Is there a packaged_task::set_exception equivalent?

Question

My assumption is that packaged_task has a promise underneath. If my task throws an exception, how do I route that to the associated future? With just a promise I could call set_exception – how do I do the same for packaged_task?

Answer 1

An std::packaged_task has an associated std::future object that will hold the exception (or the result of the task). You can retrieve that future by calling the get_future() member function of std::packaged_task.

This means that it is enough to throw an exception inside the function associated with the packaged task in order for that exception to be caught by the task's future (and re-thrown when get() is called on the future object).

For instance:

#include <thread>
#include <future>
#include <iostream>

int main()
{
    std::packaged_task<void()> pt([] () { 
        std::cout << "Hello, "; 
        throw 42; // <== Just throw an exception...
    });

    // Retrieve the associated future...
    auto f = pt.get_future();

    // Start the task (here, in a separate thread)
    std::thread t(std::move(pt));

    try
    {
        // This will throw the exception originally thrown inside the
        // packaged task's function...
        f.get();
    }
    catch (int e)
    {
        // ...and here we have that exception
        std::cout << e;
    }

    t.join();
}