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POboost fusion and const correctness
 

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    <p>I am learning boost fusion and am trying to take a view of a <code>std::vector&lt;boost::fusion::vector&lt;int,double,double&gt; &gt;</code>. The code appears simple but I appear to be running into some problems with const. I clearly misunderstand something about const and would love for someone to explain where I am going wrong.</p> <p>The code:</p> <pre><code>template&lt;int N, class T&gt; struct viewTraits{ typedef typename T::value_type etype; typedef typename boost::fusion::result_of::as_nview&lt;etype, N&gt;::type netype; typedef std::vector&lt;netype&gt; result_type; }; template &lt;int N, typename T&gt; typename viewTraits&lt;N,T&gt;::result_type c( T const &amp;t ) { typename viewTraits&lt;N, T&gt;::result_type retVal; for (typename T::const_iterator it(t.begin());it&lt;t.end();++it){ retVal.push_back(fusion::as_nview&lt;N&gt;(*it)); } return retVal; } template &lt;typename Container&gt; typename Container::value_type sum( Container const &amp;container ) { typedef typename Container::value_type value_type; return std::accumulate( container.begin(), container.end(), value_type() ); } int main(){ typedef fusion::vector&lt;int, double, double&gt; row; typedef std::vector&lt;row&gt; container; container x; b( x, 200 ); std::cout &lt;&lt; sum(c&lt;1&gt;(x)) &lt;&lt; std::endl; } </code></pre> <p>The code fails to compile at <code>retVal.push_back()</code> because of an issue with casting const. I tried several permutations of removing and adding some const keywords but have not been successful at programming by permutation and would rather understand what I am doing.</p> <p>Anyone have any thoughts? BTW the <code>const</code> in the function definition of <code>sum</code> and <code>c</code> must stay.</p> <p>EDIT: I forgot to mention that <code>b</code> fills <code>x</code> which is a <code>std::vector&lt;fusion::vector&lt;int, double, double&gt; &gt;</code></p> <p>EDIT2: The corrected code:</p> <pre><code>template&lt;int N, class T&gt; struct viewTraits{ typedef typename T::value_type etype; typedef typename fusion::result_of::as_nview&lt;etype, N&gt;::type netype; typedef typename fusion::result_of::at_c&lt;netype,0&gt;::type reference; typedef typename boost::decay&lt;reference&gt;::type value_type; typedef std::vector&lt;value_type&gt; result_type; }; template &lt;int N, typename T&gt; typename viewTraits&lt;N,T&gt;::result_type c( T const &amp;t ) { typename viewTraits&lt;N,T&gt;::result_type retVal; for(typename T::const_iterator it(t.begin()); it&lt;t.end();++it){ retVal.push_back(fusion::deref(fusion::begin(fusion::as_nview&lt;N&gt;(*it)))); } return retVal; } </code></pre>
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