1 <?xml version="1.0" encoding="utf-8"?>

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     7     Nokia Corporation - initial contribution.

     8 Contributors: 

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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-6D6DFC3A-8940-531A-A319-922317D19B51" xml:lang="en"><title>How to

    13 define the interface to a polymorphic interface DLL</title><shortdesc>Explains how to use a polymorphic interface DLL.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>The interface is defined by an abstract API which can be implemented by

    15 a DLL. The following code fragment defines the API in terms of a pure virtual

    16 abstract C++ class; this is an example class called <codeph>CMessenger</codeph>.</p>

    17 <codeblock id="GUID-68199E32-6801-5964-9B3E-56DAB3369E41" xml:space="preserve">class CMessenger : public CBase

    18     {

    19 public:

    20     virtual void ConstructL(CConsoleBase* aConsole, const TDesC& aName)=0;

    21     virtual void ShowMessage()=0;

    22 private:

    23     CConsoleBase* iConsole;

    24     HBufC*        iName;

    25     };</codeblock>

    26 <codeblock id="GUID-039AF89C-AF41-5F69-AA47-8A0D66DBAAB7" xml:space="preserve">

    27 // The UID for Messenger DLLs.

    28 // The client imposes this on DLLs which satisfy the protocol.

    29 

    30 const TInt KMessengerUidValue=0x10004262;

    31 const TUid KMessengerUid={KMessengerUidValue};</codeblock>

    32 <p>The purpose of the example is to be able to issue a simple greeting message.

    33 Any number of DLLs can be created, each containing a different implementation

    34 of the class.</p>

    35 <section id="GUID-6C3A1A07-6140-4A41-9E83-8348CDC81222"><title>Notes:</title> <p>Because the API can be implemented differently

    36 in different DLLs, note the following when declaring the class:</p> <ul>

    37 <li id="GUID-2D946F12-48A1-57B0-ADE0-7BF143DE6A8A"><p>The constructor is not

    38 declared; the default C++ constructor is used, and is not exported from the

    39 DLL. Instead, a function is exported from the DLL which constructs an object

    40 of the concrete type using <codeph>new (ELeave)</codeph> semantics.</p> </li>

    41 <li id="GUID-302202A4-DB5A-5A75-B036-9E664A915809"><p>All functions are pure

    42 virtual; a DLL <i>must</i> provide an implementation for all such functions.</p> </li>

    43 <li id="GUID-61D75525-ACA5-5419-AB64-323A5BB0EAFD"><p>All functions are <codeph>public</codeph> since

    44 the published API is all there to be used, there is no need for <codeph>private</codeph> specifications.</p> </li>

    45 <li id="GUID-A8CF8455-8860-5C64-BA75-FD3880DD8619"><p>It is possible to use

    46 polymorphic interface DLLs in a more advanced way that allows some of these

    47 rules to be changed.</p> </li>

    48 </ul> </section>

    49 </conbody></concept>