C++11 起的多线程编程笔记

C++11 标准库并发组件：<thread> / <mutex> / <condition_variable> / <future> / <atomic> / <chrono>
目标：并发执行（性能） + 正确同步（安全） + 生命周期可控（可维护）。

1. 时间与计时 std::chrono

1.1 三个核心类型

• clock：时钟（如 steady_clock、system_clock）
• time_point：时间点（clock::time_point）
• duration：时长（seconds/milliseconds/...）

1.2 计时：推荐 steady_clock（不会被系统调时影响）

#include <iostream>
#include <chrono>
#include <cstdint>

int main() {
    auto t0 = std::chrono::steady_clock::now();
    auto t1 = t0 + std::chrono::seconds(30);

    auto dt = t1 - t0; // duration
    std::int64_t sec = std::chrono::duration_cast<std::chrono::seconds>(dt).count();
    std::cout << "dt = " << sec << " s\n";
    return 0;
}

1.3 常见时间单位

• std::chrono::seconds(x)
• std::chrono::milliseconds(x)
• std::chrono::microseconds(x)
• std::chrono::nanoseconds(x)

1.4 测耗时并转 double 毫秒

#include <iostream>
#include <chrono>

int main() {
    auto t0 = std::chrono::steady_clock::now();

    std::uint64_t acc = 0;
    for (int i = 0; i < 1'000'000; ++i) acc += i;

    auto t1 = std::chrono::steady_clock::now();
    using double_ms = std::chrono::duration<double, std::milli>;
    double ms = std::chrono::duration_cast<double_ms>(t1 - t0).count();

    std::cout << "cost = " << ms << " ms, acc=" << acc << "\n";
}

---

2. 线程休眠：避免忙等

2.1 sleep_for

#include <thread>
#include <chrono>

std::this_thread::sleep_for(std::chrono::milliseconds(400));

2.2 sleep_until（对齐节拍更合适）

#include <thread>
#include <chrono>

auto t = std::chrono::steady_clock::now() + std::chrono::milliseconds(400);
std::this_thread::sleep_until(t);

2.3 实例：固定频率循环（推荐写法）

#include <iostream>
#include <chrono>
#include <thread>

int main() {
    using clock = std::chrono::steady_clock;
    auto next = clock::now();

    for (int i = 0; i < 10; ++i) {
        next += std::chrono::milliseconds(100);
        std::cout << "tick " << i << "\n";
        std::this_thread::sleep_until(next);
    }
}

---

3. 基础线程 std::thread

3.1 启动线程与 join（必须）

#include <thread>
#include <iostream>

void work(int id) { std::cout << "worker " << id << "\n"; }

int main() {
    std::thread t(work, 1);
    t.join(); // 必须 join 或 detach，否则析构会 std::terminate
}

• join()：等待线程结束（最常用）
• detach()：放飞线程（风险高：对象生命周期/退出时机难控）

---

4. 异步与返回值：std::async / std::future

4.1 基本用法（带返回值）

#include <iostream>
#include <future>
#include <thread>
#include <chrono>

int download() {
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    return 404;
}

int main() {
    auto fut = std::async(std::launch::async, [] { return download(); });
    int ret = fut.get(); // get 只能调用一次
    std::cout << "ret=" << ret << "\n";
}

4.2 启动策略

• std::launch::async：倾向于并行执行
• std::launch::deferred：延迟到 get()/wait() 才在当前线程执行

4.3 wait / wait_for / wait_until

auto fut = std::async(std::launch::async, []{
    std::this_thread::sleep_for(std::chrono::seconds(2));
    return 123;
});

fut.wait(); // 等到完成

auto st = fut.wait_for(std::chrono::milliseconds(100));
if (st == std::future_status::timeout) {
    // 还没完成
}

4.4 异常会在 get() 处重抛（非常实用）

auto fut = std::async(std::launch::async, []() -> int {
    throw std::runtime_error("boom");
});

try { fut.get(); }
catch (const std::exception& e) { std::cout << e.what() << "\n"; }

---

5. 互斥锁 std::mutex：保护共享数据

5.1 lock_guard（RAII 自动解锁）

#include <mutex>
#include <thread>
#include <iostream>

int main() {
    std::mutex m;
    int counter = 0;

    auto inc = [&] {
        for (int i = 0; i < 100000; ++i) {
            std::lock_guard<std::mutex> lk(m);
            ++counter;
        }
    };

    std::thread t1(inc), t2(inc);
    t1.join(); t2.join();
    std::cout << "counter=" << counter << "\n";
}

5.2 死锁规避（两个锁）

C++11 可用 std::lock + std::adopt_lock：

#include <mutex>

std::mutex m1, m2;

void f() {
    std::lock(m1, m2);
    std::lock_guard<std::mutex> lk1(m1, std::adopt_lock);
    std::lock_guard<std::mutex> lk2(m2, std::adopt_lock);
    // 临界区
}

