Mastering Concurrency in Modern C++: Architecture, Pitfalls, and Correctness by Example.

Designing a Thread-Safe Task Processing System in Modern C++ (C++20/23)

This example demonstrates:

• Cooperative thread cancellation (std::jthread, std::stop_token)

• Thread-safe shared state

• Proper synchronization

• RAII-based lifetime management

• Clean separation of responsibilities

• No data races, no leaks, no undefined behavior

Design Overview

Components

• TaskQueue — thread-safe queue

• WorkerPool — manages worker threads

• main() — submits work and controls lifetime

Key Guarantees

• No busy waiting

• No manual thread joining

• Deterministic shutdown

• Exception safety

Thread-Safe Task Queue

#include <queue>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <optional>

class TaskQueue {
public:
    using Task = std::function<void()>;

    void push(Task task) {
        {
            std::lock_guard<std::mutex> lock(m_mutex);
            m_tasks.push(std::move(task));
        }
        m_cv.notify_one();
    }

    std::optional<Task> pop(std::stop_token stop) {
        std::unique_lock<std::mutex> lock(m_mutex);

        m_cv.wait(lock, stop, [this] {
            return !m_tasks.empty();
        });

        if (stop.stop_requested() || m_tasks.empty())
            return std::nullopt;

        Task task = std::move(m_tasks.front());
        m_tasks.pop();
        return task;
    }

private:
    std::queue<Task> m_tasks;
    std::mutex m_mutex;
    std::condition_variable_any m_cv;
};

Worker Pool Using std::jthread

#include <vector>
#include <thread>

class WorkerPool {
public:
    explicit WorkerPool(std::size_t threadCount)
        : m_workers(threadCount) {
        for (auto& worker : m_workers) {
            worker = std::jthread([this](std::stop_token stop) {
                workerLoop(stop);
            });
        }
    }

    void submit(TaskQueue::Task task) {
        m_queue.push(std::move(task));
    }

private:
    void workerLoop(std::stop_token stop) {
        while (!stop.stop_requested()) {
            if (auto task = m_queue.pop(stop)) {
                (*task)();
            }
        }
    }

    TaskQueue m_queue;
    std::vector<std::jthread> m_workers;
};

Why this is correct

• std::jthread automatically joins on destruction

• std::stop_token enables cooperative cancellation

• No explicit join()

• No detached threads

• No shared mutable state without protection

Example Usage

#include <iostream>
#include <chrono>

int main() {
    WorkerPool pool(4);

    for (int i = 0; i < 10; ++i) {
        pool.submit([i] {
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
            std::cout << "Task " << i << " executed on thread "
                      << std::this_thread::get_id() << '\n';
        });
    }

    std::this_thread::sleep_for(std::chrono::seconds(2));
    return 0;
}

Why This Example Represents Best Practice

1. Correct Thread Ownership

Threads are owned by objects, not by free functions or global state.

2. RAII Everywhere

Threads, locks, and synchronization are automatically released.

3. No Undefined Behavior

All shared data is protected. No races. No dangling references.

4. Cooperative Cancellation

Threads stop cleanly when the pool is destroyed.

5. Modern C++20/23 Style

• std::jthread

• std::stop_token

• condition_variable_any

• No legacy patterns

Common Mistakes This Example Avoids

• Raw std::thread without joining

• Global mutexes

• Busy loops

• Atomic misuse

• Detached threads

• Manual lifetime control

When This Design Is Appropriate

• Task processing systems

• Server backends

• Job systems

• Background workers

• I/O pipelines