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    10 <!DOCTYPE concept

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

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

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

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

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

    16 Such classes are intended for derivation.</p>

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

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

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

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

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

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

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

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

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

    26 more than one active object.</p>

    27 </conbody></concept>