<?xml version="1.0" encoding="utf-8"?>

<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->

<!-- This component and the accompanying materials are made available under the terms of the License 

"Eclipse Public License v1.0" which accompanies this distribution, 

and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->

<!-- Initial Contributors:

    Nokia Corporation - initial contribution.

Contributors: 

-->

<!DOCTYPE concept

  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

<concept id="GUID-7809407D-EDC0-5907-B932-0D5726A7ECA2" xml:lang="en"><title>Simple

encapsulation of a service provider</title><shortdesc>This document describes how to use derived active object classes

to encapsulate services.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>Some active object classes do not implement a <codeph>RunL()</codeph> function.

Such classes are intended for derivation.</p>

<p>For example, the general purpose <codeph>CTimer</codeph> class provides

encapsulation of the timer services offered by the <codeph>RTimer</codeph> class,

but provides no <codeph>RunL()</codeph> to handle their completion. To use

the <codeph>CTimer</codeph> class, you must derive from it again and provide

a <codeph>RunL()</codeph> function.</p>

<p>Some asynchronous service providers may provide facilities for more than

one request to be outstanding simultaneously. However, a single active object

can only have one request outstanding. If all the facilities of such an asynchronous

service provider must be used in a program, then it must be encapsulated by

more than one active object.</p>