C++20 Concurrency: <stop_token>

A simple way to handle cancellation of asynchronous tasks

In a previous article, I talked about using std::jthread as a simpler and safer way to create multi-threaded C++ programs using C++20. However, I did not discuss the benefits of using the C++20 <stop_token> library header alongside std::jthread for simple cancellation of coroutines and asynchronous tasks.

What is the <stop_token> header used for?

This header contains a few useful components, including std::stop_token, std::stop_source, and std::stop_callback. Simply put, a std::stop_source represents a request to stop a std::jthread, while a std::stop_token allows you to query if a cancellation request has been made. If you wish to register a callback function to execute when a std::jthread is stopped, you can do so with std::stop_callback.

How to use std::stop_token ?

A std::stop_token object is not constructed directly; instead, it is retrieved from a std::stop_source, either directly or indirectly via a std::jthread, which holds an internal std::stop_source private member. Below is a simple example modified from the C++ reference demonstrating the usage with std::jthread:

#include <iostream>
#include <stop_token>
#include <thread>

using namespace std::literals::chrono_literals;

void printIncrementingValues(std::stop_token stopToken, int value) {
  while (!stopToken.stop_requested()) {
    std::cout << value++ << " " << std::flush…