[libc++] Don't assume that string_view::const_iterator is a raw pointer

[ldionne]

Summary

Our implementation of std::format assumed that string_view's iterators
were raw pointers in various places. If we want to introduce a checked
iterator in debug mode, that won't be true anymore. This patch removes
that assumption.

[ldionne]

@Mordante Please let me know if you like this approach. In a few places, we have the choice of blindly taking _Iterator or explicitly stating basic_string_view::const_iterator -- let me know what you prefer and I'll change it.

[ldionne]

Fix some CI issues

[Mordante]

Nice catch, I thought the iterators were required to be character pointers, but they indeed are not. So I feel this it the better way to do it. (Unless we want to "unwrap our iterators" in the top-level format functions.)

In D138795#3953174, @ldionne wrote:

@Mordante Please let me know if you like this approach. In a few places, we have the choice of blindly taking _Iterator or explicitly stating basic_string_view::const_iterator -- let me know what you prefer and I'll change it.

In general I like the approach, but I left some comments. I really think we should constrain the iterator types since the code makes assumptions on which operations are allowed.

Otherwise LGTM!

libcxx/include/__format/format_string.h

31

The current name is misleading after the change.

The original name was based on <charconv> which uses char pointers. When switching to iterators I prefer to use a more generic name.

102

I think it would be good to add consistent constrains to _Iterator. This line requires a random access iterator.
In some places you used contiguous_iterator, so I think it would be good to do that consistently.
(All code is C++20 so nothing we can use concepts.)

libcxx/include/__format/formatter_output.h

253–255

In combination with the next suggestion.

258

Wouldn't this work?

269

Likewise.

[ldionne]

libcxx/include/__format/formatter_output.h

258

Yes it would work, but don't you feel like it's more readable to introduce a separate name for the type?

[ldionne]

Apply most comments.

[Mordante]

libcxx/include/__format/formatter_output.h

258

My main concern it that by giving the type a name, it's intended to be configurable by the caller, which isn't intended.
Maybe lifting this requirement to a requires instead?

[ldionne]

libcxx/include/__format/formatter_output.h

280

I stumbled upon this, but this is never tested with non-char* iterators, it seems. Same for a few functions below. It is probably worth investigating to add some test coverage.

[ldionne]

Address comments.

[ldionne]

CI issues seem unrelated, shipping since @Mordante was happy with a prior version and I addressed the remaining comments.

libcxx/include/__format/format_functions.h

#      endif
   case __arg_t::__ptr:
     return __format::__compile_time_validate_argument<_CharT, const void*>(__parse_ctx, __ctx);
   case __arg_t::__handle:
     __throw_format_error("Handle should use __compile_time_validate_handle_argument");
   }
   __throw_format_error("Invalid argument");
 }
 
-template <class _CharT, class _ParseCtx, class _Ctx>
-_LIBCPP_HIDE_FROM_ABI constexpr const _CharT*
-__handle_replacement_field(const _CharT* __begin, const _CharT* __end,
+template <class _Iterator, class _ParseCtx, class _Ctx>
+_LIBCPP_HIDE_FROM_ABI constexpr _Iterator
+__handle_replacement_field(_Iterator __begin, _Iterator __end,
                            _ParseCtx& __parse_ctx, _Ctx& __ctx) {
+  using _CharT = iter_value_t<_Iterator>;
   __format::__parse_number_result __r = __format::__parse_arg_id(__begin, __end, __parse_ctx);
 
   bool __parse = *__r.__ptr == _CharT(':');
   switch (*__r.__ptr) {
   case _CharT(':'):
     // The arg-id has a format-specifier, advance the input to the format-spec.
     __parse_ctx.advance_to(__r.__ptr + 1);
     break;

 }
 
 template <class _ParseCtx, class _Ctx>
 _LIBCPP_HIDE_FROM_ABI constexpr typename _Ctx::iterator
 __vformat_to(_ParseCtx&& __parse_ctx, _Ctx&& __ctx) {
   using _CharT = typename _ParseCtx::char_type;
   static_assert(same_as<typename _Ctx::char_type, _CharT>);
 
