Boost.Spirit Qi: different entry points to the same grammar?

Question

I have a recursive grammar and want to parse starting from the different rules from it. Is it possible without rewriting the same grammar several times?

Example: I have json parser:

template <typename It, typename Skipper = qi::space_type>
struct grammar : qi::grammar<It, value (), Skipper>
{
  grammar () : grammar::base_type (value_)
  {
    using namespace qi;

    static auto const null_ = proto::deep_copy ("null" >> qi::attr (null {}));

    static auto const bool_ = proto::deep_copy (
      "true" >> qi::attr (true) | "false" >> qi::attr (false));

    static auto const text_ = proto::deep_copy (
      '"' >> qi::raw [*('\\' >> qi::char_ | ~qi::char_('"'))] >> '"');

    value_  = null_ | bool_ | text_ | double_ | object_ | array_;
    member_ = text_ >> ':' >> value_;
    object_ = '{' >> -(member_ % ',') >> '}';
    array_  = '[' >> -(value_ % ',') >> ']';

    BOOST_SPIRIT_DEBUG_NODES((value_)(member_)(object_)(array_))
  }

private:
  qi::rule<It, json:: value (), Skipper> value_;
  qi::rule<It, json::member (), Skipper> member_;
  qi::rule<It, json::object (), Skipper> object_;
  qi::rule<It, json:: array (), Skipper> array_;
};

Usually I need to parse input as json value, but sometimes I need to parse it as json array or json object. Can I do it without rewriting the same grammar over and over where the only difference between these grammars is the entry point?

Answer 1

The closest solution I found was to split the grammar into base and derived classes, and use the different start rules in the derived classes. It does not duplicate the source code a lot, but still looks as overweight solution.

template <typename It, typename Data, typename Skipper = qi::space_type>
struct base : qi::grammar<It, Data (), Skipper>
{
  using jbase_type = base;

  template <typename Member>
  base (Member& member) : base::base_type (member)
  {
    using namespace qi;
    using namespace Json;

    value_  = null_ | bool_ | text_ | double_ | object_ | array_;
    member_ = text_ >> ':' >> value_;
    object_ = '{' >> -(member_ % ',') >> '}';
    array_  = '[' >> -(value_ % ',') >> ']';

    BOOST_SPIRIT_DEBUG_NODES((value_)(member_)(object_)(array_))
  }

protected:
  escaped_string_grammar<It> text_;

  qi::rule<It, Json:: Value (), Skipper> value_;
  qi::rule<It, Json::Member (), Skipper> member_;
  qi::rule<It, Json::Object (), Skipper> object_;
  qi::rule<It, Json:: Array (), Skipper> array_;
};

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//
template <typename It, typename Skipper = qi::space_type>
struct value : base<It, Json::Value, Skipper>
{
  value () : value::jbase_type (value::jbase_type::value_) {}
};

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//
template <typename It, typename Skipper = qi::space_type>
struct array : base<It, Json::Array, Skipper>
{
  array () : array::jbase_type (array::jbase_type::array_) {}
};

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//
template <typename It, typename Skipper = qi::space_type>
struct object : base<It, Json::Object, Skipper>
{
  object () : object::jbase_type (object::jbase_type::object_) {}
};