C++11 并发指南------std::thread 详解

参考： https://github.com/forhappy/Cplusplus-Concurrency-In-Practice/blob/master/zh/chapter3-Thread/Introduction-to-Thread.md#stdthread-%E8%AF%A6%E8%A7%A3

本节将详细介绍 std::thread 的用法。

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

<thread> 头文件摘要

<thread> 头文件声明了 std::thread 线程类及 std::swap (交换两个线程对象)辅助函数。另外命名空间 std::this_thread 也声明在 <thread> 头文件中。下面是 C++11 标准所定义的 <thread> 头文件摘要：

参见 N3242=11-0012 草案第 30.3 节 Threads(p1133)。

namespace std {
    #define __STDCPP_THREADS__ __cplusplus
    class thread;
    void swap(thread& x, thread& y);
    namespace this_thread {
        thread::id get_id();
        void yield();

        template <class Clock, class Duration>
        void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);

        template <class Rep, class Period>
        void sleep_for(const chrono::duration<Rep, Period>& rel_time);
    }        
}

<thread> 头文件主要声明了 std::thread 类，另外在 std::this_thread 命名空间中声明了get_id，yield，sleep_until 以及 sleep_for 等辅助函数，本章稍微会详细介绍 std::thread 类及相关函数。

std::thread 类摘要

std::thread 代表了一个线程对象，C++11 标准声明如下：

namespace std {
    class thread {
        public:
            // 类型声明:
            class id;
            typedef implementation-defined native_handle_type;

            // 构造函数、拷贝构造函数和析构函数声明:
            thread() noexcept;
            template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
            ~thread();
            thread(const thread&) = delete;
            thread(thread&&) noexcept;
            thread& operator=(const thread&) = delete;
            thread& operator=(thread&&) noexcept;

            // 成员函数声明:
            void swap(thread&) noexcept;
            bool joinable() const noexcept;
            void join();
            void detach();
            id get_id() const noexcept;
            native_handle_type native_handle();

            // 静态成员函数声明:
            static unsigned hardware_concurrency() noexcept;
    };
}

std::thread 中主要声明三类函数：(1). 构造函数、拷贝构造函数及析构函数；(2). 成员函数；(3). 静态成员函数。另外，std::thread::id 表示线程 ID，同时 C++11 声明如下：

namespace std {
    class thread::id {
        public:
            id() noexcept;
    };

    bool operator==(thread::id x, thread::id y) noexcept;
    bool operator!=(thread::id x, thread::id y) noexcept;
    bool operator<(thread::id x, thread::id y) noexcept;
    bool operator<=(thread::id x, thread::id y) noexcept;
    bool operator>(thread::id x, thread::id y) noexcept;
    bool operator>=(thread::id x, thread::id y) noexcept;

    template<class charT, class traits>
    basic_ostream<charT, traits>&
        operator<< (basic_ostream<charT, traits>& out, thread::id id);


    // Hash 支持
    template <class T> struct hash;
    template <> struct hash<thread::id>;
}

std::thread 详解

std::thread 构造和赋值

std::thread 构造函数

1. 默认构造函数(1)，创建一个空的 std::thread 执行对象。
2. 初始化构造函数(2)，创建一个 std::thread 对象，该 std::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)(3)，意味着 std::thread 对象不可拷贝构造。
4. Move 构造函数(4)，move 构造函数(move 语义是 C++11 新出现的概念，详见附录)，调用成功之后 x 不代表任何std::thread 执行对象。

注意：可被 joinable 的 std::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‘;
}

std::thread 赋值操作

1. Move 赋值操作(1)，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，则会调用 terminate() 报错。
2. 拷贝赋值操作(2)，被禁用，因此 std::thread 对象不可拷贝赋值。

