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POTemplate Return Types / Cast as function of Template
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  1. POTemplate Return Types / Cast as function of Template
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    copied!<p>I'm working with some generated classes with broken polymorphism. For every generated class T, there are a handful of T_type_info, T_writer, T_reader classes which are only related to T conceptually. </p> <p>What I'm trying to do is something like this:</p> <pre><code>template &lt;class T&gt; class Wrapper { public: template &lt;class W&gt; W topic_cast(BrokenBaseClassWriter* p); // other operations with the same problem ... }; template &lt;&gt; class Wrapper&lt;MyTopic&gt; { public: template &lt;&gt; MyTopicWriter* topic_cast(BrokenBaseClassWriter* p) { ... } }; </code></pre> <p>So that I can do things like:</p> <pre><code>void Write(const Wrapper&lt;T&gt;&amp; topic) { BrokenBaseClassWriter p = not_important; topic.topic_cast(p)-&gt;do_stuff(); } </code></pre> <p>My T classes are generated from an IDL and are concepts that exist in application space. They don't derive from anything. In my example above, W is not really an independent parameter, it's "Something Not T that depends on T". I'm trying to keep all knowledge of T in the app, and all knowledge of T' (without knowing about T) in the backend.</p> <p>The compiler however says my topic_cast function is not a template function - I think because the template occurs in the return type and it wouldn't be distinguishable from any other instantiations. I know that that (templates differ only by return type) is not legal. Only in my case it really would be unique because W is not an independent parameter. But arguing with the compiler is seldom helpful.</p> <p>Can I do this, or is there another way to do this "cast as function of template type"?</p>
 

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