UNIX(多线程)：03--- 认识std::thread

用户3479834

发布于 2021-02-03 14:22:10

8690

发布于 2021-02-03 14:22:10

文章被收录于专栏：游戏开发司机

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

默认构造函数 (1)	thread() noexcept;
初始化构造函数 (2)	template <class Fn, class... Args>explicit thread(Fn&& fn, Args&&... args);
拷贝构造函数 [deleted] (3)	thread(const thread&) = delete;
Move 构造函数 (4)	thread(thread&& x) noexcept;

默认构造函数(1)，创建一个空的 std::thread 执行对象。
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。
拷贝构造函数(被禁用)(3)，意味着 std::thread 对象不可拷贝构造。
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

Move 赋值操作 (1)	thread& operator=(thread&& rhs) noexcept;
拷贝赋值操作 [deleted] (2)	thread& operator=(const thread&) = delete;

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。
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：

*this 不再代表任何的线程执行实例。

joinable() == false

get_id() == std::this_thread::id()

#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

本文参与腾讯云自媒体同步曝光计划，分享自微信公众号。

原始发表：2020-12-30，如有侵权请联系 cloudcommunity@tencent.com 删除

c++

本文分享自游戏开发司机微信公众号，前往查看

如有侵权，请联系 cloudcommunity@tencent.com 删除。

本文参与腾讯云自媒体同步曝光计划，欢迎热爱写作的你一起参与！

c++

登录后参与评论

0 条评论

热度

UNIX(多线程)：03--- 认识std::thread

UNIX(多线程)：03--- 认识std::thread

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