Class parser

extern/json/json.hpp:8993–9270

    This class implements a recursive decent parser.
    */
    class parser
    {
      public:
        /// a parser reading from a string literal
        parser(const char* buff, const parser_callback_t cb = nullptr)
            : callback(cb),
              m_lexer(reinterpret_cast<const typename lexer::lexer_char_t*>(buff), strlen(buff))
        {}

        /// a parser reading from an input stream
        parser(std::istream& is, const parser_callback_t cb = nullptr)
            : callback(cb), m_lexer(is)
        {}

        /// a parser reading from an iterator range with contiguous storage
        template<class IteratorType, typename std::enable_if<
                     std::is_same<typename std::iterator_traits<IteratorType>::iterator_category, std::random_access_iterator_tag>::value
                     , int>::type
                 = 0>
        parser(IteratorType first, IteratorType last, const parser_callback_t cb = nullptr)
            : callback(cb),
              m_lexer(reinterpret_cast<const typename lexer::lexer_char_t*>(&(*first)),
                      static_cast<size_t>(std::distance(first, last)))
        {}

        /// public parser interface
        basic_json parse()
        {
            // read first token
            get_token();

            basic_json result = parse_internal(true);
            result.assert_invariant();

            expect(lexer::token_type::end_of_input);

            // return parser result and replace it with null in case the
            // top-level value was discarded by the callback function
            return result.is_discarded() ? basic_json() : std::move(result);
        }

      private:
        /// the actual parser
        basic_json parse_internal(bool keep)
        {
            auto result = basic_json(value_t::discarded);

            switch (last_token)
            {
                case lexer::token_type::begin_object:
                {
                    if (keep and (not callback
                                  or ((keep = callback(depth++, parse_event_t::object_start, result)) != 0)))
                    {
                        // explicitly set result to object to cope with {}
                        result.m_type = value_t::object;
                        result.m_value = value_t::object;
                    }