-  const _CharT* __begin = __parse_ctx.begin();
-  const _CharT* __end = __parse_ctx.end();
+  auto __begin = __parse_ctx.begin();
+  auto __end = __parse_ctx.end();
   typename _Ctx::iterator __out_it = __ctx.out();
   while (__begin != __end) {
     switch (*__begin) {
     case _CharT('{'):
       ++__begin;
       if (__begin == __end)
         __throw_format_error("The format string terminates at a '{'");
 

libcxx/include/__format/format_string.h

 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 namespace __format {
 
-template <class _CharT>
+template <class _Iterator>
 struct _LIBCPP_TEMPLATE_VIS __parse_number_result {
-  const _CharT* __ptr;
+  _Iterator __ptr;
   uint32_t __value;
 };
 
-template <class _CharT>
-__parse_number_result(const _CharT*, uint32_t) -> __parse_number_result<_CharT>;
+template <class _Iterator>
+__parse_number_result(_Iterator, uint32_t) -> __parse_number_result<_Iterator>;
 
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
-__parse_number(const _CharT* __begin, const _CharT* __end);
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_Iterator>
+__parse_number(_Iterator __begin, _Iterator __end);
 
 /**
  * The maximum value of a numeric argument.
  *
  * This is used for:
  * * arg-id
  * * width as value or arg-id.
  * * precision as value or arg-id.
  *
  * The value is compatible with the maximum formatting width and precision
  * using the `%*` syntax on a 32-bit system.
  */
 inline constexpr uint32_t __number_max = INT32_MAX;
 
 namespace __detail {
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
-__parse_zero(const _CharT* __begin, const _CharT*, auto& __parse_ctx) {
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_Iterator>
+__parse_zero(_Iterator __begin, _Iterator, auto& __parse_ctx) {
   __parse_ctx.check_arg_id(0);
   return {++__begin, 0}; // can never be larger than the maximum.
 }
 
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
-__parse_automatic(const _CharT* __begin, const _CharT*, auto& __parse_ctx) {
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_Iterator>
+__parse_automatic(_Iterator __begin, _Iterator, auto& __parse_ctx) {
   size_t __value = __parse_ctx.next_arg_id();
   _LIBCPP_ASSERT(__value <= __number_max,
                  "Compilers don't support this number of arguments");
 
   return {__begin, uint32_t(__value)};
 }
 
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
-__parse_manual(const _CharT* __begin, const _CharT* __end, auto& __parse_ctx) {
-  __parse_number_result<_CharT> __r = __parse_number(__begin, __end);
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_Iterator>
+__parse_manual(_Iterator __begin, _Iterator __end, auto& __parse_ctx) {
+  __parse_number_result<_Iterator> __r = __parse_number(__begin, __end);
   __parse_ctx.check_arg_id(__r.__value);
   return __r;
 }
 
 } // namespace __detail
 
 /**
  * Parses a number.
  *
  * The number is used for the 31-bit values @em width and @em precision. This
  * allows a maximum value of 2147483647.
  */
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
-__parse_number(const _CharT* __begin, const _CharT* __end_input) {
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_Iterator>
+__parse_number(_Iterator __begin, _Iterator __end_input) {
+  using _CharT = iter_value_t<_Iterator>;
   static_assert(__format::__number_max == INT32_MAX,
                 "The algorithm is implemented based on this value.");
   /*
    * Limit the input to 9 digits, otherwise we need two checks during every
    * iteration:
    * - Are we at the end of the input?
    * - Does the value exceed width of an uint32_t? (Switching to uint64_t would
    *   have the same issue, but with a higher maximum.)
    */
-  const _CharT* __end = __end_input - __begin > 9 ? __begin + 9 : __end_input;
+  _Iterator __end = __end_input - __begin > 9 ? __begin + 9 : __end_input;
   uint32_t __value = *__begin - _CharT('0');
   while (++__begin != __end) {
     if (*__begin < _CharT('0') || *__begin > _CharT('9'))
       return {__begin, __value};
 
