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POPermutations of a List of Types Using boost::mpl
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  1. POPermutations of a List of Types Using boost::mpl
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    copied!<p>I am trying to create a list containing the permutations of a given type list. </p> <p>The below code seems to function, though without the intended result, when I use a specified list instead of generating a new list by removing from the actual input. This is demonstrated by the difference between permutation_helper and broken_helper below.</p> <p>Does anyone know why <code>mpl::remove</code> doesn't seem to function as expected in this circumstance?</p> <pre><code>#include &lt;boost/mpl/list.hpp&gt; #include &lt;boost/mpl/transform.hpp&gt; #include &lt;boost/mpl/fold.hpp&gt; #include &lt;boost/mpl/push_front.hpp&gt; #include &lt;boost/mpl/joint_view.hpp&gt; #include &lt;boost/mpl/remove.hpp&gt; #include &lt;boost/mpl/assert.hpp&gt; #include &lt;boost/mpl/equal.hpp&gt; namespace mpl = boost::mpl; struct test_type1 {}; struct test_type2 {}; struct test_type3 {}; template&lt; typename T &gt; struct permutations; template &lt;typename value&gt; struct permutations&lt;mpl::list1&lt; value &gt; &gt;: mpl::list1&lt;mpl::list1&lt; value &gt; &gt; {}; template&lt; typename value, typename T&gt; struct permutation_helper: mpl::transform&lt; typename permutations&lt; mpl::list1&lt;test_type3&gt; &gt;::type, mpl::push_front&lt; mpl::_1, value&gt; &gt; { }; template&lt; typename value, typename T&gt; struct broken_helper: mpl::transform&lt; typename permutations&lt; mpl::remove&lt;T, value&gt; &gt;::type, mpl::push_front&lt; mpl::_1, value&gt; &gt; { }; template&lt; typename T &gt; struct permutations: mpl::fold&lt; T, mpl::list0&lt;&gt;, mpl::joint_view&lt; mpl::_1, broken_helper&lt;mpl::_2, T &gt; &gt; &gt; { }; typedef mpl::list2&lt;test_type1, test_type2&gt; typelist; typedef permutations&lt;typelist&gt;::type perms; int main() { BOOST_MPL_ASSERT(( mpl::equal&lt; perms, typelist &gt; )); return 0; } </code></pre> <p>I used the assert to determine what is being returned from the function, typelist is not the expected result. This is the message the assert returns for broken_helper:</p> <pre><code>testcase.cpp: In function ‘int main()’: testcase.cpp:45: error: no matching function for call to ‘assertion_failed(mpl_::failed************ boost::mpl::equal&lt;boost::mpl::joint_view&lt;boost::mpl::joint_view&lt;boost::mpl::list0&lt;mpl_::na&gt;, boost::mpl::l_end&gt;, boost::mpl::l_end&gt;, boost::mpl::list2&lt;test_type1, test_type2&gt;, boost::is_same&lt;mpl_::arg&lt;-0x00000000000000001&gt;, mpl_::arg&lt;-0x00000000000000001&gt; &gt; &gt;::************)’ </code></pre> <p>The output using permutation_helper is an actual list:</p> <pre><code>testcase.cpp: In function ‘int main()’: testcase.cpp:45: error: no matching function for call to ‘assertion_failed(mpl_::failed************ boost::mpl::equal&lt;boost::mpl::list2&lt;test_type1, test_type2&gt;, boost::mpl::joint_view&lt;boost::mpl::joint_view&lt;boost::mpl::list0&lt;mpl_::na&gt;, boost::mpl::l_item&lt;mpl_::long_&lt;1l&gt;, boost::mpl::l_item&lt;mpl_::long_&lt;2l&gt;, test_type1, boost::mpl::list1&lt;test_type3&gt; &gt;, boost::mpl::l_end&gt; &gt;, boost::mpl::l_item&lt;mpl_::long_&lt;1l&gt;, boost::mpl::l_item&lt;mpl_::long_&lt;2l&gt;, test_type2, boost::mpl::list1&lt;test_type3&gt; &gt;, boost::mpl::l_end&gt; &gt;, boost::is_same&lt;mpl_::arg&lt;-0x00000000000000001&gt;, mpl_::arg&lt;-0x00000000000000001&gt; &gt; &gt;::************)’ </code></pre>
 

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