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    <p>Something like what you want to achieve can be accomplished with inheritance. That is, the parent class has the variable name you want your template to have.</p> <pre><code>struct ItemNull {}; template &lt;typename X, typename Y = ItemNull&gt; class Item : public X, public Y {}; template &lt;typename T&gt; struct HasPosition { T Position; }; template &lt;typename T&gt; struct HasMomentum { T Momentum; }; Item&lt; HasPosition&lt;float&gt; &gt; myItem1; myItem1.Position = 0.123f; Item&lt; HasPosition&lt;float&gt;, HasMomentum&lt;float&gt; &gt; myItem2; myItem2.Position = 0.1f; myItem2.Momentum = 0.2f; </code></pre> <p>The optional second argument allows for composition, as illustrated in <code>myItem2</code>. To add a third field, you could add to the tail, or expand from the front:</p> <pre><code>template &lt;typename T&gt; struct HasName { T Name; }; Item &lt; HasPosition&lt;float&gt;, Item&lt; HasMomentum&lt;float&gt;, HasName&lt;std::string&gt; &gt; &gt; myItem3; myItem3.Position = 0.1f; myItem3.Momentum = 0.2f; myItem3.Name = "Adam"; Item &lt; Item &lt; HasPosition&lt;float&gt;, HasMomentum&lt;float&gt; &gt;, HasName&lt;std::string&gt; &gt; myItem4; myItem4.Position = 0.1f; myItem4.Momentum = 0.2f; myItem4.Name = "Adam"; </code></pre> <p>My personal preference is for the former of the two methods, because I find it to be a more intuitive way to extend it beyond 3 fields. The <code>Item</code> template syntax could probably be simplified using variadic template arguments.</p> <p>The <code>Has...</code> templates could be machine generated or a macro could be created to make adding new fields a relatively simple task. </p> <pre><code>#define MAKE_HAS(X) template &lt;typename T&gt; struct Has##X { T X; } MAKE_HAS(Position); MAKE_HAS(Momentum); MAKE_HAS(Name); </code></pre>
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