C++11中<thread>的使用

C++11中加入了<thread>头文件，此头文件主要声明了std::thread线程类。C++11的标准类std::thread对线程进行了封装。std::thread代表了一个线程对象。应用C++11中的std::thread便于多线程程序的移值。

<thread>是C++标准程序库中的一个头文件，定义了C++11标准中的一些表示线程的类、用于互斥访问的类与方法等。

类std::thread表示一个线程。初始化时给出该线程的执行函数(或是可以调用的对象)。线程对象构造后即开始运行。默认情况下，C++11的子线程必须与主线程会合，即在主线程中调用thread::join()函数，这避免了子线程还在执行，主线程已经执行结束而撤销的情况。

如果子线程的执行函数需要参数，可把实参列表写在std::thread对象构造函数的参数列表中。如果把可调用对象(callable object)作为参数传给子线程的构造函数，则把该可调用对象复制一份给子线程。如果需要传递可调用对象的左值引用给子线程，则采用std::ref()来产生对象的引用、然后把引用值再传进去给子线程。

C++11所定义的线程是和操作系统的线程一一对应的，也就是说我们生成的线程都是直接接受操作系统的调度的，一个进程所能创建的线程数目以及一个操作系统所能创建的总的线程数据等都由运行时操作系统限定。

std::thread中主要声明三类函数：(1)、构造函数、拷贝构造函数(拷贝构造函数被禁用，意味着thread不可被拷贝构造，但能被转移(move)或者互换(swap))及析构函数；(2)、成员函数；(3)、静态成员函数(hardware_concurrency，检测硬件并发特性, Returns the number of hardware thread contexts)。

std::thread类成员函数：

(1)、get_id：获取线程ID，返回一个类型为std::thread::id的对象。

(2)、joinable：检查线程是否可被join。检查thread对象是否标识一个活动(active)的可行性线程。缺省构造的thread对象、已经完成join的thread对象、已经detach的thread对象都不是joinable。

(3)、join：调用该函数会阻塞当前线程。阻塞调用者(caller)所在的线程直至被join的std::thread对象标识的线程执行结束。

(4)、detach：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。

(5)、native_handle：该函数返回与std::thread具体实现相关的线程句柄。native_handle_type是连接thread类和操作系统SDK API之间的桥梁，如在Linux g++(libstdc++)里，native_handle_type其实就是pthread里面的pthread_t类型，当thread类的功能不能满足我们的要求的时候(比如改变某个线程的优先级)，可以通过thread类实例的native_handle()返回值作为参数来调用相关的pthread函数达到目录。This member function is only present in class thread if the library implementation supports it. If present, it returns a value used to access implementation-specific information associated to the thread.

(6)、swap：交换两个线程对象所代表的底层句柄。

(7)、operator=：moves the thread object

(8)、hardware_concurrency：静态成员函数，返回当前计算机最大的硬件并发线程数目。基本上可以视为处理器的核心数目。

另外，std::thread::id表示线程ID，定义了在运行时操作系统内唯一能够标识该线程的标识符，同时其值还能指示所标识的线程的状态。Values of this type are returned by thread::get_id and this_thread::get_id to identify threads.

有时候我们需要在线程执行代码里面对当前调用者线程进行操作，针对这种情况，C++11里面专门定义了一个命名空间this_thread，此命名空间也声明在<thread>头文件中，其中包括get_id()函数用来获取当前调用者线程的ID；yield()函数可以用来将调用者线程跳出运行状态，重新交给操作系统进行调度，即当前线程放弃执行，操作系统调度另一线程继续执行；sleep_until()函数是将线程休眠至某个指定的时刻(time point),该线程才被重新唤醒；sleep_for()函数是将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，实际休眠实际可能比sleep_duration所表示的时间片更长。

std::thread：Class to represent individual threads of execution. A thread of execution is a sequence of instructions that can be executed concurrently with other such sequences in multithreading environments, while sharing a same address space.

std::this_thread：This namespace groups a set of functions that access the current thread.

