【C++】多线程与并发【一】

part 0:多线程简介

Thread是一个指令序列，它可以在多线程环境中与其他这样的序列同时执行，同时共享同一个地址空间。

Thread is a sequence of instructions that can be executed concurrently with other such sequences in multithreading environments, while sharing a same address spac.

part 1:多线程构造

它用于构造线程对象。

It is used to constructs a thread object.

以下是std :: thread :: thread函数的声明。

thread() noexcept;
template <class Fn, class... Args>
explicit thread (Fn&& fn, Args&&... args);
thread (const thread&) = delete;	
thread (thread&& x) noexcept;

参量

• **fn-**它是函数的指针，成员的指针或任何可移动构造的函数对象。
• **args …-**传递给fn调用的参数。
• **x-**这是一个线程对象。

#include <thread> //头文件
#include <iostream>
using namespace std;void fn1(void)
{cout << "fn1" << endl;
}void fn2(int a)
{cout << "fn2 " << a << endl;
}void fn3(int& a)
{cout << "fn3 " << a << endl;
}class cls
{
public:void fn1(int a){cout << "cls::fn1" << endl;}
};void test01() {std::thread t1(&fn1); //线程对象构造后，即开始执行//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 detachedt1.join(); //必须执行join，不然运行期错误std::thread t2(&fn2, 2); //传入函数参数t2.join();int n = 2;std::thread t3(&fn3, std::ref(n)); //传入引用参数t3.join();cls c;//线程入口为类成员函数std::thread t4(&cls::fn1, &c, 2);t4.join();
}
int main()
{test01();getchar();return 0;
}

cpp多线程并发 头文件的使用，正则化实现

#include <iostream>
#include <thread>int main(void)
{std::thread x([]()->void {int i = 4;while (i--){std::this_thread::sleep_for(std::chrono::seconds(2));std::cout << "*" << std::endl;}return;});std::thread y([]()->void {int i = 4;while (i--){std::this_thread::sleep_for(std::chrono::seconds(5));std::cout << "-" << std::endl;}return;});std::cout << std::thread::hardware_concurrency() << std::endl; //检测计算机并发数std::cout << "x.hand " << x.native_handle() << std::endl;std::cout << "y.hand " << y.native_handle() << std::endl;//脱离主线程后不能直接获取句柄，所以放输出语句后面后面x.detach();y.detach();std::this_thread::sleep_for(std::chrono::seconds(30));//等待程序执行完成return 0;
}

part 2:多线程析构

它破坏线程对象。

It destroys the thread object. Following is the declaration for std::thread::~thread function.

~thread();

part 3:多线程operator=

以下是std :: thread :: operator =函数的声明。

It is used to move-assign thread. Following is the declaration for std::thread::operator= function.

thread& operator= (thread&& rhs) noexcept;	
thread& operator= (const thread&) = delete;

参量 Parameters

rhs − It is a othread object.

RHS -这是一个othread对象。

返回值

它返回* this

Data races

rhs和对象均被修改。

Both rhs and the object are modified.

part 4:joinable 它返回线程ive对象是否可连接，则返回true，否则返回false。 表示的是否可连接状态.

It returns whether the thread object is joinable.

It returns true if the thread is joinable or else false.

#include <iostream>
#include <thread>void foo02() {std::this_thread::sleep_for(std::chrono::seconds(2));
}void test02() {//joinablestd::thread t2;//创建了线程对象std::cout << "before joneable: " << t2.joinable() << std::endl;t2 = std::thread(foo02);//joinable  //实例化std::thread对象时传递了“函数名/可调用对象”std::cout << "after joneable: " << t2.joinable() << std::endl;t2.join();//等待std::cout << "after joining,joinable: " << t2.joinable()<<std::endl;
}int main() {	test02();getchar();return 0;
}

part 5:多线程std :: thread :: get_id

Declaration

下面是std::thread::get_id函数的声明。 Following is the declaration for std::thread::get_id function.

id get_id() const noexcept;

C++11

id get_id() const noexcept;

Return Value:It returns the thread id.

Exceptions:No-throw guarantee − never throws exceptions.

Data races:he object is accessed.

#include <iostream>
#include <thread>
#include <chrono>void foo() {std::this_thread::sleep_for(std::chrono::seconds(1));
}int main() {std::thread sample(foo);std::thread::id sample_id = sample.get_id();std::thread sample2(foo);std::thread::id sample2_id = sample2.get_id();std::cout << "sample's id: " << sample_id << '\n';std::cout << "sample2's id: " << sample2_id << '\n';sample.join();sample2.join();
}

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part 6:native_handle 返回底层实现定义的线程句柄

Declaration:Following is the declaration for std::thread::native_handle function.

native_handle_type native_handle();

Return Value:It returns a value of member type thread::native_handle_type.

part 7:hardware_concurrency 返回实现支持的并发线程数

Description

It returns the number of hardware thread contexts.

static unsigned hardware_concurrency() noexcept;

Return Value

It returns the number of hardware thread contexts.

part 8:join 线程执行完成后返回

It returns when the thread execution has completed.

void foo03() {std::this_thread::sleep_for(std::chrono::seconds(2));
}
void bar03() {std::this_thread::sleep_for(std::chrono::seconds(2));
}
void test03() {//join 线程执行完成后返回//It returns when the thread execution has completed.std::cout << " starting helping ...." << std::endl;std::thread helper3(foo02);std::cout << " starting another helping ...." << std::endl;std::thread helper3_2(bar03);std::cout << " waiting for helpers finish...." << std::endl;helper3.join();//线程执行完成后返回helper3_2.join();std::cout << "  done!" << std::endl;
}int main() {test03();getchar();return 0;
}

part 9:detach 容许线程从线程句柄独立开来执行

Description

It returns when the thread execution has completed.

