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    <p>Here you go:</p> <p><a href="http://ideone.com/AdEfl" rel="nofollow">http://ideone.com/AdEfl</a></p> <p>And the code:</p> <pre><code>#include &lt;iostream&gt; template &lt;int I&gt; struct Traits { struct inner{}; }; template &lt;&gt; struct Traits&lt;1&gt; { struct inner{ template&lt;class T1&gt; struct impl{ impl() { std::cout &lt;&lt; "impl&lt;T1&gt;" &lt;&lt; std::endl; } }; }; }; template &lt;&gt; struct Traits&lt;2&gt; { struct inner{ template&lt;class T1, class T2&gt; struct impl{ impl() { std::cout &lt;&lt; "impl&lt;T1, T2&gt;" &lt;&lt; std::endl; } }; }; }; template&lt;class T&gt; struct Test{}; template&lt;class T, class K&gt; struct Foo{}; template&lt;int I&gt; struct arg{}; template&lt; template&lt;class, class&gt; class T, class P1, int I &gt; struct Test&lt; T&lt;P1, arg&lt;I&gt; &gt; &gt;{ typedef typename Traits&lt;I&gt;::inner inner; }; template&lt; template&lt;class, class&gt; class T, class P2, int I &gt; struct Test&lt; T&lt;arg&lt;I&gt;, P2 &gt; &gt;{ typedef typename Traits&lt;I&gt;::inner inner; }; // and a bunch of other partial specializations int main(){ typename Test&lt;Foo&lt;int, arg&lt;1&gt; &gt; &gt;::inner::impl&lt;int&gt; b; typename Test&lt;Foo&lt;int, arg&lt;2&gt; &gt; &gt;::inner::impl&lt;int, double&gt; c; } </code></pre> <p>Explanation: Basically it's an extension of the idea of partial specialization, however the difference is that rather than specializing within <code>Test</code>, delegate to a specific class that can be specialized on <code>I</code> alone. That way you only need to define versions of <code>inner</code> for each <code>I</code> <em>once</em>. Then multiple specializations of <code>Test</code> can re-use. The <code>inner</code> holder is used to make the <code>typedef</code> in the <code>Test</code> class easier to handle. </p> <p>EDIT: here is a test case that shows what happens if you pass in the wrong number of template arguments: <a href="http://ideone.com/QzgNP" rel="nofollow">http://ideone.com/QzgNP</a></p>
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    1. POConditional compile-time inclusion/exclusion of code based on template argument(s)?
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    1. COI have posted the same simplified code, but questioner is saying that template will not be as simple as `template<int I>`.
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    2. CO@iammilind, I said it's an extension, the code is slightly different, in the above, the specialization of `I` is delegated to the `Traits` class, and the `Test` class uses that, in your answer, you were talking about specializing `Test` (or `X`) for each `I` - which the OP is not willing to do.
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    3. COyou are correct. I am sorry, I missed that. Thanks for poining.
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