[libc++] Add C++20 stringstream::view()

[philnik]

Please make sure you did everything listed at https://libcxx.llvm.org/Contributing.html#pre-commit-check-list. Feel free to ask here or on Discord (in the #libcxx channel) if you have any questions.

libcxx/include/sstream

896

This should be _LIBCPP_HIDE_FROM_ABI instead.

[pfusik]

s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/
Added the test.

[pfusik]

In D148641#4277950, @philnik wrote:

Please make sure you did everything listed at https://libcxx.llvm.org/Contributing.html#pre-commit-check-list. Feel free to ask here or on Discord (in the #libcxx channel) if you have any questions.

I have an error in my setup: https://libcxx.llvm.org/BuildingLibcxx.html#cmake-ninja-mingw

CMake Error at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:59 (add_library):
  No SOURCES given to target: libcxx-abi-headers-generate-private-headers

[pfusik]

git clang-format

[pfusik]

Move the tests to libcxx/test/std/input.output/string.streams

[pfusik]

Could you please help me with my local setup?

C:\0\src\llvm-project>cmake -G Ninja -S runtimes -B buildlib -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_ENABLE_RUNTIMES=libcxx -DLIBCXX_CXX_ABI=libstdc++
-- The C compiler identification is Clang 16.0.1
-- The CXX compiler identification is Clang 16.0.1
-- The ASM compiler identification is Clang with GNU-like command-line
-- Found assembler: C:/msys64/mingw64/bin/clang.exe
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/msys64/mingw64/bin/clang.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/msys64/mingw64/bin/clang++.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Could NOT find LLVM (missing: LLVM_DIR)
-- Could NOT find Clang (missing: Clang_DIR)
-- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG
-- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG - Success
-- Performing Test C_SUPPORTS_START_NO_UNUSED_ARGUMENTS
-- Performing Test C_SUPPORTS_START_NO_UNUSED_ARGUMENTS - Success
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG - Success
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG - Success
-- Performing Test SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG
-- Performing Test SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG - Success
-- Performing Test C_SUPPORTS_WERROR_DATE_TIME
-- Performing Test C_SUPPORTS_WERROR_DATE_TIME - Success
-- Performing Test CXX_SUPPORTS_WERROR_DATE_TIME
-- Performing Test CXX_SUPPORTS_WERROR_DATE_TIME - Success
-- Performing Test C_SUPPORTS_WERROR_UNGUARDED_AVAILABILITY_NEW
-- Performing Test C_SUPPORTS_WERROR_UNGUARDED_AVAILABILITY_NEW - Success
-- Performing Test CXX_SUPPORTS_WERROR_UNGUARDED_AVAILABILITY_NEW
-- Performing Test CXX_SUPPORTS_WERROR_UNGUARDED_AVAILABILITY_NEW - Success
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-- Performing Test C_SUPPORTS_IMPLICIT_FALLTHROUGH_FLAG
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-- Performing Test C_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG
-- Performing Test C_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG - Success
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-- Performing Test CXX_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG - Success
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-- Performing Test CXX_SUPPORTS_CLASS_MEMACCESS_FLAG - Failed
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-- Performing Test CXX_SUPPORTS_NOEXCEPT_TYPE_FLAG - Success
-- Performing Test CXX_WONT_WARN_ON_FINAL_NONVIRTUALDTOR
-- Performing Test CXX_WONT_WARN_ON_FINAL_NONVIRTUALDTOR - Success
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-- Performing Test CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG - Success
-- Performing Test CXX_WSUGGEST_OVERRIDE_ALLOWS_ONLY_FINAL
-- Performing Test CXX_WSUGGEST_OVERRIDE_ALLOWS_ONLY_FINAL - Success
-- Performing Test C_WCOMMENT_ALLOWS_LINE_WRAP
-- Performing Test C_WCOMMENT_ALLOWS_LINE_WRAP - Failed
-- Performing Test C_SUPPORTS_STRING_CONVERSION_FLAG
-- Performing Test C_SUPPORTS_STRING_CONVERSION_FLAG - Success
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-- Performing Test CXX_SUPPORTS_STRING_CONVERSION_FLAG - Success
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-- Performing Test C_SUPPORTS_FDATA_SECTIONS
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-- Performing Test CXX_SUPPORTS_FDATA_SECTIONS
-- Performing Test CXX_SUPPORTS_FDATA_SECTIONS - Success
-- Looking for os_signpost_interval_begin
-- Looking for os_signpost_interval_begin - not found
-- Found Python3: C:/msys64/mingw64/bin/python3.10.exe (found version "3.10.11") found components: Interpreter
-- LLVM host triple: x86_64-w64-windows-gnu
-- LLVM default target triple: x86_64-w64-windows-gnu
-- Using libc++ testing configuration: C:/0/src/llvm-project/libcxx/test/configs/llvm-libc++-mingw.cfg.in
-- Looking for fopen in c
-- Looking for fopen in c - not found
-- Looking for __gcc_personality_v0 in gcc_s
-- Looking for __gcc_personality_v0 in gcc_s - not found
-- Performing Test C_SUPPORTS_COMMENT_LIB_PRAGMA
-- Performing Test C_SUPPORTS_COMMENT_LIB_PRAGMA - Failed
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Looking for ccos in m
-- Looking for ccos in m - found
-- Looking for clock_gettime in rt
-- Looking for clock_gettime in rt - not found
-- Looking for __atomic_fetch_add_8 in atomic
-- Looking for __atomic_fetch_add_8 in atomic - found
CMake Warning at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:54 (message):
  Failed to find cxxabi.h in
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


