c++ multithreading code

Simple example of defining work as () operator overload on a struct.

#include <thread>
#include <iostream>

struct work{
 void operator ()() const{
 std::cout << "this is test from work" << std::endl;
 }
};

int main(){
   work w;
   std::thread t([&](){
     w();
   });
   t.join(); // wait in the main thread for finishing the thread t.
   return 0;
}

std::thread objects are not copyable but they are movable.

std::thread t1([]{std::cout << std::this_thread::get_id() << std::endl;});
std::thread t2 = t1; // NOT going to work since constructor 
for thread class is private
std::thread t2 = std::move(t1); // this work!

Detach thread: background/long running jobs which does not to report the status back to parent threads can be detached.

t.detach(); // once detached, can not be joined.

RAII applied to thread class

 

#include <thread>
#include <iostream>

struct RThread{
   RThread(){
     std::thread t([](){std::cout << "Hello from thread " << std::this_thread::get_id() << std::endl;});
     t_= std::move(t);
   }

   // Rule of 5
  ~RThread(){
     if(t_.joinable()){
       t_.join();
    }
   }

  // delete the copy constructor and copy assignment operator
  RThread(RThread const &) = delete;
  RThread& operator=(RThread const &) = delete;

  // use default move operator and move assignment operator
  RThread(RThread const &&) = default;
  RThread & operator=(RThread const && ) = default;

  private:
   std::thread t_;
};

int main(){
 RThread();
 return 0;
}

Threads pool and Atomic shared variable

#include <thread>
#include <iostream>
#include <vector>
#include <atomic>

std::atomic<int> total(0);

int main(){
   std::vector<std::thread> workers;
   for(int i = 0; i <= 20; ++i){
     workers.emplace_back(
       std::thread([](){
         for(int j = 0; j <= 20; ++j){
           // can also use ++total same effect
           // Memory order does not matter on x86
           total.fetch_add(1, std::memory_order_relaxed);
         }
       }));
     }

    std::for_each(
       workers.begin(),
       workers.end(),
       [](auto & w){
         if(w.joinable()){
           w.join();
         }
       }
     );

  std::cout << total << std::endl;
  return 0;
}

Using non-atomic variables with mutexes to prevent data races

#include <mutex>
#include <thread>
#include <iostream>
#include <vector>

static int total = 0;
static int worker_count = 20;
std::mutex mtx;

int main(){
  std::vector<std::thread> workers;
  workers.reserve(worker_count);
  for(int i = 0; i < worker_count; ++i){
    workers.emplace_back(std::thread([](){
    for(int j = 0; j < worker_count; ++j){ 
      std::lock_guard<std::mutex> lkg(mtx);
      ++total;
     }
   })
   );
 }

for(auto & worker: workers){
   if (worker.joinable()){
   worker.join();
  }
 }

std::cout << total << std::endl;
 return 0;
}

DCLP – Double Checked Linked Pattern

As per the paper by Scott Meyers and Andrei Alexandrescu, the DCLP pattern implementation is given below.

In order to get instance of a singleton class, there are two checks, first check without the lock, then the lock is acquired and then again checked if the instance exists.

Singleton* Singleton::instance() { 
if (pInstance == 0) { // 1st test 
    Lock lock; // acquire the lock 
    if (pInstance == 0) { // 2nd test 
      pInstance = new Singleton; 
    } 
  } 
  return pInstance; 
}

In order to simplify this situation, c++ standard came with call_once and once_flag.

std::once_flag pInstance_initialized; // Flag
std::call_once(pInstance_initialized, {pInstance =new Singleton;});

Here is an example code called once as per the once flag:

#include <thread>
#include <iostream>
#include <vector>
#include <chrono>
#include <atomic>

int worker_count = 20;

int main(){
  std::vector<std::thread> workers;
  workers.reserve(worker_count);
  std::once_flag is_called;
  workers.reserve(worker_count);
  for(int i = 0; i < worker_count; ++i){
    workers.emplace_back(std::thread([&](){
      std::call_once(is_called, [](){
      std::cout << "once_flag is set in the thread id " << std::this_thread::get_id() << std::endl;
    });
   })
   );
  }

  for(auto & worker: workers){
   if (worker.joinable()){
     worker.join();
   }
  }

  return 0;
}

Condition Variables: used for intercommunication between the threads. Condition Variables work with mutex as a way of synchronization between threads.

Following is a classic example of printing number series like 0102030405… using 3 threads. One thread prints 0, another prints all odd numbers and 3rd thread prints even numbers. Condition variables are used to solve this problem.

#include <thread>
#include <iostream>
#include <mutex>
#include <condition_variable>

std::condition_variable cv;
std::mutex mtx;
int n = 1;
int max = 100;
bool digit_printed = false;

int main(){
  std::thread t_zero([](){
  while(n < max){
    std::unique_lock<std::mutex> lk(mtx);
    cv.wait(lk, [](){
      return digit_printed == true;
    });
    std::cout << 0<< "\n";
    digit_printed = false;
    cv.notify_all();
   }
 });

std::thread t_odd([](){
  while(n < max){
    std::unique_lock<std::mutex> lk(mtx);
    cv.wait(lk, [](){
      return digit_printed == false && n%2 == 1;
    });
    std::cout << n++ << "\n";
    digit_printed = true;
    cv.notify_all();
   }
 });

std::thread t_even([](){
  while(n < max){
    std::unique_lock<std::mutex> lk(mtx);
    cv.wait(lk, [](){
      return digit_printed == false && n%2 == 0;
    });
    std::cout << n++ << "\n";
    digit_printed = true;
    cv.notify_all();
   }
 });
 
 t_zero.join();
 t_odd.join();
 t_even.join();
 return 0;
}