[asio] writing composed operations example

From: Stephan Menzel (stephan.menzel_at_[hidden])
Date: 2019-09-02 06:29:50

Hi everyone,

as a long time asio user I often had needs to wrap up repeating patterns of
async calls for re-use. Normally I went for some kind of specialized object
that wraps smaller pieces but I recently came across a series of examples
which try to explain how own composed operations may be created.
https://www.boost.org/doc/libs/1_71_0/doc/html/boost_asio/examples/cpp11_examples.html

Now, looking at the examples they are of course way over my head but since
own composed operations would be very much exactly what I need I thought
I'd give it a shot. But as I now look at this example here:
https://www.boost.org/doc/libs/1_71_0/doc/html/boost_asio/example/cpp11/operations/composed_8.cpp
I notice something that I hope somebody can explain.

In the coroutine implementation it says:

delay_timer_->expires_after(std::chrono::seconds(1));
yield delay_timer_->async_wait(std::move(self));
if (error)
    break;

yield boost::asio::async_write(socket_,
    boost::asio::buffer(*encoded_message_), std::move(self));

I'm puzzled about the double std::move() of self. In the comments above it
says that self is a reference to an intermediate completion handler. How
can this be moved twice?
Can anyone give me hint about what this means?

I would also appreciate any hints in general about own composed operations.
Perhaps there are other approaches that are a bit less intimidating?

Cheers,
Stephan