c++ 11 之后有了标准的线程库：std::thread。通过c++11中的线程库创建线程，极为方便，且跨平台，是语言层面的。之前跨平台的多线程开发中，多使用boost 相关第三方库。
1.std::thread 构造函数
默认构造函数 thread() noexcept;
初始化构造函数 template <class Fn, class... Args>
explicit thread(Fn&& fn, Args&&... args);
拷贝构造函数 [deleted] thread(const thread&) = delete;
Move 构造函数 thread(thread&& x) noexcept;
(1).默认构造函数，创建一个空的 std::thread 执行对象。
(2).初始化构造函数，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)，意味着 std::thread 对象不可拷贝构造。
(4).Move 构造函数，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：
#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;
std::thread t2(f1, n + 1);
std::thread t3(f2, std::ref(n));
std::thread t4(std::move(t3));
t2.join();
t4.join();
std::cout << "Final value of n is " << n << '\n';
}
2.std::thread 赋值操作
Move 赋值操作 thread& operator=(thread&& rhs) noexcept;
拷贝赋值操作 [deleted] thread& operator=(const thread&) = delete;
(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>
#include <iostream>
#include <thread>
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;
}
3.其他成员函数
3.1 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();
return 0;
}
从std::thread::id取得int值线程id
std::thread::id tid = std::this_thread::get_id();
_Thrd_t t = *(_Thrd_t*)(char*)&tid ;
unsigned int nId = t._Id
参考资料：https://www.cnblogs.com/yc-only-blog/p/9178935.html
3.2 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: " << std::boolalpha << t.joinable() << '\n';
t = std::thread(foo);
std::cout << "after starting, joinable: " << t.joinable() << '\n';
t.join();
std::cout << "after joining, joinable: " << t.joinable() << '\n';
}
std::thread “terminate called without an active exception”请参考如下接：
https://www.cnblogs.com/little-ant/p/3312841.html
3.3 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::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));
}
3.4 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
3.5 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）。
3.6 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";
}
4.std::this_thread 命名空间中相关辅助函数介绍
4.1 get_id()
获取线程 ID。
#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();
}
4.2 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>
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";
}
4.3 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 );
4.4 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。
#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";
}
5.std::thread堆栈使用
#include <iostream>
#include <thread>
using namespace std;
void show()
{
cout << "hello cplusplus!" << endl;
}
int main()
{
thread t1(show);
thread t2(show);
thread t3(show);
t1.join();
t2.join();
t3.join();
thread threads[3]{thread(show), thread(show), thread(show)};
for (auto& t: threads)
{
t.join();
}
thread *pt1(new thread(show));
thread *pt2(new thread(show));
thread *pt3(new thread(show));
pt1->join();
pt2->join();
pt3->join();
thread *pthreads(new thread[3]{thread(show), thread(show), thread(show)});
delete pt1;
delete pt2;
delete pt3;
int threadNum = 3;
thread* t = new thread[threadNum];
if(t)
{
for ( int i = 0; i < threadNum; i++){
t[i] = thread(show);
}
for ( int i = 0; i < threadNum; i++){
t[i].join();
}
}
delete [] t;
return 0;
}
注意事项：
1.std::thread在栈内创建后，需要在同一个作用域内调用join() 或者 detach()， 否则退出作用域后，程序会异常退出， 具体原因如下
~thread()
{
if (joinable())
std::terminate();
}
其中的std::terminate()就是用来终止进程的调用。由于std::thread创建后默认是joinable的， 所以需要调用join()或者detach() 来使线程变成非joinable的状态， 具体使用join(）还是detach() 取决于实际需求， 如果需要等待线程完成才能继续，那就使用join()来等待， 如果需要立刻返回继续其他操作， 那就调用detach()来脱离对线程的管理， 两者必须选一个。
2.调用new 创建的std::thread， 禁止直接使用delete， 需要在调用delete之前，调用 join()或者 detach() (如果创建线程后立刻调用了这两个函数中任意一个， 可以不再调用， 但是为了保险起见， 需要加上if(joinable()) 的判断)， 原因见上一条。
6.std::thread 线程执行体
std::thread 执行体类似boost::thread， 并不要求是普通的函数，任何可调用的对象都可，具体接受下面四种：
- 普通函数
- 函数对象
- 类成员函数
- lambda 函数（匿名函数）
分别使用举例如下：
(1) 普通函数
void hello()
{
}
void hello(std::string name)
{
}
std::thread thread1(hello);
std::thread thread2(hello, "test");
(2) 函数对象
class Hello
{
public: Hello(){}
void operator()() const
{
}
};
class Hello {
public: Hello() {}
void operator()(const std::string name) const
{
}
};
std::thread thread3(Hello());
std::thread thread4(Hello(), "test");
(3) 类成员函数
class Hello {
public: entity()
{
}
void entity(const std::string name)
{
}
};
Hello obj;