     __value = __value * 10 + *__begin - _CharT('0');
   }
 

 }
 
 /**
  * Multiplexer for all parse functions.
  *
  * The parser will return a pointer beyond the last consumed character. This
  * should be the closing '}' of the arg-id.
  */
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
-__parse_arg_id(const _CharT* __begin, const _CharT* __end, auto& __parse_ctx) {
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_Iterator>
+__parse_arg_id(_Iterator __begin, _Iterator __end, auto& __parse_ctx) {
+  using _CharT = iter_value_t<_Iterator>;
   switch (*__begin) {
   case _CharT('0'):
     return __detail::__parse_zero(__begin, __end, __parse_ctx);
 
   case _CharT(':'):
     // This case is conditionally valid. It's allowed in an arg-id in the
     // replacement-field, but not in the std-format-spec. The caller can
     // provide a better diagnostic, so accept it here unconditionally.

libcxx/include/__format/formatter_output.h

if (__size >= __specs.__width_)
     return __formatter::__copy(__str, _VSTD::move(__out_it));
 
   __padding_size_result __padding = __formatter::__padding_size(__size, __specs.__width_, __specs.__std_.__alignment_);
   __out_it                        = __formatter::__fill(_VSTD::move(__out_it), __padding.__before_, __specs.__fill_);
   __out_it                        = __formatter::__copy(__str, _VSTD::move(__out_it));
   return __formatter::__fill(_VSTD::move(__out_it), __padding.__after_, __specs.__fill_);
 }
 
-template <class _CharT, class _ParserCharT>
+template <contiguous_iterator _Iterator, class _ParserCharT,
+  class _CharT = iter_value_t<_Iterator>>
 _LIBCPP_HIDE_FROM_ABI auto
-__write(const _CharT* __first,
-        const _CharT* __last,
+__write(_Iterator __first,
+        _Iterator __last,
         output_iterator<const _CharT&> auto __out_it,
         __format_spec::__parsed_specifications<_ParserCharT> __specs,
         ptrdiff_t __size) -> decltype(__out_it) {
   _LIBCPP_ASSERT(__first <= __last, "Not a valid range");
   return __formatter::__write(basic_string_view{__first, __last}, _VSTD::move(__out_it), __specs, __size);
 }
 
 /// \overload
 ///
 /// Calls the function above where \a __size = \a __last - \a __first.
-template <class _CharT, class _ParserCharT>
+template <contiguous_iterator _Iterator, class _ParserCharT,
+  class _CharT = iter_value_t<_Iterator>>
 _LIBCPP_HIDE_FROM_ABI auto
-__write(const _CharT* __first,
-        const _CharT* __last,
+__write(_Iterator __first,
+        _Iterator __last,
         output_iterator<const _CharT&> auto __out_it,
         __format_spec::__parsed_specifications<_ParserCharT> __specs) -> decltype(__out_it) {
   _LIBCPP_ASSERT(__first <= __last, "Not a valid range");
   return __formatter::__write(__first, __last, _VSTD::move(__out_it), __specs, __last - __first);
 }
 
 template <class _CharT, class _ParserCharT, class _UnaryOperation>
 _LIBCPP_HIDE_FROM_ABI auto __write_transformed(const _CharT* __first, const _CharT* __last,
                                                output_iterator<const _CharT&> auto __out_it,
                                                __format_spec::__parsed_specifications<_ParserCharT> __specs,
                                                _UnaryOperation __op) -> decltype(__out_it) {
   _LIBCPP_ASSERT(__first <= __last, "Not a valid range");
 
   ptrdiff_t __size = __last - __first;
   if (__size >= __specs.__width_)
     return __formatter::__transform(__first, __last, _VSTD::move(__out_it), __op);
_LIBCPP_HIDE_FROM_ABI auto __write_string_no_precision(
   size_t __size =
       __format_spec::__estimate_column_width(__str, __specs.__width_, __format_spec::__column_width_rounding::__up)
           .__width_;
   return __formatter::__write(__str, _VSTD::move(__out_it), __specs, __size);
 }
 
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI int __truncate(basic_string_view<_CharT>& __str, int __precision) {
-  __format_spec::__column_width_result<_CharT> __result =
+  __format_spec::__column_width_result __result =
       __format_spec::__estimate_column_width(__str, __precision, __format_spec::__column_width_rounding::__down);
   __str = basic_string_view<_CharT>{__str.begin(), __result.__last_};
   return __result.__width_;
 }
 