#include <iostream>
#include <chrono>
#include <thread>void independentThread() {std::cout << "Starting thread.\n";std::this_thread::sleep_for(std::chrono::seconds(2));std::cout << "Exiting previous 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));
}

part 10:swap 交换二个 thread 对象

Description

It swaps the state of the object with that of x.

Declaration

Following is the declaration for std::thread::swap function.

void swap (thread& x) noexcept;

part 11:std::swap 特化 std::swap 算法

Description

It is used to exchanges the state of the thread objects x and y.

Declaration

Following is the declaration for std::thread::swap function.

void swap (thread& x, thread& y) noexcept;

Parameters

x,y − It is a thread objects.

实例：

//thread1.cpp  创建线程，并观察线程的并发执行与阻塞等待#include <iostream>
#include <thread>
#include <chrono>using namespace std;void thread_function(int n)
{std::thread::id this_id = std::this_thread::get_id();			//获取线程IDfor(int i = 0; i < 5; i++){    cout << "Child function thread " << this_id<< " running : " << i+1 << endl;std::this_thread::sleep_for(std::chrono::seconds(n));   	//进程睡眠n秒}
}class Thread_functor
{
public:// functor行为类似函数,C++中的仿函数是通过在类中重载()运算符实现，使你可以像使用函数一样来创建类的对象void operator()(int n){std::thread::id this_id = std::this_thread::get_id();for(int i = 0; i < 5; i++){cout << "Child functor thread " << this_id << " running: " << i+1 << endl;std::this_thread::sleep_for(std::chrono::seconds(n));   //进程睡眠n秒}}	
};int main()
{thread mythread1(thread_function, 1);      // 传递初始函数作为线程的参数if(mythread1.joinable()) //判断是否可以成功使用join()或者detach()，返回true则可以，false则不可mythread1.join();                     // 使用join()函数阻塞主线程直至子线程执行完毕Thread_functor thread_functor;			 //函数对象实例化一个对象thread mythread2(thread_functor, 3);     // 传递初始函数作为线程的参数if(mythread2.joinable())mythread2.detach();  // 使用detach()函数让子线程和主线程并行运行，主线程也不再等待子线程//lambda表达式格式：[capture list] (params list) mutable exception-> return type { function body }auto thread_lambda = [](int n){			std::thread::id this_id = std::this_thread::get_id();for(int i = 0; i < 5; i++){cout << "Child lambda thread " << this_id << " running: " << i+1 << endl;std::this_thread::sleep_for(std::chrono::seconds(n));   //进程睡眠n秒}       };thread mythread3(thread_lambda, 4);     // 传递初始函数作为线程的参数if(mythread3.joinable())mythread3.join();                     // 使用join()函数阻塞主线程直至子线程执行完毕std::thread::id this_id = std::this_thread::get_id();for(int i = 0; i < 5; i++){cout << "Main thread " << this_id << " running: " << i+1 << endl;std::this_thread::sleep_for(std::chrono::seconds(1));}getchar();return 0;
}

线程创建的参数是函数对象，函数对象不止是函数指针或成员函数指针，同时还包括函数对象（仿函数）与lambda表达式。上面的代码分别用三种函数对象创建了三个线程**，其中第一个线程mythread1阻塞等待其执行完后继续往下执行**，第二个线程mythread2不阻塞等待在后台与后面的第三个线程mythread3并发执行，第三个线程继续阻塞等待其完成后再继续往下执行主线程任务。

为了便于观察并发过程，对三个线程均用了睡眠延时this_thread::sleep_for(duration)函数，且延时时间作为参数传递给该函数。这里的参数是支持C++泛型模板的，STL标准容器类型（比如Array/Vector/Deque/List/Set/Map/String等）都可以作为参数传递，但这里的参数默认是以拷贝的方式传递参数的，当期望传入一个引用时，要使用std::ref进行转换，实例见part 1中实例。

如果想要线程mythread2独立运行，修改如下

mythread2.detach(); ---》   mythread2.join();

线程mythread2和线程mythread3与主线程main 同步运行

mythread2.detach(); ---》   mythread2.join();
mythread3.join();   ---》   mythread3.detach();

小结：

当线程不需要相互依赖，不会产生数据竞争，或不是流水的实现思路下，用detach()；

当可能产生数据竞争，数据之间相互依赖，算法实现设计为流水的情况下，使用join()函数，使其他线程进入阻塞状态。

针对任何线程（包括主线程），< thread > 还声明了一个命名空间std::this_thread，用以提高线程专属的全局函数。函数声明和效果见下表。 上面的代码就是利用了std::this_thread提供的函数获得当前线程的ID，让当前线程睡眠一段时间（一般需要< chrono >头文件提供duration或timepoint）的功能 。

参考资料：
https://zh.cppreference.com/w/cpp/thread/thread
https://www.tutorialspoint.com/cpp_standard_library
https://blog.csdn.net/m0_37621078/article/details/104909834