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POVariadic variadic template templates
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  1. POVariadic variadic template templates
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    copied!<p>I'm currently struggling with the following code, the intent of which is to implement variadic variadic template templates:</p> <pre><code>template &lt; template &lt;typename... HeadArgs&gt; class Head, template &lt;typename... TailArgs&gt; class... &gt; struct join&lt;Head&lt;typename HeadArgs...&gt;, Head&lt;typename TailArgs...&gt;...&gt; { typedef Head&lt;typename HeadArgs..., typename TailArgs......&gt; result; }; </code></pre> <p>Ideally, I'd be able to use this template metafunction to achieve the following:</p> <pre><code>template &lt;typename...&gt; struct obj1 {}; template &lt;typename...&gt; struct obj2 {}; typedef join &lt; obj1&lt;int, int, double&gt;, obj1&lt;double, char&gt;, obj1&lt;char*, int, double, const char*&gt; &gt;::result new_obj1; typedef join &lt; obj2&lt;int, int, double&gt;, obj2&lt;double, char&gt;, obj2&lt;char*, int, double, const char*&gt; &gt;::result new_obj2; /* This should result in an error, because there are different encapsulating objects typedef join &lt; obj1&lt;int, int, double&gt;, obj1&lt;double, char&gt;, obj2&lt;char*, int, double, const char*&gt; &gt;::result new_obj; */ </code></pre> <p>The output of the above would hopefully create <code>new_obj1</code> and <code>new_obj2</code> in the form <code>template&lt;int, int, double, double, char, char*, int, double, const char*&gt; struct new_obj[1|2] {};</code></p> <p>I'm using gcc 4.6.2 on Windows, which outputs an "expected parameter pack before '...'" for the expansion of "<strong><code>Head&lt;typename TailArgs...&gt;</code></strong>...".</p> <p><a href="http://ideone.com/Km2ra">This error is reproducable with gcc 4.5.1</a>. </p>
 

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