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<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>

</conbody></concept>