std::stop_t...op_possible(3)

💖 NAME 💖

okay bestie, you found it: std::stop_t...op_possible(3) from the C++ Standard Libary manual. The actual thing being documented is std::stop_token::stop_possible - std::stop_token::stop_possible. Yes, the name is giving "same thing twice" and honestly? iconic. ✨

This is the member function that tells you whether a stop_token can ever be asked to stop. It's the vibe check for stop-state, basically. 💅

✨ Synopsis ✨

[[nodiscard]] bool stop_possible() const noexcept;

(since C++20)

It 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.

It's also [[nodiscard]] and noexcept — both right there in the signature. 🎀

🎀 Parameters 🎀

(none) — bestie, there are literally zero parameters here. Not a single one. Slay. 💅

🦋 Return value 🦋

It returns false if the stop_token object has no associated stop-state, or if it did not yet receive a stop request and there are no associated std::stop_source object(s). It returns true otherwise. Simple as that, fr fr. ✨

🌸 Notes 🌸

• If the stop_token object has associated stop-state and a stop request has already been made, this function still returns true. No take-backs, no "well actually it's done now" — still 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. So yeah: jthread tokens are always possible-to-stop, even when nobody's checking. 💀

🔥 Example 🔥

The example even has that little // Run this code marker, and here's the whole thing, right here:

#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

💖 source: http://cppreference.com · version/date: 2024.06.10 · now go be iconic with your stop tokens, bestie 🫶