n";
            }
        });

        // A worker thread that will only stop when completed
        auto inf_worker = std::jthread([]()
        {
            for (int i = 5; i; --i)
            {
                std::this_thread::sleep_for(300ms);
                std::cout << "  Run as long as we want\n";
            }
        });

        std::stop_token def_token;
        std::stop_token stop_token = stop_worker.get_stop_token();
        std::stop_token inf_token = inf_worker.get_stop_token();
        print("def_token ", def_token);
        print("stop_token", stop_token);
        print("inf_token ", inf_token);

        std::cout << "\nRequest and join stop_worker:\n";
        stop_worker.request_stop();
        stop_worker.join();

        std::cout << "\nRequest and join inf_worker:\n";
        inf_worker.request_stop();
        inf_worker.join();
        std::cout << '\n';

        print("def_token ", def_token);
        print("stop_token", stop_token);
        print("inf_token ", inf_token);
    }

Possible output:

    def_token : stop_possible = false, stop_requested = false
    stop_token: stop_possible = true, stop_requested = false
    inf_token : stop_possible = true, stop_requested = false

    Request and join stop_worker:
      Run as long as we want
      Sleepy worker is requested to stop

    Request and join inf_worker:
      Run as long as we want
      Run as long as we want
      Run as long as we want
      Run as long as we want

    def_token : stop_possible = false, stop_requested = false
    stop_token: stop_possible = true, stop_requested = true
    inf_token : stop_possible = true, stop_requested = true

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