ThreadPool.h

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

using namespace std;

class ThreadPool
{
    int workers;
    queue<function<void()>> tasks;
    vector<thread> threads;
    mutex m;// this is to ensure synchronous or unique access to a shared resource 
    condition_variable cv;// this helps prevent the busy waiting and replace it with the sleep mechanism 
    bool stop; // this one is to indicate that the program is finished so that all threads can be joined and then the program gets terminated after all the threads being closed
    void lookup()
    {
        function<void()> task;
        while (true)
        {
            //in this part we would need to lock and unlock the internally for conditional variable so we need unique lock .. 
            unique_lock<mutex> lock(m);
            {
                cout<<"looking\n";
                cv.wait(lock, [&]
                        { return stop || !tasks.empty(); });

                if (stop && tasks.empty())
                    return;

                task = std::move(tasks.front());
                tasks.pop();
            }
            task();
        }
    }

public:
    ThreadPool()
    {
        this->workers = 1;
        this->stop=false;
    }
    ThreadPool(int workers)
    {
        this->workers = workers;
        this->stop=false;

        for(int i=0;i<workers;i++)
        {
            threads.emplace_back([this] {
                lookup();
            });
        }
    }
    void addToPool(function<void()> task)
    {
        //this lock can't be manually unlocked it would be automatically unlocked once the program goes out of scope 
        //we could have also use the unique_lock here but it would be heavier 
        // unique_lock<mutex> lock(m);
        lock_guard<mutex> lock(m);
        tasks.push(task);
        cv.notify_one();
    }
    ~ThreadPool()
    {
        {
            lock_guard<mutex> lock(m);
            stop=true;
        }
        //comment out the notify_all() to see what happens and try to explain why 
        // cv.notify_all();
        for(auto &t:threads)
        {
            if (t.joinable())
            t.join();
        }
    }
};