下面是从其他文章中copy的<thread>测试代码，详细内容介绍可以参考对应的reference：

#include "thread2.hpp"#include <iostream>#include <vector>#include<functional>#include<memory>#include<list>#include<mutex>#include<condition_variable>#include <atomic>#include <thread>#include <chrono>#include <iomanip>#include <ctime>///////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/thread/thread/std::atomic<int> global_counter(0);static void increase_global(int n) { for (int i = 0; i<n; ++i) ++global_counter; }static void increase_reference(std::atomic<int>& variable, int n) { for (int i = 0; i<n; ++i) ++variable; }struct C : std::atomic<int> {C() : std::atomic<int>(0) {}void increase_member(int n) { for (int i = 0; i<n; ++i) fetch_add(1); }};int test_thread_thread(){// thread::thread: Constructs a thread objectstd::vector<std::thread> threads;std::cout << "increase global counter with 10 threads...\n";for (int i = 1; i <= 10; ++i)threads.push_back(std::thread(increase_global, 1000));std::cout << "increase counter (foo) with 10 threads using reference...\n";std::atomic<int> foo(0);for (int i = 1; i <= 10; ++i)threads.push_back(std::thread(increase_reference, std::ref(foo), 1000));std::cout << "increase counter (bar) with 10 threads using member...\n";C bar;for (int i = 1; i <= 10; ++i)threads.push_back(std::thread(&C::increase_member, std::ref(bar), 1000));std::cout << "synchronizing all threads...\n";for (auto& th : threads) th.join();std::cout << "global_counter: " << global_counter << '\n';std::cout << "foo: " << foo << '\n';std::cout << "bar: " << bar << '\n';return 0;}////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/thread/detach/static void pause_thread(int n){std::this_thread::sleep_for(std::chrono::seconds(n));std::cout << "pause of " << n << " seconds ended\n";}int test_thread_detach(){// thread::detach: Detaches the thread represented by the object from the calling thread,// allowing them to execute independently from each other.std::cout << "Spawning and detaching 3 threads...\n";std::thread(pause_thread, 1).detach();std::thread(pause_thread, 2).detach();std::thread(pause_thread, 3).detach();std::cout << "Done spawning threads.\n";std::cout << "(the main thread will now pause for 5 seconds)\n";// give the detached threads time to finish (but not guaranteed!):pause_thread(5);return 0;}//////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/thread/get_id/std::thread::id main_thread_id = std::this_thread::get_id();static void is_main_thread(){// this_thread::get_id: Returns the thread id of the calling thread.// This value uniquely identifies the thread.if (main_thread_id == std::this_thread::get_id())std::cout << "This is the main thread.\n";elsestd::cout << "This is not the main thread.\n";}int test_thread_get_id(){// thread::get_id: Returns the thread id// If the thread object is joinable, the function returns a value that uniquely identifies the thread.// If the thread object is not joinable, the function returns a default - constructed object of member type thread::id.is_main_thread();std::thread th(is_main_thread);th.join();return 0;}/////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/thread/join/int test_thread_join(){// thread::join: The function returns when the thread execution has completed.std::cout << "Spawning 3 threads...\n";std::thread t1(pause_thread, 1);std::thread t2(pause_thread, 2);std::thread t3(pause_thread, 3);std::cout << "Done spawning threads. Now waiting for them to join:\n";t1.join();t2.join();t3.join();std::cout << "All threads joined!\n";return 0;}///////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/thread/joinable/static void mythread(){// do stuff...}int test_thread_joinable(){// thread::joinable: Returns whether the thread object is joinable.// A thread object is joinable if it represents a thread of execution.std::thread foo; // 缺省构造函数，线程不可执行std::thread bar(mythread);std::cout << "Joinable after construction:\n" << std::boolalpha;std::cout << "foo: " << foo.joinable() << '\n';std::cout << "bar: " << bar.joinable() << '\n';if (foo.joinable()) foo.join();if (bar.joinable()) bar.join();std::cout << "Joinable after joining:\n" << std::boolalpha;std::cout << "foo: " << foo.joinable() << '\n';std::cout << "bar: " << bar.joinable() << '\n';return 0;}//////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/thread/operator=/int test_thread_operator(){// thread::operator=: Move-assign thread// If the object is currently not joinable, it acquires the thread of execution represented by rhs(if any).// If it is joinable, terminate() is called.// After the call, rhs no longer represents any thread of execution(as if default - constructed).std::thread threads[5];                         // default-constructed threadsstd::cout << "Spawning 5 threads...\n";for (int i = 0; i<5; ++i)threads[i] = std::thread(pause_thread, i + 1);   // move-assign threadsstd::cout << "Done spawning threads. Now waiting for them to join:\n";for (int i = 0; i<5; ++i)threads[i].join();std::cout << "All threads joined!\n";return 0;}//////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/this_thread/sleep_for/int test_this_thread_sleep_for(){// this_thread::sleep_for: Blocks execution of the calling thread during the span of time specified by rel_time.// The execution of the current thread is stopped until at least rel_time has passed from now.