std packaged task and std future

std::packaged_task bundles a function and the associated promise for its return type:

template<typename F>
auto async_deferred(F&& func) -> std::future<decltype(func())>
{
    auto task   = std::packaged_task<decltype(func())()>(std::forward<F>(func));
    auto future = task.get_future();

    std::thread(std::move(task)).detach();

    return std::move(future);
}

The thread starts running immediately. We can either detach it, or have join it at the end of the scope. When the function call to std::thread finishes, the result is ready.

Note that this is slightly different from std::async where the returned std::future when destructed will actually block until the thread is finished.