std::async | Threading Cpp11

NOTES:

1. It runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result.

2. There are three launch policies for creating task:

     a. std::launch::async

     b. std::launch::deffered

     c. std::launch::async | std::launch::async

HOW IT WORKS:

1. It automatically creates a thread (Or picks from internal thread pool) and a promise object for us.

2. Then passes the std::promise object to thread function and returns the associated std::future object.

3. When our passed argument function exits then its value will be set in this promise object, so eventually return value will be available in std::future object.

SIDE NOTES:

1. We can send functor and lambda function as callback to std::async, it will work the same.

PROGRAM:

#include <iostream>

#include <thread>

#include <chrono>

#include <algorithm>

#include <future>

using namespace std;

using namespace std::chrono;

typedef long int  ull;

ull findOdd(ull start, ull end) {

    ull OddSum = 0;

    cout << "ThreadID of findOdd" << std::this_thread::get_id() << endl;

    for (ull i = start; i <= end; ++i) { if (i & 1) { OddSum += i; } }

    return OddSum;

}

int main() {

    

    ull start = 0, end = 1900000000;

    cout << "ThreadID" << std::this_thread::get_id() << endl;

    cout << "Thread created if policy is std::launch::async!!" << endl;

    std::future<ull> OddSum = std::async(std::launch::deferred, findOdd, start, end);

    

    cout << "Waiting For Result!!" << endl;

    cout << "OddSum : " << OddSum.get() << endl;

    cout << "Completed!!" << endl;

    return 0;

}