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POWhat's the idiomatic way to traverse a boost::mpl::list?
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  1. POWhat's the idiomatic way to traverse a boost::mpl::list?
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    copied!<p><strong>Edit:</strong> I've edited the sample to better resemble the problem I have, now the function depends on a regular parameter (and not only on template parameters) which means that the computations <strong>can't</strong> be made at compile time. </p> <hr> <p>I wrote some code with a hand written <a href="http://www.ddj.com/cpp/184403813" rel="nofollow noreferrer">typelist</a> and now we've started using <a href="http://www.boost.org" rel="nofollow noreferrer">boost</a> and I'm trying to move it to the <code>mpl</code> library.</p> <p>I can't seem to find any decent documentation for <code>mpl::list</code> and I'm even failing to port the code to <code>boost::mpl</code>. I've got the feeling that even when (if?) I do succeed in porting the code it will still not be idiomatic. Can' you please let me know how the following should be written with <code>boost</code> (note that this is not the actual code, it's a contrived simplification).</p> <p><strong>Original code</strong> <a href="http://codepad.org/0unH4XxA" rel="nofollow noreferrer">(codepad.org paste)</a></p> <pre><code>class nil {}; template &lt;class Head, class Tail = nil&gt; struct type_list { typedef Head head; typedef Tail tail; }; template &lt;class List&gt; struct foo; template &lt;class Head, class Tail&gt; struct foo&lt;type_list&lt;Head, Tail&gt; &gt;{ template &lt;class T&gt; static void* bar(T* obj, size_t size) { if (sizeof(Head) == size) return reinterpret_cast&lt;Head*&gt;(obj); // Otherwise check the rest of the list return foo&lt;Tail&gt;::bar(obj, size); } }; template &lt;&gt; struct foo&lt;nil&gt; { template &lt;class T&gt; static void* bar(T*, size_t) { return NULL; } }; #include &lt;iostream&gt; int main() { int n = 3; void *p = foo&lt;type_list&lt;char, type_list&lt;bool, type_list&lt;double, type_list&lt;long&gt; &gt; &gt; &gt; &gt;::bar(&amp;n, 4); std::cout&lt;&lt; p &lt;&lt; std::endl; } </code></pre> <p><strong>Failed Attempt to use Boost</strong> <a href="http://codepad.org/rXj1Aq2u" rel="nofollow noreferrer">(codepad.org paste)</a></p> <pre><code>#include &lt;boost/mpl/list.hpp&gt; template &lt;class List&gt; struct foo{ template &lt;class T&gt; static void* bar(T* obj, size_t size) { typedef typename boost::mpl::front&lt;List&gt;::type type; if (sizeof(type) == size) return reinterpret_cast&lt;type*&gt;(obj); // Otherwise check the rest of the list return foo&lt;typename List::next&gt;::bar(obj, size); } }; template &lt;&gt; struct foo&lt;boost::mpl::list0&lt;boost::mpl::na&gt; &gt; { template &lt;class T&gt; static void* bar(T*) { return NULL; } }; #include &lt;iostream&gt; int main() { int n = 3; void *p = foo&lt;boost::mpl::list&lt;char, bool, double, long&gt; &gt;::bar(&amp;n, 4); std::cout &lt;&lt; p &lt;&lt; std::endl; } </code></pre>
 

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