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  1. PO
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    <p>You could convert classes that create messages (MessageOne, MessageTwo ...) into message factories and register them with top level MessageFactory on initialization. </p> <p>Message factory could hold map of MessageX::Header -> instance of MessageXFactory kind of map.</p> <p>In CreateMessage you would find instance of MessageXFactory based on message header, retrieve the reference to MessageXFactory and then call it's method that would return instance of the actual MessageX.</p> <p>With new messages you no longer have to modify the 'switch', you just need to add an instance of new MessageXFactory to the TopMessageFactory.</p> <p>example:</p> <pre><code>#include &lt;iostream&gt; #include &lt;map&gt; #include &lt;string&gt; using namespace std; struct Message { static const int id = 99; virtual ~Message() {} virtual int msgId() { return id; } }; struct NullMessage : public Message { static const int id = 0; virtual int msgId() { return id; } }; struct MessageOne : public Message { static const int id = 1; virtual int msgId() { return id; } }; struct MessageTwo : public Message { static const int id = 2; virtual int msgId() { return id; } }; struct MessageThree : public Message { static const int id = 3; virtual int msgId() { return id; } }; struct IMessageFactory { virtual ~IMessageFactory() {} virtual Message * createMessage() = 0; }; struct MessageOneFactory : public IMessageFactory { MessageOne * createMessage() { return new MessageOne(); } }; struct MessageTwoFactory : public IMessageFactory { MessageTwo * createMessage() { return new MessageTwo(); } }; struct TopMessageFactory { Message * createMessage(const string&amp; data) { map&lt;string, IMessageFactory*&gt;::iterator it = msgFactories.find(data); if (it == msgFactories.end()) return new NullMessage(); return (*it).second-&gt;createMessage(); } bool registerFactory(const string&amp; msgId, IMessageFactory * factory) { if (!factory) return false; msgFactories[msgId] = factory; return true; } map&lt;string, IMessageFactory*&gt; msgFactories; }; int main() { TopMessageFactory factory; MessageOneFactory * mof = new MessageOneFactory(); MessageTwoFactory * mtf = new MessageTwoFactory(); factory.registerFactory("one", mof); factory.registerFactory("two", mtf); Message * msg = factory.createMessage("two"); cout &lt;&lt; msg-&gt;msgId() &lt;&lt; endl; msg = factory.createMessage("one"); cout &lt;&lt; msg-&gt;msgId() &lt;&lt; endl; } </code></pre>
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    1. POIs it possible to make a factory in C++ that complies with the open/closed principle?
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    1. USstefanB
    1. USstefanB
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    1. COBut that just replaces switching on message type with a map that required registering of each message type... still violates the open/closed principle.
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      1. USNeil Williams
    2. CO@Neil: No it does not violate the open/close. Because you can add more types by loading a new DLL (shared Library). As the DLL load it registers the message constructor with the factory map. Thus you can dynamically expand the application by just dropping a DLL into the application directory.
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      1. USMartin York
    3. CO@Martin: The answer never mentions DLLs, even in its updated form. @stefan: The duplication has now just moved to the factory.registerFactory lines.
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      1. USNeil Williams
 

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