std::thread thread5(&Hello::entity, &obj);
std::thread thread6(&Hello::entity, &obj, "test");
此种构造也可以借助std::bind使用另外一种方式：
std::thread thread5(std::bind(&Hello::entity, &obj));
std::thread thread6(std::bind(&Hello::entity, &obj, "test"));
(4) lambda 函数
std::thread thread7([](T var1, T var2){
}, realValue1, readValue2);
7.原子变量与线程安全
示例1：
#include <iostream>
#include <thread>
using namespace std;
const int N = 100000000;
int num = 0;
void run()
{
for (int i = 0; i < N; i++)
{
num++;
}
}
int main()
{
clock_t start = clock();
thread t1(run);
thread t2(run);
t1.join();
t2.join();
clock_t end = clock();
cout << "num=" << num << ",用时 " << end - start << " us" << endl;
return 0;
}
从上述代码执行的结果，发现结果并不是预计的200000000，这是由于线程之间发生冲突，从而导致结果不正确。为了解决此问题（多线程同时操作同一份数据），有以下方法：
（1）互斥量。
#include <iostream>
#include <thread>
#include <mutex>
using namespace std;
const int N = 100000000;
int num = 0;
mutex m;
void run()
{
for (int i = 0; i < N; i++)
{
m.lock();
num++;
m.unlock();
}
}
int main()
{
clock_t start = clock();
thread t1(run);
thread t2(run);
t1.join();
t2.join();
clock_t end = clock();
cout << "num=" << num << ",用时 " << end - start << " us" << endl;
return 0;
}
通过互斥量后运算结果正确，但是计算速度很慢，原因主要是互斥量加解锁需要时间。
互斥量详细内容 请参考C++11 并发之std::mutex。
（2）原子变量。
例如：
#include <iostream>
#include <thread>
#include <atomic>
using namespace std;
const int N = 100000000;
atomic_int num{ 0 };
void run()
{
for (int i = 0; i < N; i++)
{
num++;
}
}
int main()
{
clock_t start = clock();
thread t1(run);
thread t2(run);
t1.join();
t2.join();
clock_t end = clock();
cout << "num=" << num << ",用时 " << end - start << " us" << endl;
return 0;
}
通过原子变量后运算结果正确，计算速度一般。
（3）通过控制 join位置 。
#include <iostream>
#include <thread>
#include <atomic>
using namespace std;
const int N = 100000000;
int num = 0;
void run()
{
for (int i = 0; i < N; i++)
{
num++;
}
}
int main()
{
clock_t start = clock();
thread t1(run);
t1.join();
thread t2(run);
t2.join();
clock_t end = clock();
cout << "num=" << num << ",用时 " << end - start << " us" << endl;
return 0;
}
运算结果正确，计算速度也很理想。看似没有什么问题，实际上适用场景非常有限，仅适用于一个线程去控制一份数据的情况，大型项目中线程层级关系较多，还是通过数据互斥锁或者设置原子变量的方法来控制
编译 g++ -std=c++11 -pthread test.cpp -o test.exe
参考资料：
- C++ std::thread
- C++11并发之std::thread
- std::thread 学习记录
- std::thread使用注意事项
示例
#include <iostream>
#include <thread>
#include <atomic>
#include <functional>
#include <mutex>
class CTestThread
{
private:
public:
void startThread()
{
std::lock_guard<std::mutex> g(m_threadMtx);
if(!m_p_thread){
m_running = true;
m_p_thread = new std::thread(std::bind(&CTestThread::runThread,this));
}
}
void stopThread()
{
std::lock_guard<std::mutex> g(m_threadMtx);
if(m_p_thread){
m_running = false;
m_p_thread->join();
delete m_p_thread;
m_p_thread = nullptr;
}
}
void runThread()
{
int num = 0;
while(m_running)
{
std::cout << "thread id:" << std::this_thread::get_id() << " num: " << num++ << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(2));
}
}
void startThread2()
{
if(!m_threadrunning){
m_threadrunning = true;
m_thread = std::move(std::thread(std::bind(&CTestThread::runThread,this)));
}
}
void stopThread2()
{
if(m_threadrunning){
m_threadrunning = false;
if (m_thread.joinable())
m_thread.join();
}
}
void startThread3()
{
if(!m_threadrunning){
m_threadrunning = true;
m_thread = std::move(std::thread([this](){runThread();}));
}
}
void stopThread3()
{
if(m_threadrunning){
m_threadrunning = false;
if (m_thread.joinable())
m_thread.join();
}
}
private:
std::mutex m_threadMtx;
std::thread *m_p_thread = nullptr;
bool m_running = false;
std::thread m_thread;
bool m_threadrunning = false;
};
void hello(std::string msg)\
{
std::cout << "hello " << msg << std::endl;
}
int main(int argc, char* argv[])
{
CTestThread testThread;
testThread.startThread();
testThread.startThread2();
CTestThread testThread3;
testThread3.startThread3();
std::thread th([]()->void{
int count = 0;
while(1){
count++;
std::cout << "count: " << count << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(5));
}
});
th.detach();
std::thread th2([=](){
int age = 0;
while(1){
age++;
std::cout << "age: " << age << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(5));
}
});
th2.join();
std::thread th4(hello, "LiMing");
}
本文内容由网友自发贡献，版权归原作者所有，本站不承担相应法律责任。如您发现有涉嫌抄袭侵权的内容，请联系:hwhale#tublm.com(使用前将#替换为@)