// Other threads continue their execution.std::cout << "countdown:\n";for (int i = 10; i>0; --i) {std::cout << i << std::endl;std::this_thread::sleep_for(std::chrono::seconds(1));}std::cout << "Lift off!\n";return 0;}/////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/this_thread/sleep_until/int test_this_thread_sleep_until(){// this_thread::sleep_until: Blocks the calling thread until abs_time.// The execution of the current thread is stopped until at least abs_time, while other threads may continue to advance.using std::chrono::system_clock;std::time_t tt = system_clock::to_time_t(system_clock::now());struct std::tm * ptm = std::localtime(&tt);std::cout << "Current time: " << std::put_time(ptm, "%X") << '\n';std::cout << "Waiting for the next minute to begin...\n";++ptm->tm_min; ptm->tm_sec = 0;std::this_thread::sleep_until(system_clock::from_time_t(mktime(ptm)));std::cout << std::put_time(ptm, "%X") << " reached!\n";return 0;}/////////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/thread/this_thread/yield/std::atomic<bool> ready(false);static void count1m(int id){while (!ready) {             // wait until main() sets ready...std::this_thread::yield();}for (volatile int i = 0; i<1000000; ++i) {}std::cout << id << std::endl;}int test_this_thread_yield(){// this_thread::yield: The calling thread yields, offering the implementation the opportunity to reschedule.// This function shall be called when a thread waits for other threads to advance without blocking.std::thread threads[10];std::cout << "race of 10 threads that count to 1 million:\n";for (int i = 0; i<10; ++i) threads[i] = std::thread(count1m, i);ready = true;               // go!for (auto& th : threads) th.join();std::cout << '\n';return 0;}//////////////////////////////////////////////////////////////// reference: https://zh.wikibooks.org/zh-hans/C%2B%2B/STL/Threadtemplate<typename T>class SyncQueue {public:SyncQueue(int maxSize) :m_maxSize(maxSize), m_needStop(false) {}void Put(const T&x) { Add(x); }void Put(T&&x) { Add(std::forward<T>(x)); }void Take(std::list<T>& list){std::unique_lock<std::mutex> locker(m_mutex);m_notEmpty.wait(locker, [this] {return m_needStop || NotEmpty(); });if (m_needStop) return;list = std::move(m_queue);m_notFull.notify_one();}void Take(T& t){std::unique_lock<std::mutex> locker(m_mutex);m_notEmpty.wait(locker, [this] {return m_needStop || NotEmpty(); });if (m_needStop) return;t = m_queue.front();m_queue.pop_front();m_notFull.notify_one();}void Stop(){{std::lock_guard<std::mutex> locker(m_mutex);m_needStop = true;}m_notFull.notify_all();m_notEmpty.notify_all();}bool Empty(){std::lock_guard<std::mutex> locker(m_mutex);return m_queue.empty();}bool Full(){std::lock_guard<std::mutex> locker(m_mutex);return m_queue.size() == m_maxSize;}size_t Size(){std::lock_guard<std::mutex> locker(m_mutex);return m_queue.size();}int Count(){return m_queue.size();}private:bool NotFull() const{bool full = m_queue.size() >= m_maxSize;if (full)std::cout << "full, waiting，thread id: " << std::this_thread::get_id() << std::endl;return !full;}bool NotEmpty() const{bool empty = m_queue.empty();if (empty)std::cout << "empty,waiting，thread id: " << std::this_thread::get_id() << std::endl;return !empty;}template<typename F>void Add(F&&x){std::unique_lock< std::mutex> locker(m_mutex);m_notFull.wait(locker, [this] {return m_needStop || NotFull(); });if (m_needStop) return;m_queue.push_back(std::forward<F>(x));m_notEmpty.notify_one();}private:std::list<T> m_queue; //缓冲区std::mutex m_mutex; //互斥量和条件变量结合起来使用std::condition_variable m_notEmpty;//不为空的条件变量std::condition_variable m_notFull; //没有满的条件变量int m_maxSize; //同步队列最大的sizebool m_needStop; //停止的标志};const int MaxTaskCount = 100;class ThreadPool {public:using Task = std::function<void()>;ThreadPool(int numThreads = std::thread::hardware_concurrency()) : m_queue(MaxTaskCount){Start(numThreads);}~ThreadPool(void){//如果没有停止时则主动停止线程池Stop();}void Stop(){std::call_once(m_flag, [this] {StopThreadGroup(); }); //保证多线程情况下只调用一次StopThreadGroup}void AddTask(Task&&task){m_queue.Put(std::forward<Task>(task));}void AddTask(const Task& task){m_queue.Put(task);}private:void Start(int numThreads){m_running = true;//创建线程组for (int i = 0; i <numThreads; ++i) {m_threadgroup.push_back(std::make_shared<std::thread>(&ThreadPool::RunInThread, this));}}void RunInThread(){while (m_running) {//取任务分别执行std::list<Task> list;m_queue.Take(list);for (auto& task : list) {if (!m_running)return;task();}}}void StopThreadGroup(){m_queue.Stop(); //让同步队列中的线程停止m_running = false; //置为false，让内部线程跳出循环并退出for (auto thread : m_threadgroup) { //等待线程结束if (thread)thread->join();}m_threadgroup.clear();}std::list<std::shared_ptr<std::thread>> m_threadgroup; //处理任务的线程组SyncQueue<Task> m_queue; //同步队列     std::atomic_bool m_running; //是否停止的标志std::once_flag m_flag;};void TestThdPool(){ThreadPool pool; bool runing = true;std::thread thd1([&pool, &runing] {while (runing) {std::cout << "produce " << std::this_thread::get_id() << std::endl;pool.AddTask([] {std::cout << "consume " << std::this_thread::get_id() << std::endl;});}});std::this_thread::sleep_for(std::chrono::seconds(10));runing = false;pool.Stop();thd1.join();getchar();}int test_thread_pool(){TestThdPool();return 0;}//////////////////////////////////////////////////////////////int test_thread_hardware_concurrency(){std::cout << " the number of hardware thread contexts: " << std::thread::hardware_concurrency() << std::endl;return 0;}

GitHub：https://github.com/fengbingchun/Messy_Test