In our previous discussion, we explored a task-based while loop employing custom callables that passed copies of themselves to the next iteration, which follows the continuation-passing style used in the Parallel Patterns Library (PPL). In this session, we will implement the same function using a more traditional recursive approach, aiming to simplify the sharing of state between lambda callables in PPL-style programming.

On Writing Loops in PPL and Continuation-passing Style, Part 2

By Raymond Chen

From the article:

Last time, we came up with task-based while loop that involved creating a custom callable that passed copies of itself to the next iteration.

This time, we’ll implement the function in terms of a more traditional recursion.

template<typename Callable>
task<void> do_while_task(
    std::shared_ptr<Callable> const& f)
{
    return (*f)().then([f](bool loop) {
        return loop ? do_while_task(f) :
                      task_from_result();
    });
}

template<typename Callable, typename =
    std::enable_if_t<std::is_invocable_v<Callable>>>
task<void> do_while_task(Callable&& callable)
{
    using Decayed = std::decay_t<Callable>;
    return do_while_task(
        std::make_shared<Decayed>(
            std::forward<Callable>(callable)));
}

The real work happens in the first overload, which takes a ready-made shared_ptr. The second overload is a convenience method that lets you pass a callable, and it will wrap it in a shared_ptr for you.