Using std::async in C++

/*

* aysn_test.cpp

*

*/

#include <iostream>

#include <algorithm>

#include <future>

using namespace std;

struct X{

void thread_func(int o){

cout<< "thread func " <<o <<endl;

}

};

int thread_func(int o){

cout<< "thread func " <<o <<endl;

return 100+o;

}

int main(){

X x;

/**

* synchronous call -> next line will not be executed untill

* the thread function is executed. so "thread func 1" will be printed

* first followed by "after ->async(thread_func,1)"

**/

auto a = async(&X::thread_func,&x,1);

cout<< "after ->async(thread_func,1)" <<endl;

/**

* asynchronous call -> next line will be executed mostly even before

* the thread function is executed. so "thread func 2" will be printed

* after "after ->async(std::launch::async,thread_func,2)"

*

* a1.get() will get the returned value of the thread, if the thread

* function has not been scheduled it will wait until the thread is

* scheduled and thevalue is returned

**/

auto a1 = async(std::launch::async,thread_func,2);

cout<< "after ->async(std::launch::async,thread_func,2)" <<endl;

std::cout << a1.get() << '\n';

/**

* asynchronous call -> next line will be executed mostly even before

* the thread function is executed. so "thread func 3" will be printed

* only after "after ->async(std::launch::deferred,thread_func,3"

*

* thread function will be scheduled only after a2.wait() or a2.get()

* is called. so to schedule and get the return value wait() or get()

* has to be called otherwise thread will never be executed(thats why

* its lazy evaluation)

**/

auto a2 = async(std::launch::deferred,thread_func,3);

cout<< "after ->async(std::launch::deferred,thread_func,3)" <<endl;

//a2.wait();

//std::cout << a2.get() << '\n';;

}

compile command in linux

"g++ aysn_test.cpp -lpthread"

Output::

after ->async(thread_func,1)

thread func 1

after ->async(std::launch::async,thread_func,2)

thread func 2

102

after ->async(std::launch::deferred,thread_func,3)

thread func 2 ----a2.wait() or std::cout << a2.get() << '\n';;

103 ----std::cout << a2.get() << '\n';;