 /// Writes a string using format's width estimation algorithm.
 ///
 /// \note When \c _LIBCPP_HAS_NO_UNICODE is defined the function assumes the

libcxx/include/__format/parser_std_format_spec.h

 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 namespace __format_spec {
 
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __format::__parse_number_result< _CharT>
-__parse_arg_id(const _CharT* __begin, const _CharT* __end, auto& __parse_ctx) {
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __format::__parse_number_result<_Iterator>
+__parse_arg_id(_Iterator __begin, _Iterator __end, auto& __parse_ctx) {
+  using _CharT = iter_value_t<_Iterator>;
   // This function is a wrapper to call the real parser. But it does the
   // validation for the pre-conditions and post-conditions.
   if (__begin == __end)
     __throw_format_error("End of input while parsing format-spec arg-id");
 
   __format::__parse_number_result __r = __format::__parse_arg_id(__begin, __end, __parse_ctx);
 
   if (__r.__ptr == __end || *__r.__ptr != _CharT('}'))

 /// set to zero. That way they can be repurposed if a future revision of the
 /// Standards adds new fields to std-format-spec.
 template <class _CharT>
 class _LIBCPP_TEMPLATE_VIS __parser {
 public:
   _LIBCPP_HIDE_FROM_ABI constexpr auto __parse(basic_format_parse_context<_CharT>& __parse_ctx, __fields __fields)
       -> decltype(__parse_ctx.begin()) {
 
-    const _CharT* __begin = __parse_ctx.begin();
-    const _CharT* __end = __parse_ctx.end();
+    auto __begin = __parse_ctx.begin();
+    auto __end = __parse_ctx.end();
     if (__begin == __end)
       return __begin;
 
     if (__parse_fill_align(__begin, __end) && __begin == __end)
       return __begin;
 
     if (__fields.__sign_ && __parse_sign(__begin) && __begin == __end)
       return __begin;
_LIBCPP_HIDE_FROM_ABI constexpr bool __parse_alignment(_CharT __c) {
 
     case _CharT('>'):
       __alignment_ = __alignment::__right;
       return true;
     }
     return false;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_fill_align(const _CharT*& __begin, const _CharT* __end) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_fill_align(_Iterator& __begin, _Iterator __end) {
     _LIBCPP_ASSERT(__begin != __end, "when called with an empty input the function will cause "
                                      "undefined behavior by evaluating data not in the input");
     if (__begin + 1 != __end) {
       if (__parse_alignment(*(__begin + 1))) {
         if (*__begin == _CharT('{') || *__begin == _CharT('}'))
           __throw_format_error("The format-spec fill field contains an invalid character");
 
         __fill_ = *__begin;
         __begin += 2;
         return true;
       }
     }
 
     if (!__parse_alignment(*__begin))
       return false;
 
     ++__begin;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_sign(const _CharT*& __begin) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_sign(_Iterator& __begin) {
     switch (*__begin) {
     case _CharT('-'):
       __sign_ = __sign::__minus;
       break;
     case _CharT('+'):
       __sign_ = __sign::__plus;
       break;
     case _CharT(' '):
       __sign_ = __sign::__space;
       break;
     default:
       return false;
     }
     ++__begin;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_alternate_form(const _CharT*& __begin) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_alternate_form(_Iterator& __begin) {
     if (*__begin != _CharT('#'))
       return false;
 
     __alternate_form_ = true;
     ++__begin;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_zero_padding(const _CharT*& __begin) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_zero_padding(_Iterator& __begin) {
     if (*__begin != _CharT('0'))
       return false;
 
     if (__alignment_ == __alignment::__default)
       __alignment_ = __alignment::__zero_padding;
     ++__begin;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_width(const _CharT*& __begin, const _CharT* __end, auto& __parse_ctx) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_width(_Iterator& __begin, _Iterator __end, auto& __parse_ctx) {
     if (*__begin == _CharT('0'))
       __throw_format_error("A format-spec width field shouldn't have a leading zero");
 
     if (*__begin == _CharT('{')) {
       __format::__parse_number_result __r = __format_spec::__parse_arg_id(++__begin, __end, __parse_ctx);
       __width_as_arg_ = true;
       __width_ = __r.__value;
       __begin = __r.__ptr;
       return true;
     }
 
     if (*__begin < _CharT('0') || *__begin > _CharT('9'))
       return false;
 
