理解C++：C++11 多线程

理解

以前多线程只能用系统的API，无法解决跨平台问题，一套代码进行平台移植，对应多线程也必须要修改。C++11中只需要使用语言层面的thread可以解决这个问题。

参考：包含在#include< thread >

单个cpu内核的多个线程：一个时间片运行一个线程的代码，并不是真正意义的并行计算。

多个cpu或者多个内核：可以做到真正的并行计算

简单体验

下面的测试我在QT上面测试的，不知道为什么不能在网页上面进行

// thread example
#include <iostream>       // std::cout
#include <thread>         // std::thread
using namespace std;
void thread_1()
{
    
    
    cout<<"thread_1"<<endl;
}
void thread_2(int x)
{
    
    
    cout<<"x:"<<x<<endl;
  cout<<"thread_2"<<endl;
}
int main()
{
    
    
  thread first ( thread_1);     // spawn new thread that calls thread_1() 开启线程，并调用thread_1
  thread second (thread_2,100);  // spawn new thread that calls thread_2(100)开启线程，并调用thread_2(100)
  std::cout << "主线程\n";
  // synchronize threads:
  first.join();                // pauses until first finishes 这个操作完了之后才能destroyed
  second.join();               // pauses until second finishes//join完了之后，才能往下执行。
  std::cout << "thread_1&thread_2线程结束.\n";//必须join完成
  return 0;
}

获取线程id

(thread::id & this_thread::get::get_id())

// thread::get_id / this_thread::get_id
#include <iostream>       // std::cout
#include <thread>         // std::thread, std::thread::id, std::this_thread::get_id
#include <chrono>         // std::chrono::seconds
std::thread::id main_thread_id = std::this_thread::get_id();
void is_main_thread() {
    
    
  if ( main_thread_id == std::this_thread::get_id() )
    std::cout << "This is the main thread.\n";
  else
    std::cout << "This is not the main thread.\n";
}
int main()
{
    
    
  is_main_thread();//普通函数调用
  std::thread th (is_main_thread);//子线程调用
   //或者可以用"="进行赋值
//std::thread th;
//th = std::thread(is_main_thread);
  th.join();
}

detach

不需要join，可以直接destroyed

#include <iostream>       // std::cout
#include <thread>         // std::thread, std::this_thread::sleep_for
#include <chrono>         // std::chrono::seconds

void pause_thread(int n)
{
    
    
  std::this_thread::sleep_for (std::chrono::seconds(n));
  std::cout << "pause of " << n << " seconds ended\n";
}

int main()
{
    
    
  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;
}

joinable

可以使用这个参数查看是否可以join

// example for thread::joinable
#include <iostream>       // std::cout
#include <thread>         // std::thread
void mythread()
{
    
    
  // do stuff...
}
int main()
{
    
    
  std::thread foo;
  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;
}

运行结果：

Joinable after construction: 
foo: false 
bar: true 
Joinable after joining: 
foo: false 
bar: false