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POUsing a boost::fusion::map in boost::spirit::karma
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  1. POUsing a boost::fusion::map in boost::spirit::karma
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    copied!<p>I am using boost spirit to parse some text files into a data structure and now I am beginning to generate text from this data structure (using spirit karma).</p> <p>One attempt at a data structure is a boost::fusion::map (as suggested in an answer to <a href="https://stackoverflow.com/questions/409827/boosttuples-vs-structs-for-return-values">this</a> question). But although I can use boost::spirit::qi::parse() and get data in it easily, when I tried to generate text from it using karma, I failed.</p> <p>Below is my attempt (look especially at the "map_data" type). After some reading and playing around with other fusion types, I found boost::fusion::vector and BOOST_FUSION_DEFINE_ASSOC_STRUCT. I succeeded to generate output with both of them, but they don't seem ideal: in vector you cannot access a member using a name (it is like a tuple) -- and in the other solution, I don't think I need both ways (member name and key type) to access the members. </p> <pre><code>#include &lt;iostream&gt; #include &lt;string&gt; #include &lt;boost/spirit/include/karma.hpp&gt; #include &lt;boost/fusion/include/map.hpp&gt; #include &lt;boost/fusion/include/make_map.hpp&gt; #include &lt;boost/fusion/include/vector.hpp&gt; #include &lt;boost/fusion/include/as_vector.hpp&gt; #include &lt;boost/fusion/include/transform.hpp&gt; struct sb_key; struct id_key; using boost::fusion::pair; typedef boost::fusion::map &lt; pair&lt;sb_key, int&gt; , pair&lt;id_key, unsigned long&gt; &gt; map_data; typedef boost::fusion::vector &lt; int, unsigned long &gt; vector_data; #include &lt;boost/fusion/include/define_assoc_struct.hpp&gt; BOOST_FUSION_DEFINE_ASSOC_STRUCT( (), assocstruct_data, (int, a, sb_key) (unsigned long, b, id_key)) namespace karma = boost::spirit::karma; template &lt;typename X&gt; std::string to_string ( const X&amp; data ) { std::string generated; std::back_insert_iterator&lt;std::string&gt; sink(generated); karma::generate_delimited ( sink, karma::int_ &lt;&lt; karma::ulong_, karma::space, data ); return generated; } int main() { map_data d1(boost::fusion::make_map&lt;sb_key, id_key&gt;(234, 35314988526ul)); vector_data d2(boost::fusion::make_vector(234, 35314988526ul)); assocstruct_data d3(234,35314988526ul); std::cout &lt;&lt; "map_data as_vector: " &lt;&lt; boost::fusion::as_vector(d1) &lt;&lt; std::endl; //std::cout &lt;&lt; "map_data to_string: " &lt;&lt; to_string(d1) &lt;&lt; std::endl; //*FAIL No 1* std::cout &lt;&lt; "at_key (sb_key): " &lt;&lt; boost::fusion::at_key&lt;sb_key&gt;(d1) &lt;&lt; boost::fusion::at_c&lt;0&gt;(d1) &lt;&lt; std::endl &lt;&lt; std::endl; std::cout &lt;&lt; "vector_data: " &lt;&lt; d2 &lt;&lt; std::endl; std::cout &lt;&lt; "vector_data to_string: " &lt;&lt; to_string(d2) &lt;&lt; std::endl &lt;&lt; std::endl; std::cout &lt;&lt; "assoc_struct as_vector: " &lt;&lt; boost::fusion::as_vector(d3) &lt;&lt; std::endl; std::cout &lt;&lt; "assoc_struct to_string: " &lt;&lt; to_string(d3) &lt;&lt; std::endl; std::cout &lt;&lt; "at_key (sb_key): " &lt;&lt; boost::fusion::at_key&lt;sb_key&gt;(d3) &lt;&lt; d3.a &lt;&lt; boost::fusion::at_c&lt;0&gt;(d3) &lt;&lt; std::endl; return 0; } </code></pre> <p>Including the commented line gives lots of pages of compilation errors, among which notably something like: no known conversion for argument 1 from ‘boost::fusion::pair’ to ‘double’ no known conversion for argument 1 from ‘boost::fusion::pair’ to ‘float’</p> <p>Might it be that to_string needs the values of the map_data, and not the pairs? Though I am not good with templates, I tried to get a vector from a map using transform in the following way</p> <pre><code>template &lt;typename P&gt; struct take_second { typename P::second_type operator() (P p) { return p.second; } }; // ... inside main() pair &lt;char, int&gt; ff(32); std::cout &lt;&lt; "take_second (expect 32): " &lt;&lt; take_second&lt;pair&lt;char,int&gt;&gt;()(ff) &lt;&lt; std::endl; std::cout &lt;&lt; "transform map_data and to_string: " &lt;&lt; to_string(boost::fusion::transform(d1, take_second&lt;&gt;())); //*FAIL No 2* </code></pre> <p>But I don't know what types am I supposed to give when instantiating take_second and anyway I think there must be an easier way to get (iterate over) the values of a map (is there?)</p> <p>If you answer this question, please also give your opinion on whether using an ASSOC_STRUCT or a map is better.</p>
 

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