     __format::__parse_number_result __r = __format::__parse_number(__begin, __end);
     __width_ = __r.__value;
     _LIBCPP_ASSERT(__width_ != 0, "A zero value isn't allowed and should be impossible, "
                                   "due to validations in this function");
     __begin = __r.__ptr;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_precision(const _CharT*& __begin, const _CharT* __end,
-                                                         auto& __parse_ctx) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_precision(_Iterator& __begin, _Iterator __end, auto& __parse_ctx) {
     if (*__begin != _CharT('.'))
       return false;
 
     ++__begin;
     if (__begin == __end)
       __throw_format_error("End of input while parsing format-spec precision");
 
     if (*__begin == _CharT('{')) {
_LIBCPP_HIDE_FROM_ABI constexpr bool __parse_precision(_Iterator& __begin, _Iterator __end, auto& __parse_ctx) {
 
     __format::__parse_number_result __r = __format::__parse_number(__begin, __end);
     __precision_ = __r.__value;
     __precision_as_arg_ = false;
     __begin = __r.__ptr;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_locale_specific_form(const _CharT*& __begin) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __parse_locale_specific_form(_Iterator& __begin) {
     if (*__begin != _CharT('L'))
       return false;
 
     __locale_specific_form_ = true;
     ++__begin;
     return true;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr void __parse_type(const _CharT*& __begin) {
+  template <class _Iterator>
+  _LIBCPP_HIDE_FROM_ABI constexpr void __parse_type(_Iterator& __begin) {
     // Determines the type. It does not validate whether the selected type is
     // valid. Most formatters have optional fields that are only allowed for
     // certain types. These parsers need to do validation after the type has
     // been parsed. So its easier to implement the validation for all types in
     // the specific parse function.
     switch (*__begin) {
     case 'A':
       __type_ = __type::__hexfloat_upper_case;
_LIBCPP_HIDE_FROM_ABI constexpr void __process_display_type_pointer(__format_spec::__type __type) {
   case __format_spec::__type::__pointer:
     break;
 
   default:
     std::__throw_format_error("The format-spec type has a type not supported for a pointer argument");
   }
 }
 
-template <class _CharT>
+template <contiguous_iterator _Iterator>
 struct __column_width_result {
   /// The number of output columns.
   size_t __width_;
   /// One beyond the last code unit used in the estimation.
   ///
   /// This limits the original output to fit in the wanted number of columns.
-  const _CharT* __last_;
+  _Iterator __last_;
 };
 
-template <class _CharT>
-__column_width_result(size_t, const _CharT*) -> __column_width_result<_CharT>;
+template <contiguous_iterator _Iterator>
+__column_width_result(size_t, _Iterator) -> __column_width_result<_Iterator>;
 
 /// Since a column width can be two it's possible that the requested column
 /// width can't be achieved. Depending on the intended usage the policy can be
 /// selected.
 /// - When used as precision the maximum width may not be exceeded and the
 ///   result should be "rounded down" to the previous boundary.
 /// - When used as a width we're done once the minimum is reached, but
 ///   exceeding is not an issue. Rounding down is an issue since that will

 /// The general case, accepting all values.
 _LIBCPP_HIDE_FROM_ABI constexpr int __column_width(uint32_t __c) noexcept {
   if (__c < 0x10000)
     return __detail::__column_width_3(__c);
 
   return __detail::__column_width_4(__c);
 }
 
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_CharT> __estimate_column_width_grapheme_clustering(
-    const _CharT* __first, const _CharT* __last, size_t __maximum, __column_width_rounding __rounding) noexcept {
+template <class _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_Iterator> __estimate_column_width_grapheme_clustering(
+    _Iterator __first, _Iterator __last, size_t __maximum, __column_width_rounding __rounding) noexcept {
+  using _CharT = iter_value_t<_Iterator>;
   __unicode::__extended_grapheme_cluster_view<_CharT> __view{__first, __last};
 
