std::async with wait c++11

std::async

It is an API provided by c++ standard library to execute the task (that is passed as argument) either asynchronously (create separate thread)  or synchronously (normal call to function). It depends upon launch policy (please check this std::async with std::launch).

std::future

It is class, it provides access to outcome of task i.e return values or exception.

There are four member functions of Future Class, which are used to interact with std::async function.

• get() :
  • It is blocking call ( it waits for the function to be finished)
  • return the result (return value or exception of function) if present.
  • It invalidates the state so that second call of get() will give run time error.
  • if deferred policy then get() will also trigger the function call
• wait() :
  • It is blocking call ( it waits for the function to be finished)
  • It don’t touch the state so that we can call wait() multiple time.
  • if deferred policy then wait() will also trigger the function call
• wait_for(duration) :
  • It is blocking call for specific duration(given duration)
  • It don’t touch the state so that we can call wait_for() multiple time.
  • if deferred policy then this will NOT trigger the function call
• wait_until(time point):
  • It is blocking call till given time point
  • It don’t touch the state so that we can call wait_untill() multiple time.
  • if deferred policy then this will NOT trigger the function call

source code and example:

/*
Program: async with wait
Author: Alpha Master
Date:30 Jan 2021
*/

//Header File
#include<iostream>
#include<future>
#include<exception>

//Thread1
int AddThread(int a, int b)
{
    std::cout<<"Add Thread"<<std::endl;
    return a+b;
}

//Thread2
int MulThread(int a, int b)
{
    std::cout<<"Mul Thread"<<std::endl;
    return a*b;
}


int main()
{
    std::cout<<"Main thread to display the data"<<std::endl;

    //Using async to run the AddThread
    std::future<int>result1 = std::async(AddThread, 3, 3);

    //wait is blocking call, it will ONLY wait for function to be finished.
    //if deferref policy then wait will also trigger the function call.
    result1.wait();

    //we can call wait more than one time, second call of get() will
    //give run time error.
    result1.wait();

    //get is blocking call + get the result or exception
    std::cout<<"Add result:"<<result1.get()<<std::endl;

    //We can also use auto instead of future.
    //call asyn with async policy
    auto result2 = std::async(std::launch::async, MulThread, 3, 3);
  
    //wait_for zero, mean continously polling
    if(result2.wait_for(std::chrono::seconds(0)) != std::future_status::deferred)
    {
        while(result2.wait_for(std::chrono::seconds(0)) != std::future_status::ready)
        {
  	    std::cout<<"Waiting"<<std::endl;
        }
        std::cout<<"Ready"<<std::endl;     
    } 

    std::cout<<"Mul Result:"<<result2.get()<<std::endl;
    return 0;
}

Output:

Main thread to display the data
Add Thread
Add result:6
Waiting
Waiting
Mul Thread
Ready
Mul Result:9