Symbian3/SDK/Source/GUID-3AD20E0C-F364-533F-9FBC-227478CA9982.dita
author Dominic Pinkman <Dominic.Pinkman@Nokia.com>
Thu, 21 Jan 2010 18:18:20 +0000
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<?xml version="1.0" encoding="utf-8"?>
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<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
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<!-- This component and the accompanying materials are made available under the terms of the License 
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"Eclipse Public License v1.0" which accompanies this distribution, 
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and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
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<!-- Initial Contributors:
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    Nokia Corporation - initial contribution.
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Contributors: 
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-->
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<!DOCTYPE concept
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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="GUID-3AD20E0C-F364-533F-9FBC-227478CA9982" xml:lang="en"><title>How
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to use TRAP</title><shortdesc>Provides code snippets to show you how to use <codeph>TRAP</codeph>.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<section id="GUID-0CE805F2-226D-4A8C-8CDF-51BF63281B10"><title>Execute leaving functions under a trap harness</title> <p>Functions
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that leave, including functions that call other functions that can leave,
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must be executed under a trap harness. </p> <p>If a call to <codeph>User::Leave()</codeph> occurs
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within the function, control will immediately be returned to the most recent <codeph>TRAP</codeph>.
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A variable is used with every trap to receive the error code specified in
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a leave. </p> <p>If no leave occurs, then when the called function ends, execution
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returns to the <codeph>TRAP</codeph>, and the leave variable has the value <codeph>KErrNone</codeph>. </p> <p>Typically
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after a <codeph>TRAP</codeph>, a function checks the leave variable to test
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whether processing returned normally or by leaving, and acts appropriately.
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Special mechanisms discussed later are provided to handle cleanup after the
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exception. </p> <codeblock id="GUID-AE3B0F19-F632-5E4C-A14A-5AD804472BA3" xml:space="preserve">TInt E32Main() 
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 {  
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 testConsole.Title(); // write out title
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 testConsole.Start(_LIT("Example")); // start a new "test"
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 // The leave variable
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 TInt r;
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 // Perform example function. If it leaves,
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 // the leave code is put in r
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 TRAP(r,doExampleL()); 
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 // Test the leave variable
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 if (r) 
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  testConsole.Printf(_LIT("Failed: leave code=%d"), r);
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 testConsole.End(); // finish
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 testConsole.Close(); // close it
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 return KErrNone; // and return
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 }</codeblock> <p><b>Notes</b> </p> <ul>
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<li id="GUID-DD570EB3-15DA-5615-930C-3CD7FD3A65F2"><p>It is not necessary
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that all L functions be <i>directly</i> invoked by a trap harness. In most
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cases, functions that can leave are called normally by other functions. It
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is only necessary that somewhere above the function in the call chain is a
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trap harness. </p> </li>
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<li id="GUID-E7231292-D852-5DF7-830D-7B2B92F8C3D3"><p>It is not recommended
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to call a function with an <codeph>LC</codeph> suffix from inside a trap harness.
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This is because if the function does not leave, the object that the function
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created and pushed onto the cleanup stack will remain on the cleanup stack
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on exiting from the trap harness. This causes a <codeph>E32USER-CBase 71</codeph> panic,
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unless the object is popped by the caller from within the trap harness, for
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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,
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then <codeph>pointer</codeph> is destroyed as part of leave processing. If
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it does not leave then <codeph>CleanupStack::Pop(pointer)</codeph> is called,
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avoiding the panic. </p> </li>
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</ul> </section>
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<section id="GUID-E21C113D-9931-5E85-9ED5-6F14BD28E042"><title>Using TRAPD</title> <p>For
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convenience, there is a <codeph>TRAPD</codeph> form of the macro which defines
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the variable to be used as the leave code. This saves a line of source code
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in the majority of situations. </p> <codeblock id="GUID-EBC2B054-A961-54A5-BBB8-3C194F208C25" xml:space="preserve">TRAPD(leaveCode,SomeFunctionL()); // call a function
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if (leaveCode!=KErrNone)          // check for error leave code
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 {
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 // some cleanup
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 }</codeblock> </section>
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<section id="GUID-BB8DEAE9-D247-5AAE-80AD-A5FB193786BB"><title>Trap harnesses
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and function return values</title> <p>Trap harnesses can be used when the
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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
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if (leaveCode!=KErrNone) // check for error leave code
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 {
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 // some cleanup
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 }
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else { // didn’t leave: value valid
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 }</codeblock> </section>
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</conbody></concept>