-  __column_width_result<_CharT> __result{0, __first};
+  __column_width_result<_Iterator> __result{0, __first};
   while (__result.__last_ != __last && __result.__width_ <= __maximum) {
     typename __unicode::__extended_grapheme_cluster_view<_CharT>::__cluster __cluster = __view.__consume();
     int __width = __detail::__column_width(__cluster.__code_point_);
 
     // When the next entry would exceed the maximum width the previous width
     // might be returned. For example when a width of 100 is requested the
     // returned width might be 99, since the next code point has an estimated
     // column width of 2. This depends on the rounding flag.

 ///
 /// \param __first    Points to the first element of the input range.
 /// \param __last     Points beyond the last element of the input range.
 /// \param __maximum  The maximum number of output columns. The returned number
 ///                   of estimated output columns will not exceed this value.
 /// \param __rounding Selects the rounding method.
 ///                   \c __down result.__width_ <= __maximum
 ///                   \c __up result.__width_ <= __maximum + 1
-template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_CharT> __estimate_column_width(
+template <class _CharT, class _Iterator = typename basic_string_view<_CharT>::const_iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_Iterator> __estimate_column_width(
     basic_string_view<_CharT> __str, size_t __maximum, __column_width_rounding __rounding) noexcept {
   // The width estimation is done in two steps:
   // - Quickly process for the ASCII part. ASCII has the following properties
   //   - One code unit is one code point
   //   - Every code point has an estimated width of one
   // - When needed it will a Unicode Grapheme clustering algorithm to find
   //   the proper place for truncation.
 
   if (__str.empty() || __maximum == 0)
     return {0, __str.begin()};
 
   // ASCII has one caveat; when an ASCII character is followed by a non-ASCII
   // character they might be part of an extended grapheme cluster. For example:
   //   an ASCII letter and a COMBINING ACUTE ACCENT
   // The truncate should happen after the COMBINING ACUTE ACCENT. Therefore we
   // need to scan one code unit beyond the requested precision. When this code
   // unit is non-ASCII we omit the current code unit and let the Grapheme
   // clustering algorithm do its work.
-  const _CharT* __it = __str.begin();
+  auto __it = __str.begin();
   if (__is_ascii(*__it)) {
     do {
       --__maximum;
       ++__it;
       if (__it == __str.end())
         return {__str.size(), __str.end()};
 
       if (__maximum == 0) {

libcxx/include/__format/unicode.h

 ///
 /// The data tables used are
 /// https://www.unicode.org/Public/UCD/latest/ucd/auxiliary/GraphemeBreakProperty.txt
 /// https://www.unicode.org/Public/UCD/latest/ucd/emoji/emoji-data.txt
 /// https://www.unicode.org/Public/UCD/latest/ucd/auxiliary/GraphemeBreakTest.txt (for testing only)
 
 inline constexpr char32_t __replacement_character = U'\ufffd';
 
-_LIBCPP_HIDE_FROM_ABI constexpr bool __is_continuation(const char* __char, int __count) {
+template <contiguous_iterator _Iterator>
+  requires same_as<iter_value_t<_Iterator>, char>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __is_continuation(_Iterator __char, int __count) {
   do {
     if ((*__char & 0b1000'0000) != 0b1000'0000)
       return false;
     --__count;
     ++__char;
   } while (__count);
   return true;
 }
 
 /// Helper class to extract a code unit from a Unicode character range.
 ///
 /// The stored range is a view. There are multiple specialization for different
 /// character types.
 template <class _CharT>
 class __code_point_view;
 
 /// UTF-8 specialization.
 template <>
 class __code_point_view<char> {
+  using _Iterator = basic_string_view<char>::const_iterator;
+
 public:
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(const char* __first, const char* __last)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(_Iterator __first, _Iterator __last)
       : __first_(__first), __last_(__last) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr bool __at_end() const noexcept { return __first_ == __last_; }
-  _LIBCPP_HIDE_FROM_ABI constexpr const char* __position() const noexcept { return __first_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iterator __position() const noexcept { return __first_; }
 
