【C/C++开发】C++11 并发指南二(std::thread 详解)

2023-01-25 11:02:42

上一篇博客《C++11 并发指南一(C++11 多线程初探)》中只是提到了 std::thread
的基本用法，并给出了一个最简单的例子，本文将稍微详细地介绍 std::thread 的用法。

std::thread 在 <thread> 头文件中声明，因此使用 std::thread 时需要包含 <thread> 头文件。

std::thread 构造

default (1)	thread() noexcept;
initialization (2)	template <class Fn, class... Args> explicit thread (Fn&& fn, Args&&... args);
copy [deleted] (3)	thread (const thread&) = delete;
move (4)	thread (thread&& x) noexcept;

(1). 默认构造函数，创建一个空的 thread 执行对象。
(2). 初始化构造函数，创建一个 thread对象，该 thread对象可被 joinable，新产生的线程会调用 fn 函数，该函数的参数由 args 给出。
(3). 拷贝构造函数(被禁用)，意味着 thread 不可被拷贝构造。
(4). move 构造函数，move 构造函数，调用成功之后 x 不代表任何 thread 执行对象。
注意：可被 joinable 的 thread 对象必须在他们销毁之前被主线程 join 或者将其设置为 detached.

std::thread 各种构造函数例子如下（参考）：

#include <iostream>

#include <utility>

#include <thread>

#include <chrono>

#include <functional>

#include <atomic>

void f1(int n)

{

    for (int i = 0; i < 5; ++i) {

        std::cout << "Thread " << n << " executing\n";

        std::this_thread::sleep_for(std::chrono::milliseconds(10));

    }

}

void f2(int& n)

{

    for (int i = 0; i < 5; ++i) {

        std::cout << "Thread 2 executing\n";

        ++n;

        std::this_thread::sleep_for(std::chrono::milliseconds(10));

    }

}

int main()

{

    int n = 0;

    std::thread t1; // t1 is not a thread

    std::thread t2(f1, n + 1); // pass by value

    std::thread t3(f2, std::ref(n)); // pass by reference

    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread

    t2.join();

    t4.join();

    std::cout << "Final value of n is " << n << '\n';

}

move 赋值操作

move (1)	thread& operator= (thread&& rhs) noexcept;
copy [deleted] (2)	thread& operator= (const thread&) = delete;

(1). move 赋值操作，如果当前对象不可 joinable，需要传递一个右值引用(rhs)给 move 赋值操作；如果当前对象可被 joinable，则 terminate() 报错。
(2). 拷贝赋值操作被禁用，thread 对象不可被拷贝。

请看下面的例子：

#include <stdio.h>

#include <stdlib.h>

#include <chrono>    // std::chrono::seconds

#include <iostream>  // std::cout

#include <thread>    // std::thread, std::this_thread::sleep_for

void thread_task(int n) {

    std::this_thread::sleep_for(std::chrono::seconds(n));

    std::cout << "hello thread "

        << std::this_thread::get_id()

        << " paused " << n << " seconds" << std::endl;

}

/*

 * ===  FUNCTION  =========================================================

 *         Name:  main

 *  Description:  program entry routine.

 * ========================================================================

 */

int main(int argc, const char *argv[])

{

    std::thread threads[5];

    std::cout << "Spawning 5 threads...\n";

    for (int i = 0; i < 5; i++) {

        threads[i] = std::thread(thread_task, i + 1);

    }

    std::cout << "Done spawning threads! Now wait for them to join\n";

    for (auto& t: threads) {

        t.join();

    }

    std::cout << "All threads joined.\n";

    return EXIT_SUCCESS;

}  /* ----------  end of function main  ---------- */