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;
}

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;
}

其他成员函数

本小节例子来自 http://en.cppreference.com

• get_id: 获取线程 ID，返回一个类型为 std::thread::id 的对象。请看下面例子：

  #include <iostream>
  #include <thread>
  #include <chrono>
  
  void foo()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  int main()
  {
      std::thread t1(foo);
      std::thread::id t1_id = t1.get_id();
  
      std::thread t2(foo);
      std::thread::id t2_id = t2.get_id();
  
      std::cout << "t1‘s id: " << t1_id << ‘\n‘;
      std::cout << "t2‘s id: " << t2_id << ‘\n‘;
  
      t1.join();
      t2.join();
  }
  

• joinable: 检查线程是否可被 join。检查当前的线程对象是否表示了一个活动的执行线程，由默认构造函数创建的线程是不能被 join 的。另外，如果某个线程 已经执行完任务，但是没有被 join 的话，该线程依然会被认为是一个活动的执行线程，因此也是可以被 join 的。

  #include <iostream>
  #include <thread>
  #include <chrono>
  
  void foo()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  int main()
  {
      std::thread t;
      std::cout << "before starting, joinable: " << t.joinable() << ‘\n‘;
  
      t = std::thread(foo);
      std::cout << "after starting, joinable: " << t.joinable() << ‘\n‘;
  
      t.join();
  }
  

• join: Join 线程，调用该函数会阻塞当前线程，直到由 *this 所标示的线程执行完毕 join 才返回。

  #include <iostream>
  #include <thread>
  #include <chrono>
  
  void foo()
  {
      // simulate expensive operation
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  void bar()
  {
      // simulate expensive operation
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  int main()
  {
      std::cout << "starting first helper...\n";
      std::thread helper1(foo);
  
      std::cout << "starting second helper...\n";
      std::thread helper2(bar);
  
      std::cout << "waiting for helpers to finish..." << std::endl;
      helper1.join();
      helper2.join();
  
      std::cout << "done!\n";
  }
  

• detach: Detach 线程。 将当前线程对象所代表的执行实例与该线程对象分离，使得线程的执行可以单独进行。一旦线程执行完毕，它所分配的资源将会被释放。

调用 detach 函数之后：

1. *this 不再代表任何的线程执行实例。
2. joinable() == false
3. get_id() == std::thread::id()

另外，如果出错或者 joinable() == false，则会抛出 std::system_error.

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

    void independentThread() 
    {
        std::cout << "Starting concurrent thread.\n";
        std::this_thread::sleep_for(std::chrono::seconds(2));
        std::cout << "Exiting concurrent thread.\n";
    }

    void threadCaller() 
    {
        std::cout << "Starting thread caller.\n";
        std::thread t(independentThread);
        t.detach();
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::cout << "Exiting thread caller.\n";
    }

    int main() 
    {
        threadCaller();
        std::this_thread::sleep_for(std::chrono::seconds(5));
    }

• swap: Swap 线程，交换两个线程对象所代表的底层句柄(underlying handles)。

  #include <iostream>
  #include <thread>
  #include <chrono>
  
  void foo()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  void bar()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  int main()
  {
      std::thread t1(foo);
      std::thread t2(bar);
  
      std::cout << "thread 1 id: " << t1.get_id() << std::endl;
      std::cout << "thread 2 id: " << t2.get_id() << std::endl;
  
      std::swap(t1, t2);
  
      std::cout << "after std::swap(t1, t2):" << std::endl;
      std::cout << "thread 1 id: " << t1.get_id() << std::endl;
      std::cout << "thread 2 id: " << t2.get_id() << std::endl;
  
      t1.swap(t2);
  
      std::cout << "after t1.swap(t2):" << std::endl;
      std::cout << "thread 1 id: " << t1.get_id() << std::endl;
      std::cout << "thread 2 id: " << t2.get_id() << std::endl;
  
      t1.join();
      t2.join();
  }
  

执行结果如下：

thread 1 id: 1892
thread 2 id: 2584
after std::swap(t1, t2):
thread 1 id: 2584
thread 2 id: 1892
after t1.swap(t2):
thread 1 id: 1892
thread 2 id: 2584