CMake Warning at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:54 (message):
  Failed to find bits/c++config.h in
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


CMake Warning at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:54 (message):
  Failed to find bits/os_defines.h in
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


CMake Warning at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:54 (message):
  Failed to find bits/cpu_defines.h in
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


CMake Warning at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:54 (message):
  Failed to find bits/cxxabi_tweaks.h in
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


CMake Warning at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:54 (message):
  Failed to find bits/cxxabi_forced.h in
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


-- Performing Test CXX_SUPPORTS_FALIGNED_ALLOCATION_FLAG
-- Performing Test CXX_SUPPORTS_FALIGNED_ALLOCATION_FLAG - Success
-- Performing Test CXX_SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG
-- Performing Test CXX_SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG - Success
-- Performing Test CXX_SUPPORTS_FVISIBILITY_EQ_HIDDEN_FLAG
-- Performing Test CXX_SUPPORTS_FVISIBILITY_EQ_HIDDEN_FLAG - Success
-- Performing Test CXX_SUPPORTS_WALL_FLAG
-- Performing Test CXX_SUPPORTS_WALL_FLAG - Success
-- Performing Test CXX_SUPPORTS_WEXTRA_FLAG
-- Performing Test CXX_SUPPORTS_WEXTRA_FLAG - Success
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-- Performing Test CXX_SUPPORTS_WERROR_EQ_RETURN_TYPE_FLAG
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-- Performing Test CXX_SUPPORTS_WEXTRA_SEMI_FLAG
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-- Performing Test CXX_SUPPORTS_WUNDEF_FLAG
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-- Performing Test CXX_SUPPORTS_WNO_USER_DEFINED_LITERALS_FLAG
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-- Performing Test CXX_SUPPORTS_WNO_COVERED_SWITCH_DEFAULT_FLAG - Success
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-- Performing Test CXX_SUPPORTS_WNO_SUGGEST_OVERRIDE_FLAG - Success
-- Performing Test CXX_SUPPORTS_WNO_ERROR_FLAG
-- Performing Test CXX_SUPPORTS_WNO_ERROR_FLAG - Success
-- Performing Test CXX_SUPPORTS_EHSC_FLAG
-- Performing Test CXX_SUPPORTS_EHSC_FLAG - Failed
-- Adding Benchmark: algorithms.partition_point.bench.cpp
-- Adding Benchmark: equal.bench.cpp
-- Adding Benchmark: lower_bound.bench.cpp
-- Adding Benchmark: make_heap.bench.cpp
-- Adding Benchmark: make_heap_then_sort_heap.bench.cpp
-- Adding Benchmark: min.bench.cpp
-- Adding Benchmark: min_max_element.bench.cpp
-- Adding Benchmark: pop_heap.bench.cpp
-- Adding Benchmark: push_heap.bench.cpp
-- Adding Benchmark: ranges_make_heap.bench.cpp
-- Adding Benchmark: ranges_make_heap_then_sort_heap.bench.cpp
-- Adding Benchmark: ranges_pop_heap.bench.cpp
-- Adding Benchmark: ranges_push_heap.bench.cpp
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-- Adding Benchmark: ranges_sort_heap.bench.cpp
-- Adding Benchmark: ranges_stable_sort.bench.cpp
-- Adding Benchmark: sort.bench.cpp
-- Adding Benchmark: sort_heap.bench.cpp
-- Adding Benchmark: stable_sort.bench.cpp
-- Adding Benchmark: dynamic_cast.bench.cpp
-- Adding Benchmark: allocation.bench.cpp
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-- Adding Benchmark: to_chars.bench.cpp
-- Adding Benchmark: unordered_set_operations.bench.cpp
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-- Adding Benchmark: variant_visit_1.bench.cpp
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-- Adding Benchmark: vector_operations.bench.cpp
-- ABI list file not generated for configuration x86_64-w64-windows-gnu.libstdc++.v1.stable.exceptions.nonew, `check-cxx-abilist` will not be available.
-- Configuring done (22.1s)
CMake Error at C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:59 (add_library):
  No SOURCES given to target: libcxx-abi-headers-generate-private-headers
Call Stack (most recent call first):
  C:/0/src/llvm-project/libcxx/cmake/Modules/HandleLibCXXABI.cmake:87 (import_private_headers)
  C:/0/src/llvm-project/libcxx/CMakeLists.txt:493 (include)


