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C++11 并发指南二(std::thread 详解)

2013-08-04 13:37 by Haippy, ... 阅读, ... 评论, 收藏, 编辑

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  ---------- */

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