C++多线程学习（一）：C++11 多线程快速入门

参考引用

• C++11 14 17 20 多线程从原理到线程池实战
• 代码运行环境：Visual Studio 2019

1. 为什么要用多线程

• 任务分解
  • 耗时的操作，任务分解，实时响应
• 数据分解
  • 充分利用多核CPU处理数据
• 数据流分解
  • 读写分离，解耦合设计

2. 第一个子线程代码示例

• first_thread.cpp

  #include <iostream>
  #include <thread>using namespace std;// 创建的子线程的入口函数
  void ThreadMain() {// 获取子线程开始时刻的 ID 号cout << "begin sub thread ID " << this_thread::get_id() << endl;for (int i = 0; i < 5; i++) {cout << "in thread " << i << endl;// 子线程睡眠（释放）CPU 资源 1000msthis_thread::sleep_for(chrono::seconds(1));  }cout << "end sub thread ID " << this_thread::get_id() << endl;
  }// 主线程的入口函数 main()
  int main(int argc, char* argv[]) {cout << "main thread ID " << this_thread::get_id() << endl;// 线程创建启动thread th(ThreadMain);cout << "begin wait sub thread" << endl;// 阻塞等待子线程退出th.join();cout << "end wait sub thread" << endl;return 0;
  }
  

• 控制台输出

  main thread ID 21580
  begin wait sub thread
  begin sub thread ID 22924
  in thread 0
  in thread 1
  in thread 2
  in thread 3
  in thread 4
  end sub thread ID 22924
  end wait sub thread
  

3. std::thread 对象生命周期、线程等待和分离

• thread_detach.cpp

  #include <iostream>
  #include <thread>using namespace std;bool is_exit = false;void ThreadMain() {cout << "begin sub thread ID " << this_thread::get_id() << endl;for (int i = 0; i < 5; i++) {if (!is_exit)break;cout << "in thread " << i << endl;this_thread::sleep_for(chrono::seconds(1));  // 子线程睡眠（释放）CPU 资源 1000ms}cout << "end sub thread ID " << this_thread::get_id() << endl;
  }int main(int argc, char* argv[]) {{//thread th(ThreadMain); // 出错，thread 对象被销毁 子线程还在运行}{thread th(ThreadMain);th.detach();  // 子线程与主线程分离 守护线程（在后台运行）// 但存在一个问题：主线程退出后 子线程不一定退出}{thread th(ThreadMain);this_thread::sleep_for(chrono::seconds(1));//1000msis_exit = true;  // 通知子线程退出cout << "主线程阻塞，等待子线程退出" << endl;th.join();       // 主线程阻塞，等待子线程退出cout << "子线程已经退出！" << endl;}getchar();return 0;
  }
  

• 控制台输出

  begin sub thread ID 25520
  end sub thread ID 25520
  begin sub thread ID 23324
  end sub thread ID 23324
  主线程阻塞，等待子线程退出
  子线程已经退出！
  

4. 全局函数作为线程入口

• thread_para.cpp

  #include <iostream>
  #include <thread>
  #include <string>using namespace std;class Para {
  public:Para() {cout << "Create Para" << endl;}Para(const Para& p) {  // 拷贝构造函数cout << "Copy Para" << endl; }~Para() {cout << "Drop Para" << endl;}string name;
  };void ThreadMain(int p1, float p2, string str, Para p4) {this_thread::sleep_for(100ms);cout << "ThreadMain " << p1 << " " << p2 << " " << str << " " << p4.name << endl;
  }int main(int argc, char* argv[]) {thread th;{float f1 = 12.1f;Para p;p.name = "test Para class";// 所有的参数做复制th = thread(ThreadMain, 101, f1, "test string para", p);}th.join();return 0;
  }
  

• 控制台输出

  Create Para
  Copy Para
  Drop Para
  Copy Para
  ThreadMain 101 12.1 test string para
  Drop Para
  Drop Para
  