CMake Generate step failed.  Build files cannot be regenerated correctly.

[pfusik]

In D148641#4279422, @pfusik wrote:

Could you please help me with my local setup?

As a workaround, I've set it up on my Mac.

[pfusik]

Remove #include <string_view>.
Use the string_view constructor with two pointers.

[pfusik]

Revert to the string_view(ptr, count) constructor.
Hide streambuf::view() from ABI.

[pfusik]

Now submitting using arc.

[Mordante]

Thanks for working on this! I mainly looked at the high-level parts not compared against the paper yet.

Does this patch implement the entire paper or not? If not please make sure you add a note on the status page to explain which parts are and are not done. (We might remove the synopsis in the future.)

libcxx/include/sstream

37

Please update libcxx/docs/Status/Cxx20.rst with the implementation status of P0408R7. It would be great to put that number+paper name in the commit message too. That makes it easier to find the associated paper.

450–458

The feature should be guarded so it is only available in C++20 and later.

The same applies to where you add the view member.

508

Please format the new code with clang-format so it matches our "normal" style.

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/str.pass.cpp

14 ↗

(On Diff #517514)

Please create a new test for the view (can be a copy past of this file) and update the synopsis here.

That test will need an // UNSUPPORTED: c++03, c++11, c++14, c++17 line since it is C++20 only.

[philnik]

libcxx/include/sstream

36

Please update the correct status paper in libcxx/docs/Status/.

260

Throughout

514

Generally, we don't spell out the this pointer if not required.

[pfusik]

Thanks!

How do I run C++03 tests locally?

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/str.pass.cpp

14 ↗

(On Diff #517514)

That would be four tests copied. Why not #if ?

[Mordante]

In D148641#4302726, @pfusik wrote:

Thanks!

How do I run C++03 tests locally?

Using the argument -Dstd=c++03 for lit.

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/str.pass.cpp

14 ↗

(On Diff #517514)

Using a different way makes it harder to find the proper test. When the view test is in str I can't find it and assume it's untested.

The tests you made are not complete for view you need to test whether it's a const member and noexcept so the number of #ifdefs will grow.

For real code duplication is bad, for tests it's not always bad to have duplication. For test easy to understand is more important.

[pfusik]

Paper reference, C++20 guards, new tests.

[pfusik]

There's more in the paper. I'd be happy to implement these in subsequent commits.

libcxx/include/sstream

508

How do I do that?

514

Ok. Here it is required (accessing class template base member).

[pfusik]

Add _LIBCPP_HIDE_FROM_ABI

[Mordante]

In D148641#4304594, @pfusik wrote:

There's more in the paper. I'd be happy to implement these in subsequent commits.

