Understanding C++: C++11 multithreading

understanding

In the past, multi-threading could only use the system's API, which could not solve cross-platform problems. A set of code was ported to the platform, and the corresponding multi-threading had to be modified. C++11 only needs to use language-level threads to solve this problem.

Reference: included in #include< thread>

Multiple threads of a single cpu core : Code that runs one thread in a time slice, which is not really parallel computing.

Multiple CPUs or multiple cores : true parallel computing can be achieved

Simple experience

The following test I tested on QT, I don’t know why it can’t be done on the web page

// 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;
}

Get thread 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

No need to join, can be destroyed directly

#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

You can use this parameter to see if you can 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;
}

operation result:

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