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PODesign pattern to avoid downcasting in message passing
 

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    <p>Base class <code>MessageHandler</code> has derived classes. They would like to pass messages to each other. Messages could be of different classes, but can be made to share a base class. How can each <code>MessageHandler</code> avoid downcasting a received message? Is it somehow possible to do something that has the effect of template-parametrizing the virtual <code>receiveMessage</code> function on MessageHandler?</p> <p>Essentially, I'm trying to replace the following code with something that does not downcast, and is hopefully a compile-time thing:</p> <pre><code>// ... virtual void MessageHandler::receiveMessage(Message &amp;msg) = 0; // ... // to receive a message void DerivedMessageHandler::receiveMessage(Message&amp; msg) { switch (msg.MsgType()) // enum { case Message::MessageType::A: MessageA&amp; = dynamic_cast&lt;MessageA&amp;&gt;(msg); break; case Message::MessageType::B: MessageB&amp; = dynamic_cast&lt;MessageB&amp;&gt;(msg); break; default: // don't process unknown messages break; } } // to send a message list&lt;MessageHandler&gt; mhList; // populate list for (MessageHandler&amp; mh : mhList) { mh.receiveMessage(msg); } </code></pre> <p>I know I can't do this, but something like</p> <pre><code>template &lt;typename M&gt; void MessageHandler::receiveMessage(M&amp; msg) {} </code></pre> <p>And have each <code>DerivedMessageHandler</code> specialize on <code>M</code>? What would be a design pattern that cleanly lets each handler work on their supported message objects?</p>
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