[PATCH] libstdc++: Ensure that _Utf_view is always a view.
Tomasz Kaminski
tkaminsk@redhat.com
Fri Nov 14 17:58:15 GMT 2025
On Fri, Nov 14, 2025 at 6:55 PM Hewill Kang <hewillk@gmail.com> wrote:
> LGTM.
>
> I know that CTAD wasn't provided due to heavy header considerations. But
> wouldn't providing CTAD be an enhancement?
> At least in terms of user convenience.
>
I care much more about the use of libstdc++ compile time performance, that
convenience of the
libstdc++ authors (only people that should use it). If we get
official Utf_view, it should
have the deduction guide in place.
>
> Tomasz Kamiński <tkaminsk@redhat.com> 於 2025年11月15日 週六 上午1:06寫道：
>
>> Previously, _Utf_view accepted any input_range, including
>> reference-to-array
>> types like char(&)[2], and stored it as the _M_base member. In such cases,
>> _Utf_view was not assignable, failing the requirement to be a view
>> concept.
>>
>> This patch addresses the issue by adding the ranges::view constraint to
>> the
>> second template parameter of _Utf_view, and for clarity renaming it from
>> _Range to _View. The constructor is also adjusted to accept its argument
>> by value (views must be O(1) move-constructible). This prevents implicitly
>> generated CTAD from deducing a reference type.
>>
>> This makes _Utf_view consistent with both other standard views and the
>> wording from P2728R8: Unicode in the Library, Part 1: UTF Transcoding [1].
>>
>> The explicit CTAD from viewable_range is not defined for _Utf_view because
>> it depends on views::all_t, views::ref_view, and views::owning_view,
>> which are declared in <ranges>. Consequently, users must explicitly cast
>> the argument to a view or specify it as a template parameter.
>>
>> [1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2728r8.html
>>
>> libstdc++-v3/ChangeLog:
>>
>> * include/bits/unicode.h (_Utf_view): Rename the template
>> parameter
>> from _Range to _View and constrain it with ranges::view.
>> (_Utf_view::_Utf_view): Accept by value instead of rvalue
>> reference.
>> * include/std/format (__format::__write_padded): Replace _Utf_view
>> over const char32_t(&)[1] with span<const char32_t, 1>.
>> * testsuite/ext/unicode/view.cc: Add checks if specialization
>> of _Utf_view satisfy view. Wrap arrays into std::span before
>> cosntructing _Utf_view.
>> ---
>> libstdc++-v3/include/bits/unicode.h | 15 ++++++++-------
>> libstdc++-v3/include/std/format | 2 +-
>> libstdc++-v3/testsuite/ext/unicode/view.cc | 16 ++++++++++------
>> 3 files changed, 19 insertions(+), 14 deletions(-)
>>
>> diff --git a/libstdc++-v3/include/bits/unicode.h
>> b/libstdc++-v3/include/bits/unicode.h
>> index 44872db4ed6..09f7c2d7bfb 100644
>> --- a/libstdc++-v3/include/bits/unicode.h
>> +++ b/libstdc++-v3/include/bits/unicode.h
>> @@ -695,13 +695,14 @@ namespace __unicode
>> friend class _Utf_iterator;
>> };
>>
>> - template<typename _ToFormat, ranges::input_range _Range>
>> + template<typename _ToFormat, ranges::input_range _View>
>> + requires ranges::view<_View>
>> class _Utf_view
>> - : public ranges::view_interface<_Utf_view<_ToFormat, _Range>>
>> + : public ranges::view_interface<_Utf_view<_ToFormat, _View>>
>> {
>> - using _Iterator = _Utf_iterator<ranges::range_value_t<_Range>,
>> - _ToFormat,
>> ranges::iterator_t<_Range>,
>> - ranges::sentinel_t<_Range>>;
>> + using _Iterator = _Utf_iterator<ranges::range_value_t<_View>,
>> + _ToFormat,
>> ranges::iterator_t<_View>,
>> + ranges::sentinel_t<_View>>;
>>
>> template<typename _Iter, typename _Sent>
>> constexpr auto
>> @@ -725,11 +726,11 @@ namespace __unicode
>> return _Iterator(__last, __last);
>> }
>>
>> - _Range _M_base;
>> + _View _M_base;
>>
>> public:
>> constexpr explicit
>> - _Utf_view(_Range&& __r) : _M_base(std::forward<_Range>(__r)) { }
>> + _Utf_view(_View __r) : _M_base(std::move(__r)) { }
>>
>> constexpr auto begin()
>> { return _M_begin(ranges::begin(_M_base), ranges::end(_M_base)); }
>> diff --git a/libstdc++-v3/include/std/format
>> b/libstdc++-v3/include/std/format
>> index 1102ac8f6e8..f64f35a202e 100644
>> --- a/libstdc++-v3/include/std/format
>> +++ b/libstdc++-v3/include/std/format
>> @@ -845,7 +845,7 @@ namespace __format
>> {
>> // Encode fill char as multiple code units of type _CharT.
