Re: Design question: asynchronous API in C++
Adam Skutt wrote:
None of the existing libraries that provide this sort of functionality are adequate? ACE, boost.asio, several others?
The question is more how much work is it to port them to all the target
platforms. And the second question is how bulky they are.
Unfortunately this is very bulky, because the caller has to implement
one class for each different call. So I would like to have a more
convenient API.
No, they don't. For example, write an implementation of that interface that takes a function as a parameter and calls that function when the result is available. ACE uses this technique in a few places, as I recall.
They only have to keep writing classes if they constantly need different state. Even that can be resolved to a large degree by templates (reducing the problem to the amount of stored state) and so on.
Or, do the sensible thing and not mandate such a complicated interface just to get an asynchronous result.
The interface is already there. It is a C API (pulseaudio port to OS/2).
But it requires a callback function to be passed and returns a request
object pointer. However these two things are not related directly which
is very error prone. E.g. one cannot call a wait function on the request
object pointer, because this object does not receive the callback. On
the other side the request object is needed to cancel an outstanding
request and prevent the callback from being called anymore.
I would like to wrap that by a C++ implementation. This should consist
of a single request object which deals with the callback as well as the
request object in a single place. Unfortunately the callback information
is input from the APIs point of view, while the request object is output.
In fact a single request has to join an object to query, the request
itself, and a request handler in one statement of code.
Look at what boost::asio does, for example.
I will do so.
Fundamentally, a callback is just a function and your API should reflect that. If they need to store state with their function, let the API user create a functor (or use boost::bind, or whatever).
I think that Lambda expressions would help. However, my compiler does
not support them.
So use boost::lambda? Yes, they certainly can help, but without details...
Hmm, I have to check whether boost:lambda compiles on my platform. I did
already use other parts of boost, but this one is untested.
Marcel