Design question: asynchronous API in C++

=?ISO-8859-1?Q?Marcel_M=FCller?= <>
Thu, 30 Dec 2010 20:15:44 +0100
I want to provide an asynchronous C++ API. I.e. placing a request will
not immediately provide a reply. Instead a callback signals the
availability of the result.

The classic solution is to provide an interface that must be implemented
by a class of the caller. This interface contains a method for the
asynchronous callback.

Advantage: the implementing classes could also be used to track the
state of the request. (In this case it is an abstract base class rather
than an interface.)

class AsyncBase
   // Notify availability of result.
   virtual void OnCompleteion() = 0;

   // Cancel an outstanding request.
   // Do not call OnCompletion once it returns.
   void Cancel();

   // Fetch the result data (and wait if necessary).
   // If called from OnCompletion() the returned data must not be used
   // after OnCompletion returned.
   // If /not/ called from OnCompletion the lifetime of Result is
   // extended until this class instance dies.
   const Result& ReceiveResult();

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.

The requirements are:
- 1. The caller places a request, does some other processing and then
waits for the reply.
- 2. The caller places a request and gets notified as soon as the reply
is available.
- 3. The caller places a request with fire and forget.

Case 2 is the most important one. Whether the notification function
already receives the reply as parameters is optional. An additional call
to a method of the tracking object that is returned when placing the
request is also fine.

In fact the returned data could be rather large and a value copy should
be avoided.

Has anyone an idea that comes with less effort for the caller? I would
prefer something with template meta programming and preferably no more
than two code lines for each call.

I think that Lambda expressions would help. However, my compiler does
not support them.


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