   _LIBCPP_HIDE_FROM_ABI constexpr char32_t __consume() noexcept {
     _LIBCPP_ASSERT(__first_ != __last_, "can't move beyond the end of input");
 
     // Based on the number of leading 1 bits the number of code units in the
     // code point can be determined. See
     // https://en.wikipedia.org/wiki/UTF-8#Encoding
     switch (_VSTD::countl_one(static_cast<unsigned char>(*__first_))) {
_LIBCPP_HIDE_FROM_ABI constexpr __consume_p2286_result __consume_p2286() noexcept {
     // An invalid number of leading ones can be garbage or a code unit in the
     // middle of a code point. By consuming one code unit the parser may get
     // "in sync" after a few code units.
     return {1, static_cast<unsigned char>(*__first_++)};
   }
 #    endif // _LIBCPP_STD_VER > 20
 
 private:
-  const char* __first_;
-  const char* __last_;
+  _Iterator __first_;
+  _Iterator __last_;
 };
 
 #    ifndef TEST_HAS_NO_WIDE_CHARACTERS
 _LIBCPP_HIDE_FROM_ABI constexpr bool __is_surrogate_pair_high(wchar_t __value) {
   return __value >= 0xd800 && __value <= 0xdbff;
 }
 
 _LIBCPP_HIDE_FROM_ABI constexpr bool __is_surrogate_pair_low(wchar_t __value) {
   return __value >= 0xdc00 && __value <= 0xdfff;
 }
 
 /// This specialization depends on the size of wchar_t
 /// - 2 UTF-16 (for example Windows and AIX)
 /// - 4 UTF-32 (for example Linux)
 template <>
 class __code_point_view<wchar_t> {
+  using _Iterator = typename basic_string_view<wchar_t>::const_iterator;
+
 public:
   static_assert(sizeof(wchar_t) == 2 || sizeof(wchar_t) == 4, "sizeof(wchar_t) has a not implemented value");
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(const wchar_t* __first, const wchar_t* __last)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(_Iterator __first, _Iterator __last)
       : __first_(__first), __last_(__last) {}
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const wchar_t* __position() const noexcept { return __first_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iterator __position() const noexcept { return __first_; }
   _LIBCPP_HIDE_FROM_ABI constexpr bool __at_end() const noexcept { return __first_ == __last_; }
 
   _LIBCPP_HIDE_FROM_ABI constexpr char32_t __consume() noexcept {
     _LIBCPP_ASSERT(__first_ != __last_, "can't move beyond the end of input");
 
     if constexpr (sizeof(wchar_t) == 2) {
       char32_t __result = *__first_++;
       // Is the code unit part of a surrogate pair? See
if constexpr (sizeof(wchar_t) == 2) {
         return {1, __result};
     }
 
     return {0, __result};
   }
 #      endif // _LIBCPP_STD_VER > 20
 
 private:
-  const wchar_t* __first_;
-  const wchar_t* __last_;
+  _Iterator __first_;
+  _Iterator __last_;
 };
 #    endif
 
 _LIBCPP_HIDE_FROM_ABI constexpr bool __at_extended_grapheme_cluster_break(
     bool& __ri_break_allowed,
     bool __has_extened_pictographic,
     __extended_grapheme_custer_property_boundary::__property __prev,
     __extended_grapheme_custer_property_boundary::__property __next) {

 
 /// Helper class to extract an extended grapheme cluster from a Unicode character range.
 ///
 /// This function is used to determine the column width of an extended grapheme
 /// cluster. In order to do that only the first code point is evaluated.
 /// Therefore only this code point is extracted.
 template <class _CharT>
 class __extended_grapheme_cluster_view {
+  using _Iterator = typename basic_string_view<_CharT>::const_iterator;
+
 public:
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __extended_grapheme_cluster_view(const _CharT* __first, const _CharT* __last)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __extended_grapheme_cluster_view(_Iterator __first, _Iterator __last)
       : __code_point_view_(__first, __last),
         __next_code_point_(__code_point_view_.__consume()),
         __next_prop_(__extended_grapheme_custer_property_boundary::__get_property(__next_code_point_)) {}
 
   struct __cluster {
     /// The first code point of the extended grapheme cluster.
     ///
     /// The first code point is used to estimate the width of the extended
     /// grapheme cluster.
     char32_t __code_point_;
 
