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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-260C7ACD-0024-59DE-B1C6-0C852E44C457" xml:lang="en"><title>Dynamically
+Loading Link Libraries Overview</title><shortdesc>Description of Polymorphic DLLs.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<section id="GUID-25D3E357-BE6E-40BD-897D-DA5EB92029C5"><title>Purpose</title> <p>Dynamically load libraries during program
+execution.</p> <p>These interfaces are used by programs that need to load
+libraries at run-time, that have a common interface, but different concrete
+behaviour. Because of this relationship between interface and behaviour, such
+libraries are called <i>polymorphic DLLs</i>. </p> </section>
+<section id="GUID-5444CFD3-5FC2-4160-BD8A-8BD7154402B3"><title>Architectural relationships</title> <p>Many parts of the system
+use polymorphic DLLs. Communications components, such as Sockets and Messaging,
+use architectures in which particular communications protocols are provided
+as polymorphic DLLs, and are loaded as needed. UI application programs are
+also polymorphic DLLs that are loaded when the user starts the program.</p> </section>
+<section id="GUID-2A379537-7B05-445F-83FC-C4396794A3CD"><title>Description</title> <p>The API has three key concepts: polymorphic
+DLL, polymorphic DLL handle, and polymorphic DLL function pointer. </p> </section>
+<section id="GUID-0A0DF81E-94C0-4D91-8F2A-1DFBE542E019"><title>Polymorphic DLL</title> <p>A program that uses a type of polymorphic
+DLLs defines their interface in terms of a single abstract class whose functions
+are declared as pure virtual. Each type of polymorphic DLL is given a unique
+identifier (UID). </p> <p>A particular concrete DLL:</p> <ul>
+<li id="GUID-E073605D-1AC4-55B9-94AA-FD1A761B5448"><p>implements the interface
+by defining and implementing a concrete class derived from the abstract class.</p> </li>
+<li id="GUID-AF95C27B-841D-53B5-836B-A55B4ED3E5F3"><p>exports a function that
+creates an object of the derived class type. All other functions in the DLL
+are virtual and called through the base class interface.</p> </li>
+</ul></section>
+<section id="GUID-825B7916-7E13-42B5-BAE4-A7009679E4F2"><title>Polymorphic DLL handle</title><p>The polymorphic DLL handle
+allows a program to load and close a particular polymorphic DLL. It also allows
+the caller to obtain pointers to functions exported by the DLL. The system
+can check that a polymorphic DLL is of the correct type by checking the type
+UID value.</p> <p>The polymorphic DLL handle interface is provided by <xref href="GUID-25327159-83D6-3507-B187-09EA4BB3727F.dita"><apiname>RLibrary</apiname></xref>.</p> <p>A
+wildcard file system search for polymorphic DLLs can be done through <xref href="GUID-1B05E2E3-B64E-36A8-BFFF-430028113BA4.dita"><apiname>TFindLibrary</apiname></xref>.</p> </section>
+<section id="GUID-C16BECEC-B59A-4FFE-94F0-548DB602A0BA"><title>Polymorphic DLL function pointer</title> <p>An exported function
+is called through a pointer to function obtained through a polymorphic DLL
+handle.</p> <p>The polymorphic DLL function pointer type is provided by the <xref href="GUID-7E0A7BA7-0EF8-3832-A1EA-B85FFADD1A9B.dita"><apiname>TLibraryFunction</apiname></xref> typedef.
+The caller casts the pointer to the real type of the exported function before
+using it.</p> </section>
+</conbody><related-links>
+<link>
+<desc><xref href="GUID-AB705D21-87A3-5F32-A4B9-17B7593DD5C3.dita">UID Manipulation
+Overview</xref></desc>
+</link>
+</related-links></concept>
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