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POPartial Specialization of tuple contents with variadic arguments
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  1. POPartial Specialization of tuple contents with variadic arguments
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    copied!<p>Currently, I'm trying to get some code to react differently to different types. This isn't the exact code, but it gets the message across.</p> <pre><code>template&lt;class A, class B&gt; struct alpha { enum { value = 0 }; }; template&lt;class T, class... Args&gt; struct alpha&lt;std::tuple&lt;Args...&gt;, T&gt; { enum { value = 1 }; }; // This gets ignored template&lt;class T, class... Args&gt; struct alpha&lt;std::tuple&lt;Args..., std::vector&lt;T&gt; &gt;, T&gt; { enum { value = 2 }; }; // This gets ignored template&lt;class T, class... Args&gt; struct alpha&lt;std::tuple&lt;Args..., T&gt;, T&gt; { enum { value = 3 }; }; template&lt;class T, class... Args&gt; struct alpha&lt;T, std::tuple&lt;Args...&gt; &gt; { enum { value = 4 }; }; template&lt;class... LArgs, class... RArgs&gt; struct alpha&lt;std::tuple&lt;LArgs...&gt;, std::tuple&lt;RArgs...&gt; &gt; { enum { value = 5 }; }; int main(int argc, char* argv[]) { std::cout &lt;&lt; alpha&lt;std::tuple&lt;int, double&gt;, double&gt;::value &lt;&lt; std::endl; // prints 1 return 0; } </code></pre> <p>I've tried more than this code shows, but nothing works so far and I ran across a problem with explicit specialization in a non-namespace scope. For reference, I'm working on gcc 4.6 (the one that comes with oneiric server), which I believe has complete variadic template support. I don't care how ugly it gets if the implementation works to detect the last argument of the parameter pack and the other types as well. Any suggestions?</p> <p>EDIT: I wanted to share the solution I used based on the answers (this is an example).</p> <pre><code>template&lt;typename T&gt; struct tuple_last; template&lt;typename T, typename U, typename... Args&gt; struct tuple_last&lt;std::tuple&lt;T,U,Args...&gt;&gt; { typedef typename tuple_last&lt;std::tuple&lt;U,Args...&gt;&gt;::type type; }; template&lt;typename T&gt; struct tuple_last&lt;std::tuple&lt;T&gt;&gt; { typedef T type; }; namespace details { // default case: template&lt;class T, class U&gt; struct alpha_impl { enum { value = 1 }; }; template&lt;class T&gt; struct alpha_impl&lt;T, T&gt; { enum { value = 101 }; }; template&lt;class T&gt; struct alpha_impl&lt;T, std::vector&lt;T&gt;&gt; { enum { value = 102 }; }; // and so on. } template&lt;class T, class... Args&gt; struct alpha&lt;std::tuple&lt;Args...&gt;, T&gt; : details::alpha_impl&lt;T, tuple_last&lt;std::tuple&lt;Args...&gt;&gt;; </code></pre>
 

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