boost :: función de enlace del espíritu que proporciona parámetros como espíritu: qi :: _ val
Es necesario proporcionar los valores de un objeto de tipo boost :: variant para un objeto std :: pair. ¿Cómo implementarías esta idea usando otros recursos? Cualquier otra forma que esto se hace a continuación?
struct aggr_pair_visitor : public ::boost::static_visitor<void>
{
public:
explicit aggr_pair_visitor( column_and_aggregate & pair_ ) : pair(pair_)
{
}
void operator()(column_name_t const & column)
{
pair.first = column;
}
void operator()(unsigned const & faggr)
{
if ( faggr > static_cast<unsigned>(sql_faggregate::SUM) || faggr < static_cast<unsigned>(sql_faggregate::AVG) )
throw std::runtime_error("Failed to parse aggregate type : Not valid integer");
else pair.second = static_cast<sql_faggregate>(faggr);
}
private:
column_and_aggregate & pair;
};
void apply_col_and_aggr_visitor( column_and_aggregate & col_and_aggr_pair, ::boost::variant< column_name_t, unsigned > const & val )
{
aggr_pair_visitor pair_visitor( col_and_aggr_pair );
::boost::apply_visitor( pair_visitor, val ); // N.B.!!! Runtime execution of operator()!
}
spirit::qi::rule< iterator, column_and_aggregate(), ascii::space_type > agg_pair =
quoted_string[::boost::bind( &apply_col_and_aggr_visitor, spirit::qi::_val, spirit::qi::_1 )]
> ':'
> spirit::int_[::boost::bind( &apply_col_and_aggr_visitor, spirit::qi::_val, spirit::qi::_1 )];
spirit::qi::rule< iterator, column_and_aggregate_container(), ascii::space_type > aggregates_parser =
'{'
> agg_pair[phoenix::push_back(spirit::qi::_val, spirit::qi::_1)] % ',' // N.B.!!! list-redux technic
> '}';