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<concept id="GUID-1BD987D8-B018-51B7-80B2-4E1988841BC2" xml:lang="en"><title>How

to cancel requests</title><shortdesc>This document describes how to cancel a previously scheduled request.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The <codeph>CActive</codeph> class provides the <codeph>Cancel()</codeph> member

function to cancel an active object’s request.</p>

<p>If no request has been issued, <codeph>Cancel()</codeph> does nothing,

otherwise it calls <codeph>DoCancel()</codeph>. This is a pure virtual function

that must be provided by derived classes and which handles specific cancellation

as required by the service provider. <codeph>Cancel()</codeph> then waits

for the completion of that request and sets the active request flag to false.</p>

<p>The service provider must guarantee that the cancellation operation completes

in a very short period of time.</p>

<p>For example, a specialized timer active object, derived from <codeph>CTimer</codeph> which

is derived from <codeph>CActive</codeph>, might implement this as:</p>

<codeblock id="GUID-2454885F-9410-5E9F-A72E-618E78D32500" xml:space="preserve">void CTimedMessenger::DoCancel()

    {

      // Base class

    CTimer::DoCancel(); 

      // Reset variable - needed if the object is later re-activated

    iTicksDone = 0;

    ...

    }</codeblock>

<p>Note that an active object's destructor should ensure that any outstanding

requests are cancelled. If the class implements a DoCancel() function, then

the destructor must call the <codeph>Cancel()</codeph> member function. For

example:</p>

<codeblock id="GUID-1C265422-8C45-5ED2-A2F8-DFF68C08BF9C" xml:space="preserve">CTimedMessenger::~CTimedMessenger()

    {

    Cancel();

    }</codeblock>

</conbody></concept>