parsing into several vector members

boost-spirit, boost-spirit-qi, c++, wavefront

Solution

There are several ways :)

- Custom attribute traits

- The same using semantic actions

- Everything in semantic actions, at detail level

1. Custom attribute traits

The cleanest, IMO would to replace the Fusion Sequence Adaptation (`BOOST_FUSION_ADAPT_STRUCT`) by custom container attribute traits for Spirit:

namespace boost { namespace spirit { namespace traits {

    template<> 
        struct is_container<ElemParseData, void> : mpl::true_ { };
    template<> 
        struct container_value<ElemParseData, void> { 
             typedef boost::variant<float, unsigned int> type;
        };
    template <>
        struct push_back_container<ElemParseData, std::vector<float>, void> {
            static bool call(ElemParseData& c, std::vector<float> const& val) {
                c.verts.insert(c.verts.end(), val.begin(), val.end());
                return true;
            }
        };
    template <>
        struct push_back_container<ElemParseData, std::vector<unsigned int>, void> {
            static bool call(ElemParseData& c, std::vector<unsigned int> const& val) {
                c.idx.insert(c.idx.end(), val.begin(), val.end());
                return true;
            }
        };
}}}

Without changes to the grammar, this will simply result in the same effect. However, now you can modify the parser to expect the desired grammar:

    vertex   = 'v' >> qi::double_ >> qi::double_ >> qi::double_;
    elements = 'f' >> qi::int_ >> qi::int_ >> qi::int_;

    start = *(vertex | elements);

And because of the traits, Spirit will "just know" how to insert into `ElemParseData`. See it live on Coliru

2. The same using semantic actions

You can wire it up in semantic actions:

    start = *(  
               vertex   [phx::bind(insert, _val, _1)] 
             | elements [phx::bind(insert, _val, _1)]
             );

With `insert` a member of type `inserter`:

struct inserter {
    template <typename,typename> struct result { typedef void type; };

    template <typename Attr, typename Vec>
        void operator()(Attr& attr, Vec const& v) const { dispatch(attr, v); }
    private:
    static void dispatch(ElemParseData& data, std::vector<float> vertices) {
        data.verts.insert(data.verts.end(), vertices.begin(), vertices.end());
    }
    static void dispatch(ElemParseData& data, std::vector<unsigned int> indices) {
        data.idx.insert(data.idx.end(), indices.begin(), indices.end());
    }
};

This looks largely the same, and it does the same: live on Coliru

3. Everything in semantic actions, at detail level

This is the only solution that doesn't require any kind of plumbing, except perhaps inclusion of `boost/spirit/include/phoenix.hpp`:

struct objGram : qi::grammar<std::string::const_iterator, ElemParseData(), iso8859::space_type>
{
    objGram() : objGram::base_type(start)
    {
        using namespace qi;

        auto add_vertex = phx::push_back(phx::bind(&ElemParseData::verts, _r1), _1);
        auto add_index  = phx::push_back(phx::bind(&ElemParseData::idx,   _r1), _1);
        vertex   = 'v' >> double_ [add_vertex] >> double_ [add_vertex] >> double_ [add_vertex];
        elements = 'f' >> int_    [add_index]  >> int_    [add_index]  >> int_    [add_index] ;

        start = *(vertex(_val) | elements(_val));
    }

    qi::rule<std::string::const_iterator, ElemParseData(), iso8859::space_type> start;
    qi::rule<std::string::const_iterator, void(ElemParseData&), iso8859::space_type> vertex, elements;
} objGrammar;

Note:

- One slight advantage here would be that there is less copying of values

- A disadvantage is that you lose 'atomicity' (if a line fails to parse after, say, the second value, the first two values will have been pushed into the `ElemParseData` members irrevocably).

Side note

There is a bug in the read loop, prefer the simpler options:

std::filebuf fb;
if (fb.open("parsetest.txt", std::ios::in))
{
    ss << &fb;
    fb.close();
}

Or consider `boost::spirit::istream_iterator`

Problem

Original source

Related problems

• Boost Spirit: "Semantic actions are evil"?