boost::xpressive::op::at — at
是一个用于对序列进行索引的多态函数对象
// In header: <boost/xpressive/regex_actions.hpp> struct at { // member classes/structs/unions template<typename Sig> struct result { }; template<typename This, typename Cont, typename Idx> struct result<This(Cont &, Idx)> { // types typedef Cont::reference type; }; template<typename This, typename Cont, typename Idx> struct result<This(Cont const &, Idx)> { // types typedef Cont::const_reference type; }; template<typename This, typename Cont, typename Idx> struct result<This(Cont, Idx)> { // types typedef Cont::const_reference type; }; // public member functions template<typename Cont, typename Idx> Cont::reference operator()(Cont &, Idx) const; template<typename Cont, typename Idx> Cont::const_reference operator()(Cont const &, Idx) const; };