>> const char32_t __arr[1]{ __fill_char };
>> - _Utf_view<_CharT, const char32_t(&)[1]> __v(__arr);
>> + _Utf_view<_CharT, span<const char32_t, 1>> __v(__arr);
>> basic_string<_CharT> __padstr(__v.begin(), __v.end());
>> __padding = __padstr;
>> while (__l-- > 0)
>> diff --git a/libstdc++-v3/testsuite/ext/unicode/view.cc
>> b/libstdc++-v3/testsuite/ext/unicode/view.cc
>> index 40c8fcf34fb..677a21d8c1f 100644
>> --- a/libstdc++-v3/testsuite/ext/unicode/view.cc
>> +++ b/libstdc++-v3/testsuite/ext/unicode/view.cc
>> @@ -7,6 +7,10 @@
>> namespace uc = std::__unicode;
>> using namespace std::string_view_literals;
>>
>> +static_assert( std::ranges::view<uc::_Utf8_view<std::string_view>> );
>> +static_assert( std::ranges::view<uc::_Utf16_view<std::string_view>> );
>> +static_assert( std::ranges::view<uc::_Utf32_view<std::string_view>> );
>> +
>> template<std::ranges::range View>
>> constexpr void
>> compare(View v, std::basic_string_view<std::ranges::range_value_t<View>>
>> s)
>> @@ -87,18 +91,18 @@ test_illformed_utf16()
>> compare(uc::_Utf16_view(s.substr(0, 1)), r);
>> compare(uc::_Utf16_view(s.substr(1, 1)), r);
>> std::array s2{ s[0], s[0] };
>> - compare(uc::_Utf16_view(s2), u"\uFFFD\uFFFD"sv);
>> + compare(uc::_Utf16_view(std::span(s2)), u"\uFFFD\uFFFD"sv);
>> std::array s3{ s[0], s[0], s[1] };
>> - compare(uc::_Utf16_view(s3), u"\uFFFD\N{CLOWN FACE}"sv);
>> + compare(uc::_Utf16_view(std::span(s3)), u"\uFFFD\N{CLOWN FACE}"sv);
>> std::array s4{ s[1], s[0] };
>> - compare(uc::_Utf16_view(s4), u"\uFFFD\uFFFD"sv);
>> + compare(uc::_Utf16_view(std::span(s4)), u"\uFFFD\uFFFD"sv);
>> std::array s5{ s[1], s[0], s[1] };
>> - compare(uc::_Utf16_view(s5), u"\uFFFD\N{CLOWN FACE}"sv);
>> + compare(uc::_Utf16_view(std::span(s5)), u"\uFFFD\N{CLOWN FACE}"sv);
>>
>> std::array<char16_t, 2> s6{ 0xDC00, 0xDC01 };
>> - compare(uc::_Utf16_view(s6), u"\uFFFD\uFFFD"sv);
>> + compare(uc::_Utf16_view(std::span(s6)), u"\uFFFD\uFFFD"sv);
>> std::array<char16_t, 2> s7{ 0xD7FF, 0xDC00 };
>> - compare(uc::_Utf16_view(s7), u"\uD7FF\uFFFD"sv);
>> + compare(uc::_Utf16_view(std::span(s7)), u"\uD7FF\uFFFD"sv);
>> }
>>
>> constexpr void
>> --
>> 2.51.0
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20251114/67211159/attachment.htm>
More information about the Libstdc++
mailing list