Class format_handler

include/fmt/format.h:3973–4018

    }

    struct format_handler : error_handler {
            basic_format_parse_context<Char> parse_context;
            buffer_context<Char> context;

            format_handler(buffer_appender<Char> p_out, basic_string_view<Char> str,
                           basic_format_args<buffer_context<Char>> p_args, locale_ref p_loc)
                : parse_context(str), context(p_out, p_args, p_loc) {}

            void on_text(const Char* begin, const Char* end) {
                auto text = basic_string_view<Char>(begin, to_unsigned(end - begin));
                context.advance_to(write<Char>(context.out(), text));
            }

            FMT_CONSTEXPR auto on_arg_id() -> int { return parse_context.next_arg_id(); }

            FMT_CONSTEXPR auto on_arg_id(int id) -> int { return parse_context.check_arg_id(id), id; }

            FMT_CONSTEXPR auto on_arg_id(basic_string_view<Char> id) -> int {
                int arg_id = context.arg_id(id);
                if (arg_id < 0) on_error("argument not found");
                return arg_id;
            }

            FMT_INLINE void on_replacement_field(int id, const Char*) {
                auto arg = get_arg(context, id);
                context.advance_to(visit_format_arg(
                    default_arg_formatter<Char> {context.out(), context.args(), context.locale()}, arg));
            }

            auto on_format_specs(int id, const Char* begin, const Char* end) -> const Char* {
                auto arg = get_arg(context, id);
                if (arg.type() == type::custom_type) {
                    parse_context.advance_to(begin);
                    visit_format_arg(custom_formatter<Char> {parse_context, context}, arg);
                    return parse_context.begin();
                }
                auto specs = detail::dynamic_format_specs<Char>();
                begin = parse_format_specs(begin, end, specs, parse_context, arg.type());
                detail::handle_dynamic_spec<detail::width_checker>(specs.width, specs.width_ref, context);
                detail::handle_dynamic_spec<detail::precision_checker>(specs.precision, specs.precision_ref, context);
                if (begin == end || *begin != '}') on_error("missing '}' in format string");
                auto f = arg_formatter<Char> {context.out(), specs, context.locale()};
                context.advance_to(visit_format_arg(f, arg));
                return begin;
            }
    };

    detail::parse_format_string<false>(fmt, format_handler(out, fmt, args, loc));
}