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POPolicy-based template design: How to access certain policies of the class?
 

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    <p>I have a class that uses several policies that are templated. It is called <code>Dish</code> in the following example. I store many of these <code>Dish</code>es in a <code>vector</code> (using a pointer to simple base class), but then I'd like to extract and use them. But I don't know their exact types. </p> <p>Here is the code; it's a bit long, but really simple:</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; struct DishBase { int id; DishBase(int i) : id(i) {} }; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; out, const DishBase&amp; d) { out &lt;&lt; d.id; return out; } // Policy-based class: template&lt;class Appetizer, class Main, class Dessert&gt; class Dish : public DishBase { Appetizer appetizer_; Main main_; Dessert dessert_; public: Dish(int id) : DishBase(id) {} const Appetizer&amp; get_appetizer() { return appetizer_; } const Main&amp; get_main() { return main_; } const Dessert&amp; get_dessert() { return dessert_; } }; struct Storage { typedef DishBase* value_type; typedef std::vector&lt;value_type&gt; Container; typedef Container::const_iterator const_iterator; Container container; Storage() { container.push_back(new Dish&lt;int,double,float&gt;(0)); container.push_back(new Dish&lt;double,int,double&gt;(1)); container.push_back(new Dish&lt;int,int,int&gt;(2)); } ~Storage() { // delete objects } const_iterator begin() { return container.begin(); } const_iterator end() { return container.end(); } }; int main() { Storage s; for(Storage::const_iterator it = s.begin(); it != s.end(); ++it){ std::cout &lt;&lt; **it &lt;&lt; std::endl; std::cout &lt;&lt; "Dessert: " &lt;&lt; *it-&gt;get_dessert() &lt;&lt; std::endl; // ?? } return 0; } </code></pre> <p>The tricky part is here, in the <code>main()</code> function:</p> <pre><code> std::cout &lt;&lt; "Dessert: " &lt;&lt; *it-&gt;get_dessert() &lt;&lt; std::endl; // ?? </code></pre> <p>How can I access the dessert? I don't even know the Dessert type (it is templated), let alone the complete type of the object that I'm getting from the storage.</p> <p>This is just a toy example, but I think my code reduces to this. I'd just like to pass those <code>Dish</code> classes around, and different parts of the code will access different parts of it (in the example: its appetizer, main dish, or dessert).</p>
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