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POgiven abstract base class X, how to create another template class D<T> where T is the type of the class deriving from X?
 

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  1. POgiven abstract base class X, how to create another template class D<T> where T is the type of the class deriving from X?
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    <p>I want to be able to accept a <code>Message&amp;</code> object which references either a <code>Message1</code> or <code>Message2</code> class. I want to be able to create a <code>MessageWithData&lt;Message1&gt;</code> or <code>MessageWithData&lt;Message2&gt;</code> based on the underlying type of the <code>Message&amp;</code> object. For example, see below:</p> <pre><code>class Message {}; class Message1 : public Message {}; class Message2 : public Message {}; template&lt;typename Message1or2&gt; class MessageWithData : public Message1or2 { public: int x, y; } class Handler() { public: void process(const Message&amp; message, int x, int y) { // create object messageWithData whose type is // either a MessageWithData&lt;Message1&gt; or a MessageWithData&lt;Message2&gt; // based on message's type.. how do I do this? // messageWithData.dispatch(...) } }; </code></pre> <p>The messageWithData class essentially contains methods inherited from Message which allow it to be dynamically double dispatched back to the handler based on its type. My best solution so far has been to keep the data separate from the message type, and pass it all the way through the dynamic dispatch chain, but I was hoping to come closer to the true idiom of dynamic double dispatch wherein the message type contains the variable data.</p> <p>(The method I'm more or less following is from <a href="http://jogear.net/dynamic-double-dispatch-and-templates" rel="nofollow">http://jogear.net/dynamic-double-dispatch-and-templates</a>)</p>
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      1. POgiven abstract base class X, how to create another template class D<T> where T is the type of the class deriving from X?
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      1. POgiven abstract base class X, how to create another template class D<T> where T is the type of the class deriving from X?
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    1. COWhat is the advantage of doing it with a template rather than a virtual dispatch function since both Message1 and Message2 derive from Message?
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      1. POgiven abstract base class X, how to create another template class D<T> where T is the type of the class deriving from X?
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    2. COsorry, it's not evident in the abbreviated code I posted -- `Message1` actually inherits from `Message<Message1>` which inherits from `MessageBase`. `MessageBase` has a pure virtual `dispatch()` method, which `Message<Message1>` implements. In `Message<Message1>::dispatch()`, a `static_cast<const Message1*>(this)` is done in order to dispatch back to the correct function in Handler. Therefore `MessageWithData` must also be templated with `Message1` or `Message2` so dispatch() works correctly.
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      1. POgiven abstract base class X, how to create another template class D<T> where T is the type of the class deriving from X?
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