     /// Points one beyond the last code unit in the extended grapheme cluster.
     ///
     /// It's expected the caller has the start position and thus can determine
     /// the code unit range of the extended grapheme cluster.
-    const _CharT* __last_;
+    _Iterator __last_;
   };
 
   _LIBCPP_HIDE_FROM_ABI constexpr __cluster __consume() {
     _LIBCPP_ASSERT(
         __next_prop_ != __extended_grapheme_custer_property_boundary::__property::__eot,
         "can't move beyond the end of input");
     char32_t __code_point = __next_code_point_;
     if (!__code_point_view_.__at_end())
       return {__code_point, __get_break()};
 
     __next_prop_ = __extended_grapheme_custer_property_boundary::__property::__eot;
     return {__code_point, __code_point_view_.__position()};
   }
 
 private:
   __code_point_view<_CharT> __code_point_view_;
 
   char32_t __next_code_point_;
   __extended_grapheme_custer_property_boundary::__property __next_prop_;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _CharT* __get_break() {
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iterator __get_break() {
     bool __ri_break_allowed         = true;
     bool __has_extened_pictographic = false;
     while (true) {
-      const _CharT* __result                                          = __code_point_view_.__position();
+      _Iterator __result                                              = __code_point_view_.__position();
       __extended_grapheme_custer_property_boundary::__property __prev = __next_prop_;
       if (__code_point_view_.__at_end()) {
         __next_prop_ = __extended_grapheme_custer_property_boundary::__property::__eot;
         return __result;
       }
       __next_code_point_ = __code_point_view_.__consume();
       __next_prop_       = __extended_grapheme_custer_property_boundary::__get_property(__next_code_point_);
 
       __has_extened_pictographic |=
           __prev == __extended_grapheme_custer_property_boundary::__property::__Extended_Pictographic;
 
       if (__at_extended_grapheme_cluster_break(__ri_break_allowed, __has_extened_pictographic, __prev, __next_prop_))
         return __result;
     }
   }
 };
 
-template <class _CharT>
-__extended_grapheme_cluster_view(const _CharT*, const _CharT*) -> __extended_grapheme_cluster_view<_CharT>;
+template <contiguous_iterator _Iterator>
+__extended_grapheme_cluster_view(_Iterator, _Iterator) -> __extended_grapheme_cluster_view<iter_value_t<_Iterator>>;
 
 #  else //  _LIBCPP_HAS_NO_UNICODE
 
 // For ASCII every character is a "code point".
 // This makes it easier to write code agnostic of the _LIBCPP_HAS_NO_UNICODE define.
 template <class _CharT>
 class __code_point_view {
+  using _Iterator = basic_string_view<_CharT>::const_iterator;
+
 public:
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(const _CharT* __first, const _CharT* __last)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(_Iterator __first, _Iterator __last)
       : __first_(__first), __last_(__last) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr bool __at_end() const noexcept { return __first_ == __last_; }
-  _LIBCPP_HIDE_FROM_ABI constexpr const _CharT* __position() const noexcept { return __first_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iterator __position() const noexcept { return __first_; }
 
   _LIBCPP_HIDE_FROM_ABI constexpr char32_t __consume() noexcept {
     _LIBCPP_ASSERT(__first_ != __last_, "can't move beyond the end of input");
     return *__first_++;
   }
 
 #    if _LIBCPP_STD_VER > 20
   _LIBCPP_HIDE_FROM_ABI constexpr __consume_p2286_result __consume_p2286() noexcept {
     _LIBCPP_ASSERT(__first_ != __last_, "can't move beyond the end of input");
 
     return {0, std::make_unsigned_t<_CharT>(*__first_++)};
   }
 #    endif // _LIBCPP_STD_VER > 20
 
 private:
-  const _CharT* __first_;
-  const _CharT* __last_;
+  _Iterator __first_;
+  _Iterator __last_;
 };
 
 #  endif //  _LIBCPP_HAS_NO_UNICODE
 
 } // namespace __unicode
 
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_UNICODE_H