5. 线程函数传递指针和引用

• 参数传递存在的问题

  • 传递空间已经销毁
  • 多线程共享访问一块空间
  • 传递的指针变量的生命周期小于线程

• thread_para.cpp

#include <thread>
#include <iostream>
#include <string>using namespace std;class Para {
public:Para() { cout << "Create Para" << endl; }Para(const Para& p) { cout << "Copy Para" << endl; }~Para() { cout << "Drop Para" << endl; }string name;
};void ThreadMain(int p1, float p2, string str, Para p4) {this_thread::sleep_for(100ms);cout << "ThreadMain " << p1 << " " << p2 << " " << str <<" "<<p4.name<< endl;
}void ThreadMainPtr(Para* p) {this_thread::sleep_for(100ms);cout << "ThreadMainPtr name = " << p->name << endl;
}void ThreadMainRef(Para& p) {this_thread::sleep_for(100ms);cout << "ThreadMainPtr name = " << p.name << endl;
}
int main(int argc, char* argv[]) {{// 传递引用Para p;p.name = "test ref";thread th(ThreadMainRef, ref(p));th.join();}getchar();{// 传递线程指针Para p;p.name = "test ThreadMainPtr name";thread th(ThreadMainPtr, &p);  th.detach();  // 错误，线程访问的 p 空间会提前释放}getchar();  // Para 释放{  // 传递线程指针Para p;p.name = "test ThreadMainPtr name";thread th(ThreadMainPtr, &p);th.join();getchar();}thread th;{float f1 = 12.1f;Para p;p.name = "test Para class";// 所有的参数做复制th =  thread(ThreadMain, 101, f1, "test string para", p);}th.join();return 0;
}

• 控制台输出

  Create Para
  ThreadMainPtr name = test ref
  Drop Para
  

6. 成员函数作为线程入口–封装线程基类接口

• XThread.h

  • 封装线程基类接口

  #pragma once#include <iostream>
  #include <thread>class XThread {
  public:virtual void Start() {is_exit_ = false;th_ = std::thread(&XThread::Main, this);}virtual void Stop() {is_exit_ = true;Wait();}virtual void Wait() {if (th_.joinable())th_.join();}bool is_exit() {return is_exit_;}private:virtual void Main() = 0;  // 纯虚函数必须要在基类中实现std::thread th_;bool is_exit_ = false;    // 符合谷歌代码风格，私有成员变量后缀加 _
  };
  

• thread_class.cpp

  #include <iostream>
  #include <thread>
  #include <string>
  #include "XThread.h"using namespace std;/*
  class MyThread {
  public:// 入口线程函数void Main() {cout << "MyThread Main" << name << ": " << age;}string name;int age = 0;
  };
  */class TestXThread : public XThread {
  public:void Main() override {cout << "TestXThread Main begin" << endl;while (!is_exit()) {this_thread::sleep_for(100ms);cout << "." << flush;  // 添加 flush 是为了确保 . 正常输出}cout << "\nTestXThread Main end" << endl;}string name;
  };int main(int argc, char* argv[]) {TestXThread testth;testth.name = "TestXThread name ";testth.Start();this_thread::sleep_for(3s);testth.Stop();testth.Wait();getchar();/*MyThread myth;myth.name = "Test name";myth.age = 20;thread th(&MyThread::Main, &myth);th.join();*/return 0;
  }
  

• 控制台输出

  TestXThread Main begin
  ............................
  TestXThread Main end
  

7. lambda 临时函数作为线程入口

• lambda 函数基本格式

  • [捕捉列表] (参数) mutable -> 返回值类型 {函数体}

• thread_lambda.cpp

  #include <iostream>
  #include <thread>
  #include <string>using namespace std;class TestLambda {
  public:void Start() {thread th([this]() {cout << "name = " << name << endl; });th.join();}string name = "Test Lambda";
  };int main(int argc, char* argv[]) {thread th([](int i) {cout << "test lambda " << i << endl;}, 123);th.join();TestLambda test;test.Start();return 0;
  }
  

• 控制台输出

  test lambda 123
  name = Test Lambda
  

8. call_once 多线程调用函数只进入一次

初始化函数可以在每一个类型的构造里都调用一遍，不用明确区分，代码可读性提升

• call_once.cpp

  #include <iostream>
  #include <thread>
  #include <string>
  #include <mutex>using namespace std;void SystemInit() {cout << "Call SystemInit" << endl;
  }void CallOnceSystemInit() {static std::once_flag flag;  // 通过 flag 区分是否只调用一次std::call_once(flag, SystemInit);
  }int main(int argc, char* argv[]) {CallOnceSystemInit();CallOnceSystemInit();for (int i = 0; i < 3; i++) {thread th(CallOnceSystemInit);th.detach();}getchar();return 0;
  }
  

• 控制台输出

  Call SystemInit