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POWhat's the best way to have a variable number of template parameters?
 

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  1. POWhat's the best way to have a variable number of template parameters?
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    <p>Please consider this -probably poorly written- example : </p> <pre><code>class Command; class Command : public boost::enable_shared_from_this&lt;Command&gt; { public : void execute() { executeImpl(); // then do some stuff which is common to all commands ... } // Much more stuff ... private: virtual void executeImpl()=0; // Much more stuff too ... }; </code></pre> <p>and : </p> <pre><code>class CmdAdd : public Command { public: CmdAdd(int howMuchToAdd); void executeImpl(); int _amountToAdd; }; // implementation isn't really important here .... </code></pre> <p>With this, I can simply add a callback using this syntax : </p> <pre><code> boost::shared_ptr&lt;Command&gt; cmdAdd(CmdAdd(someValue)); cmdAdd-&gt;execute(); </code></pre> <p>It works flawlessly. My "Command" class does much more things which are common to all commands, such as implementing undo, redo, progress report and so on, but I removed it from the code for the sake of readability.</p> <p>Now my question is simple : is there a way to rewrite the command class, so that I can replace this call : </p> <pre><code>boost::shared_ptr&lt;Command&gt; cmdAdd(CmdAdd(someValue)); cmdAdd-&gt;execute(); </code></pre> <p>by something like : </p> <pre><code>CmdAdd(someValue); // preferably or CmdAdd-&gt;execute(someValue) </code></pre> <p>I've been thinking about that a lot but I have a conceptual problem : I wanted to template my Command class like </p> <pre><code>template &lt;typename R,typename T1, typename T2, ..., typename Tn&gt; class Command { R1 execute(T1 p1, ...,Tn pn) { return executeImpl(T1 p1, ...,Tn pn); // then do some stuff which is common to all commands ... } } </code></pre> <p>but obviously, there's a problem here : the syntax <code>template &lt;typename R,typename T1, typename T2, ..., typename Tn&gt;</code> isn't legal C++ , AFAIK. </p> <p>Do I have to write n versions of Command, like :</p> <pre><code>template &lt;typename R&gt; class Command template &lt;typename R,typename T1&gt; class Command template &lt;typename R,typename T1, typename T2&gt; class Command ... </code></pre> <p>and so on ? (not even sure this is gonna work indeed) </p> <p>Or is there another, more elegant way to do this ? Is the syntax, mentioned <a href="https://stackoverflow.com/questions/3125950/c-weird-syntax-spotted-in-boost-template-parameters">here</a> of any use there ? (function f;)</p> <p>I've been looking at Loki's type lists and they seem to do the job. But I can't find anything that in Boost. I read on the web that boost::mpl is what one wants to use to implement typelists, but I'm a bit confused by MPL docs ?</p> <p>Any insights on this ? Regads, D. </p>
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