• native_handle: 返回 native handle（由于 std::thread 的实现和操作系统相关，因此该函数返回与 std::thread 具体实现相关的线程句柄，例如在符合 Posix 标准的平台下(如 Unix/Linux)是 Pthread 库）。

  #include <thread>
  #include <iostream>
  #include <chrono>
  #include <cstring>
  #include <pthread.h>
  
  std::mutex iomutex;
  void f(int num)
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  
     sched_param sch;
     int policy; 
     pthread_getschedparam(pthread_self(), &policy, &sch);
     std::lock_guard<std::mutex> lk(iomutex);
     std::cout << "Thread " << num << " is executing at priority "
               << sch.sched_priority << ‘\n‘;
  }
  
  int main()
  {
      std::thread t1(f, 1), t2(f, 2);
  
      sched_param sch;
      int policy; 
      pthread_getschedparam(t1.native_handle(), &policy, &sch);
      sch.sched_priority = 20;
      if(pthread_setschedparam(t1.native_handle(), SCHED_FIFO, &sch)) {
          std::cout << "Failed to setschedparam: " << std::strerror(errno) << ‘\n‘;
      }
  
      t1.join();
      t2.join();
  }
  

执行结果如下：

Thread 2 is executing at priority 0
Thread 1 is executing at priority 20

• hardware_concurrency [static]: 检测硬件并发特性，返回当前平台的线程实现所支持的线程并发数目，但返回值仅仅只作为系统提示(hint)。

  #include <iostream>
  #include <thread>
  
  int main() {
      unsigned int n = std::thread::hardware_concurrency();
      std::cout << n << " concurrent threads are supported.\n";
  }
  

std::this_thread 命名空间中相关辅助函数介绍

• get_id: 获取线程 ID。

  #include <iostream>
  #include <thread>
  #include <chrono>
  #include <mutex>
  
  std::mutex g_display_mutex;
  
  void foo()
  {
      std::thread::id this_id = std::this_thread::get_id();
  
      g_display_mutex.lock();
      std::cout << "thread " << this_id << " sleeping...\n";
      g_display_mutex.unlock();
  
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  
  int main()
  {
      std::thread t1(foo);
      std::thread t2(foo);
  
      t1.join();
      t2.join();
  }
  

• yield: 当前线程放弃执行，操作系统调度另一线程继续执行。

  #include <iostream>
  #include <chrono>
  #include <thread>
  
  // "busy sleep" while suggesting that other threads run 
  // for a small amount of time
  void little_sleep(std::chrono::microseconds us)
  {
      auto start = std::chrono::high_resolution_clock::now();
      auto end = start + us;
      do {
          std::this_thread::yield();
      } while (std::chrono::high_resolution_clock::now() < end);
  }
  
  int main()
  {
      auto start = std::chrono::high_resolution_clock::now();
  
      little_sleep(std::chrono::microseconds(100));
  
      auto elapsed = std::chrono::high_resolution_clock::now() - start;
      std::cout << "waited for "
                << std::chrono::duration_cast<std::chrono::microseconds>(elapsed).count()
                << " microseconds\n";
  }
  

• sleep_until: 线程休眠至某个指定的时刻(time point)，该线程才被重新唤醒。

  template< class Clock, class Duration >
  void sleep_until( const std::chrono::time_point<Clock,Duration>& sleep_time );
  

• sleep_for: 线程休眠某个指定的时间片(time span)，该线程才被重新唤醒，不过由于线程调度等原因，实际休眠时间可能比sleep_duration 所表示的时间片更长。

  template< class Rep, class Period >
  void sleep_for( const std::chrono::duration<Rep,Period>& sleep_duration );
  
  #include <iostream>
  #include <chrono>
  #include <thread>
  
  int main()
  {
      std::cout << "Hello waiter" << std::endl;
      std::chrono::milliseconds dura( 2000 );
      std::this_thread::sleep_for( dura );
      std::cout << "Waited 2000 ms\n";
  }
  

执行结果如下：

Hello waiter
Waited 2000 ms