Packaged Task | Advanced C++ (Multithreading & Multiprocessing)
The std::packaged_task class wraps any Callable objects (function, lambda expression, bind expression, or another function object) so that they can be invoked asynchronously. A packaged_task won’t start on its own, you have to invoke it, As its return value is stored in a shared state that can be called/accessed by std::future objects.
Need of packaged_task
The main advantage of a packaged task is that it can link a callable object to a future and that is very important in a flooding environment. For example, if we have an existing function that fetches the data from Database (DB) and returns it. Now there is a need to execute this function in a separate thread. This can be done using:
std::packaged_task<>
Otherwise, we’ll have to use:
std::promise<>
and have to change code but with the help of std::packaged_task<> its simple and we don’t need to do that.
Member Functions
Some of the member functions in packaged_task are:
- Operator=- it moves packaged tasks and it’s a public member function.
- Swap- It just swaps to the packaged task or you can say exchange two packaged tasks with each other.
- get_future- It returns a std::future associated with the promised result.
- reset- This public member function just resets the task.
- (constructor)- As the name suggests this public member function constructs the packaged task.
- (destructor)- Similarly, (destructor) destructs the task object.
Non-Member Functions
One of the non-member functions is:
- swap(packaged_task)- It specializes the std::swap algorithm.
Below is the C++ program to implement the above functions-
// C++ program to implement
// the functions
#include <bits/stdc++.h>
using namespace std;
// Factorial function
int factorial(int N)
{
int res = 1;
for (int i = N; i > 1; i--) {
res *= i;
}
cout << "Result is = " << res << "\n";
return res;
}
// packaged task
deque<packaged_task<int()> > task_q;
mutex mu;
condition_variable cond;
void thread1()
{
// packaged task
packaged_task<int()> t;
{
unique_lock<mutex> locker(mu);
cond.wait(locker, []() { return !task_q.empty(); });
t = move(task_q.front());
task_q.pop_front();
}
t();
}
// Driver Code
int main()
{
thread t1(thread1);
// Create a packaged_task<> that
// encapsulated the callback i.e. a function
packaged_task<int()> t(bind(factorial, 6));
// Fetch the associated future<>
// from packaged_task<>
future<int> fu = t.get_future();
{
lock_guard<mutex> locker(mu);
task_q.push_back(move(t));
}
cond.notify_one();
// Fetch the result of packaged_task<>
cout << fu.get();
// Join the thread. Its blocking and
// returns when thread is finished.
t1.join();
return 0;
}
Output:
Result is = 720
720
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