---

6. 条件变量 std::condition_variable：生产者-消费者（比 sleep 正确）

#include <mutex>
#include <condition_variable>
#include <queue>
#include <thread>
#include <iostream>

int main() {
    std::mutex m;
    std::condition_variable cv;
    std::queue<int> q;
    bool done = false;

    std::thread producer([&]{
        for (int i = 0; i < 5; ++i) {
            { std::lock_guard<std::mutex> lk(m); q.push(i); }
            cv.notify_one();
        }
        { std::lock_guard<std::mutex> lk(m); done = true; }
        cv.notify_one();
    });

    std::thread consumer([&]{
        while (true) {
            std::unique_lock<std::mutex> lk(m);
            cv.wait(lk, [&]{ return done || !q.empty(); }); // 带谓词防虚假唤醒
            while (!q.empty()) {
                std::cout << "consume " << q.front() << "\n";
                q.pop();
            }
            if (done) break;
        }
    });

    producer.join();
    consumer.join();
}

---

7. 原子 std::atomic：轻量同步（计数/标志位）

#include <atomic>
#include <thread>
#include <iostream>

int main() {
    std::atomic<int> counter{0};

    auto inc = [&] { for (int i = 0; i < 100000; ++i) ++counter; };

    std::thread t1(inc), t2(inc);
    t1.join(); t2.join();

    std::cout << "counter=" << counter.load() << "\n";
}

---

8. 线程池/任务队列（C++11 可直接用）

线程池 = 固定 worker 线程 + 任务队列（阻塞队列）
典型价值：避免频繁创建/销毁线程，提高吞吐；统一任务提交接口，支持 future 返回值与异常回传。

8.1 任务队列：BlockingQueue（阻塞队列）

特性：

• push() 入队并唤醒消费者
• pop() 阻塞等待任务；队列关闭且为空时返回 false
• close() 关闭队列并唤醒全部等待线程

#pragma once
#include <queue>
#include <mutex>
#include <condition_variable>
#include <utility>

template <class T>
class BlockingQueue {
public:
    BlockingQueue() : closed_(false) {}

    BlockingQueue(const BlockingQueue&) = delete;
    BlockingQueue& operator=(const BlockingQueue&) = delete;

    bool push(T value) {
        {
            std::lock_guard<std::mutex> lk(m_);
            if (closed_) return false;
            q_.push(std::move(value));
        }
        cv_.notify_one();
        return true;
    }

    bool pop(T& out) {
        std::unique_lock<std::mutex> lk(m_);
        cv_.wait(lk, [&]{ return closed_ || !q_.empty(); });

        if (q_.empty()) return false; // closed_ && empty
        out = std::move(q_.front());
        q_.pop();
        return true;
    }

    void close() {
        {
            std::lock_guard<std::mutex> lk(m_);
            closed_ = true;
        }
        cv_.notify_all();
    }

private:
    std::mutex m_;
    std::condition_variable cv_;
    std::queue<T> q_;
    bool closed_;
};

8.2 线程池：ThreadPool（enqueue 返回 future）

特性：

• 固定线程数 worker
• enqueue(f, args...) 返回 std::future<R>
• 任务抛异常 → 在 future.get() 时重抛
• 析构/shutdown()：停止接收新任务 + 关闭队列 + join 所有 worker

#pragma once
#include <vector>
#include <thread>
#include <future>
#include <functional>
#include <stdexcept>
#include <type_traits>
#include <atomic>
#include <utility>

#include "blocking_queue.h"

class ThreadPool {
public:
    explicit ThreadPool(size_t nthreads) : accept_(true) {
        if (nthreads == 0) nthreads = 1;
        workers_.reserve(nthreads);
        for (size_t i = 0; i < nthreads; ++i) {
            workers_.push_back(std::thread([this]{ worker_loop(); }));
        }
    }

    ThreadPool(const ThreadPool&) = delete;
    ThreadPool& operator=(const ThreadPool&) = delete;

    ~ThreadPool() { shutdown(); }

    template <class F, class... Args>
    auto enqueue(F&& f, Args&&... args)
        -> std::future<typename std::result_of<F(Args...)>::type>
    {
        typedef typename std::result_of<F(Args...)>::type R;

        if (!accept_.load()) {
            throw std::runtime_error("ThreadPool is not accepting new tasks.");
        }

        auto task_ptr = std::make_shared<std::packaged_task<R()>>(
            std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );
        std::future<R> fut = task_ptr->get_future();

        bool ok = tasks_.push([task_ptr]{ (*task_ptr)(); });
        if (!ok) throw std::runtime_error("Task queue is closed.");

        return fut;
    }

    void shutdown() {
        bool expected = true;
        if (accept_.compare_exchange_strong(expected, false)) {
            tasks_.close();
            for (auto& t : workers_) {
                if (t.joinable()) t.join();
            }
        }
    }

private:
    void worker_loop() {
        std::function<void()> task;
        while (tasks_.pop(task)) {
            task();
        }
    }

private:
    std::vector<std::thread> workers_;
    BlockingQueue<std::function<void()>> tasks_;
    std::atomic<bool> accept_;
};

8.3 线程池使用示例（返回值 + 异常回传）

#include <iostream>
#include <chrono>
#include <thread>
#include "thread_pool.h"

int slow_add(int a, int b) {
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    return a + b;
}

int main() {
    ThreadPool pool(4);

    auto f1 = pool.enqueue(slow_add, 1, 2);
    auto f2 = pool.enqueue([](int x){ return x * x; }, 12);

    auto f3 = pool.enqueue([]() -> int {
        throw std::runtime_error("boom");
    });

    std::cout << "f1=" << f1.get() << "\n";
    std::cout << "f2=" << f2.get() << "\n";

    try {
        std::cout << "f3=" << f3.get() << "\n";
    } catch (const std::exception& e) {
        std::cout << "caught: " << e.what() << "\n";
    }

    pool.shutdown(); // 可省略：析构也会 shutdown
}