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POC++ policy based design.... base class as either sigleton or not based on template parametere.
 

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  1. POC++ policy based design.... base class as either sigleton or not based on template parametere.
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    <p>The OutputPolicy of my class sometimes goes to a static/singleton object and sometime goes to a 1-to-1 object. </p> <p>Not sure if I can explain in english.. so here is the desired behavior in pseudo code:</p> <pre><code>template&lt; class Algo, template&lt;class&gt; OutputPolicy &gt; class AlgoBase : Algo, OutputPolicy&lt;Algo&gt; { }; class AlgoImp { public: AlgoImp(string topic) {} void OnData( { cout &lt;&lt; " AlgoImp::OnData"; if ( .... ) NewMsg(msg); } }; class AlgoImp2 { public: AlgoImp2(string topic) {} void OnData( { cout &lt;&lt; " AlgoImp2::OnData"; if ( .... ) NewMsg(msg); } }; </code></pre> <p>AlgoImp::OnData, does some processing and calls NewMsg. I am trying to decouple the implimentation of the NewMsg() call. </p> <pre><code>template &lt; class T&gt; class OutMessageQueueTcp { void NewMsg(string in) { //Should be a signgleton or static cout &lt;&lt; " OutMessageQueueTcp::NewMsg"; } }; </code></pre> <p>If OutputPolicy is OutMessageQueueTcp (above) then there should only be one instance of OutputPolicy. So I would either have to derive from a Singleton or use a static variable somewhere. </p> <pre><code>template &lt;class T&gt; class OutMessageAsInput : AlgoBase&lt;AlgoImp2, OutMessageQueueTcp&gt; { void NewMsg(string in) { cout &lt;&lt; " OutMessageAsInput::OnData"; AlgoBase&lt;AlgoImp2, OutMessageQueueTcp&gt;::NewMsg(in); } }; </code></pre> <p>If OutputPolicy is OutMessageAsInput (above) then the out becomes input to a sister object and then will be sent to OutMessageQueueTcp. </p> <p>Here is the main.</p> <pre><code>main { AlgoBase&lt;AlgoImp, OutMessageAsInput&gt; myalgo1; myalgo1.OnData("hi"); } </code></pre> <p>And my desired output: </p> <pre><code>AlgoImp::OnData OutMessageAsInput::NewMsg AlgoImp2::OnData OutMessageQueueTcp::NewMsg </code></pre> <p>What I really have is a collection of AlgoImp objects, each with its own topic and data. AlgoImp will generate outout by calling NewMsg. Sometimes these messages should be sent to a Socket, and sometimes they should be processed by AlgoImp2. </p> <p>This decision will happen at compile time. There will be many Algo's each doing diff things. Seems like I am trying to do some kind of Aspect oriented stuff. </p> <p><strong>Question:</strong></p> <p><strong>Is it possible to have a Base class be either a singleton or a normal object by making it a policy? Is there a design pattern for something like this? Maybe some kind of factory?</strong> </p> <p>Thanks for the help. </p>
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