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+<?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-3AD20E0C-F364-533F-9FBC-227478CA9982" xml:lang="en"><title>How
+to use TRAP</title><shortdesc>Provides code snippets to show you how to use <codeph>TRAP</codeph>.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<section id="GUID-0CE805F2-226D-4A8C-8CDF-51BF63281B10"><title>Execute leaving functions under a trap harness</title> <p>Functions
+that leave, including functions that call other functions that can leave,
+must be executed under a trap harness. </p> <p>If a call to <codeph>User::Leave()</codeph> occurs
+within the function, control will immediately be returned to the most recent <codeph>TRAP</codeph>.
+A variable is used with every trap to receive the error code specified in
+a leave. </p> <p>If no leave occurs, then when the called function ends, execution
+returns to the <codeph>TRAP</codeph>, and the leave variable has the value <codeph>KErrNone</codeph>. </p> <p>Typically
+after a <codeph>TRAP</codeph>, a function checks the leave variable to test
+whether processing returned normally or by leaving, and acts appropriately.
+Special mechanisms discussed later are provided to handle cleanup after the
+exception. </p> <codeblock id="GUID-AE3B0F19-F632-5E4C-A14A-5AD804472BA3" xml:space="preserve">TInt E32Main() 
+ {  
+ testConsole.Title(); // write out title
+ testConsole.Start(_LIT("Example")); // start a new "test"
+
+ // The leave variable
+ TInt r;
+ // Perform example function. If it leaves,
+ // the leave code is put in r
+ TRAP(r,doExampleL()); 
+ // Test the leave variable
+ if (r) 
+  testConsole.Printf(_LIT("Failed: leave code=%d"), r);
+
+ testConsole.End(); // finish
+ testConsole.Close(); // close it
+ return KErrNone; // and return
+ }</codeblock> <p><b>Notes</b> </p> <ul>
+<li id="GUID-DD570EB3-15DA-5615-930C-3CD7FD3A65F2"><p>It is not necessary
+that all L functions be <i>directly</i> invoked by a trap harness. In most
+cases, functions that can leave are called normally by other functions. It
+is only necessary that somewhere above the function in the call chain is a
+trap harness. </p> </li>
+<li id="GUID-E7231292-D852-5DF7-830D-7B2B92F8C3D3"><p>It is not recommended
+to call a function with an <codeph>LC</codeph> suffix from inside a trap harness.
+This is because if the function does not leave, the object that the function
+created and pushed onto the cleanup stack will remain on the cleanup stack
+on exiting from the trap harness. This causes a <codeph>E32USER-CBase 71</codeph> panic,
+unless the object is popped by the caller from within the trap harness, for
+example: </p> <codeblock id="GUID-933B5F12-B729-5B0F-A021-FF999EFF6A79" xml:space="preserve">TRAPD(error, pointer = SomeClass::SomeFunctionLC(); CleanupStack::Pop(pointer));</codeblock> <p>In this code, if <codeph>SomeClass::SomeFunctionLC()</codeph> leaves,
+then <codeph>pointer</codeph> is destroyed as part of leave processing. If
+it does not leave then <codeph>CleanupStack::Pop(pointer)</codeph> is called,
+avoiding the panic. </p> </li>
+</ul> </section>
+<section id="GUID-E21C113D-9931-5E85-9ED5-6F14BD28E042"><title>Using TRAPD</title> <p>For
+convenience, there is a <codeph>TRAPD</codeph> form of the macro which defines
+the variable to be used as the leave code. This saves a line of source code
+in the majority of situations. </p> <codeblock id="GUID-EBC2B054-A961-54A5-BBB8-3C194F208C25" xml:space="preserve">TRAPD(leaveCode,SomeFunctionL()); // call a function
+if (leaveCode!=KErrNone)          // check for error leave code
+ {
+ // some cleanup
+ }</codeblock> </section>
+<section id="GUID-BB8DEAE9-D247-5AAE-80AD-A5FB193786BB"><title>Trap harnesses
+and function return values</title> <p>Trap harnesses can be used when the
+function being called returns a result. </p> <codeblock id="GUID-3848F9FA-72A1-569D-B603-AB6FB4DA042B" xml:space="preserve">TRAPD(leaveCode,value=GetSomethingL()); // get a value
+if (leaveCode!=KErrNone) // check for error leave code
+ {
+ // some cleanup
+ }
+else { // didn’t leave: value valid
+ }</codeblock> </section>
+</conbody></concept>
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