--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Symbian3/SDK/Source/GUID-301193C2-666C-5A57-B898-A04EBB63E533.dita	Thu Jan 21 18:18:20 2010 +0000
@@ -0,0 +1,77 @@
+<?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-301193C2-666C-5A57-B898-A04EBB63E533" xml:lang="en"><title>How to
+use a polymorphic interface DLL</title><shortdesc>The example issues a greeting to the person whose name is passed
+in the descriptor <codeph>aName</codeph>.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>The following code can use any implementation of the interface defined
+by the example class, <codeph>CMessenger</codeph>, to issue a simple greeting.</p>
+<p> The name of the DLL, which contains the implementation to be used, is
+passed in the descriptor <codeph>aLibraryName</codeph>.</p>
+<codeblock id="GUID-3BBCFE9F-7D77-563A-AE87-4BBE2B7F195E" xml:space="preserve">void UseDllL(const TFileName&amp; aLibraryName,const TDesC&amp; aName)
+    {
+          // Use RLibrary object to interface to the DLL
+    RLibrary library;
+          // Dynamically load the DLL
+    TUidType uidType(KDynamicLibraryUid,KMessengerUid);
+    User::LeaveIfError(library.Load(aLibraryName,uidType));
+          // Function at ordinal 1 creates new CMessenger
+    TLibraryFunction entry=library.Lookup(1);
+          // Call the function to create new CMessenger
+    CMessenger* messenger=(CMessenger*) entry();
+          // Push pointer to CMessenger onto the cleanup stack
+    CleanupStack::PushL(messenger);
+          // Second-phase constructor for CMessenger
+    messenger-&gt;ConstructL(console, aName);
+          // Use Cmessenger object to issue greeting
+    messenger-&gt;ShowMessage();
+          // Pop CMessenger object off cleanup stack and destroy
+    CleanupStack::PopAndDestroy();
+          // Finished with the DLL
+    library.Close();
+    }</codeblock>
+<section id="GUID-738C30CA-1A75-4038-8BB4-68682D3C539C"><title>Notes:</title> <ul>
+<li id="GUID-01B13D4C-7028-5987-B993-213E6FE9E571"><p>the <codeph>RLibrary</codeph> class
+is the interface to dynamically loaded DLLs. It encapsulates a handle to the
+DLL.</p> </li>
+<li id="GUID-726B2AA5-7674-5DA1-B6FA-EB10F85D7D1F"><p>The DLL has a <codeph>TUidType</codeph> which
+is assigned when the DLL is built. This is ensures that the DLL that is loaded
+complies with the <codeph>CMessenger</codeph> protocol.</p> </li>
+<li id="GUID-F5F2DAE3-62B2-5690-92FF-85AF3F8C987F"><p>If a specific implementation
+of the DLL is to be used, then specify a third UID when declaring the TUidType.
+Each specific implementation of the DLL is uniquely identified by the third
+UID.</p> </li>
+<li id="GUID-C24783C0-CCFA-5AF1-9941-7FB5D0504756"><p>A pointer to the ordinal
+1 function in the DLL is fetched by calling the <codeph>Lookup()</codeph> member
+function of <codeph>RLibrary</codeph> and passing it a parameter value of
+1. The returned function has type <codeph>TLibraryFunction</codeph>; such
+a function takes no parameters and returns a <codeph>TAny*</codeph> type.</p> </li>
+<li id="GUID-C1FB1A45-A02A-5B10-B1EA-AE84982DB6BC"><p>The function pointer
+is assigned to the variable called <codeph>entry</codeph> and the function
+itself is called to construct a <codeph>CMessenger</codeph> object and return
+a pointer to that object. The implementation uses the <codeph>new (ELeave)</codeph> operator
+to allocate the new <codeph>CMessenger</codeph> object and invoke the C++
+default constructor.</p> </li>
+<li id="GUID-1A7CBF75-9059-5AED-ABA9-DF649D613988"><p>As no arguments are
+passed to the function, a second-phase constructor, <codeph>ContructL()</codeph>,
+is called to finish off construction.</p> </li>
+<li id="GUID-B58DFA44-368F-578B-B35D-7DBF1C67A83A"><p>The <codeph>CMessenger</codeph> object
+can then be used like any other C++ object. In this example, the <codeph>ShowMessage()</codeph> function
+is used to emit a greeting in whatever way the implementation chooses. In
+more complex cases, the object could have a long lifetime, and create many
+dependent objects.</p> </li>
+<li id="GUID-67D8B0AA-8425-52C1-851A-D022B7847448"><p>When the <codeph>CMessenger</codeph> object
+is no longer needed, the DLL is unloaded using <codeph>RLibrary::Close()</codeph>.
+Do not call any code in the DLL after the library has been closed. It is important
+to note, therefore, that the library must not be closed until the object constructed
+by the DLL, <i>and all its dependent objects</i>, have been destroyed.</p> </li>
+</ul> </section>
+</conbody></concept>
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