Can you for now add notes to the status page to make clear which parts are not done?

libcxx/include/sstream

508

You can use git clang-format like git clang-format-16 HEAD^ this formats the changes in the last commit.

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

24

Why did you add this helper function?

25

Using something along these lines avoid some code duplication. This also makes it easier to adapt the tests if we can use basic_stringstream with other character types like char8_t.

33

This tests the additional properties of the function.

[pfusik]

Is the failed build my fault?

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

24

To test const noexcept. I did not know static_assert(noexcept(...)).

[Mordante]

In D148641#4306046, @pfusik wrote:

Is the failed build my fault?

No unfortunately these 3 CI runners have issues at the moment.

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

24

I think that is quite subtile. I would prefer the making the object const explicitly.

[pfusik]

Documented paper status.
Removed the get_view helper functions from the tests.

[pfusik]

libcxx/include/sstream

508

fox@wiesmac llvm-project % git clang-format HEAD^
clang-format did not modify any files

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

25

Now I support your original claim that tests should be simple. There's not much duplication here.

[Mordante]

I had a closer look at the patch this time and compared it with the papers. Some new remarks, but it's getting close.

libcxx/docs/Status/Cxx20Papers.csv

102

The note needs to linked to this entry. See remark below.

195

Here is an example how to link the note.

libcxx/include/sstream

508

Odd it doesn't work for me either, normally it does. When I select this hunk manully and format it I get

template <class _CharT, class _Traits, class _Allocator>
_LIBCPP_HIDE_FROM_ABI basic_string_view<_CharT, _Traits>
basic_stringbuf<_CharT, _Traits, _Allocator>::view() const noexcept {
    if (__mode_ & ios_base::out) {
        if (__hm_ < this->pptr())
            __hm_ = this->pptr();
        return basic_string_view<_CharT, _Traits>(this->pbase(), __hm_);
    } else if (__mode_ & ios_base::in)
        return basic_string_view<_CharT, _Traits>(this->eback(), this->egptr());
    return basic_string_view<_CharT, _Traits>();
}

512

It seems the tests only validate this code path. The basic_stringbuf is always uses the default which argument. I assume the same issue applies to the original tests you copied. It would be great to improve these tests too in a separate review.

512

The same applies for the _Traits argument, it's only tested for the default traits. We don't have specific char_traits in our test frame-work but you can probably do something like

template <class CharT>
struct my_char_traits : public char_traits<CharT> {};

and use these traits.

Nowadays we try to have more extensive tests then we used to have in the past so that's why the str() tests are as they are.

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

25

This is something we do in a lot of tests, so I like to keep that pattern.

libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/view.pass.cpp

37

Please also test the const view() returns the proper value. This test would pass with a const member that returns basic_string_view<charT>{}.

[pfusik]

Added a note link.
clang-format new code.
Test with templates on CharT.
Test Traits.

libcxx/include/sstream

512

istringstream exercises the second path. I now added testing the second and third path in the stringbuf test.

[Mordante]

LGTM modulo one tiny remark. I leave the final approval to @philnik since he also had comments.

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

35

Just for completeness, please apply to all tests.

[pfusik]

Test noexcept with a non-const object.

[philnik]

libcxx/include/sstream

508

git-clang-format doesn't recognize sstream because it doesn't have a file ending.

libcxx/docs/Status/Cxx20.rst

.. note::
    .. [#note-P0645] P0645: The paper is implemented but still marked as an incomplete feature
       (the feature-test macro is not set and the libary is only available when built with ``-fexperimental-library``).
       Not yet implemented LWG-issues will cause API and ABI breakage.
    .. [#note-P0966] P0966: It was previously erroneously marked as complete in version 8.0. See `bug 45368 <https://llvm.org/PR45368>`__.
    .. [#note-P0619] P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, D.12, and D.14 remain undone.
    .. [#note-P0883.1] P0883: shared_ptr and floating-point changes weren't applied as they themselves aren't implemented yet.
    .. [#note-P0883.2] P0883: ``ATOMIC_FLAG_INIT`` was marked deprecated in version 14.0, but was undeprecated with the implementation of LWG3659 in version 15.0.
    .. [#note-P2231] P2231: Optional is complete. The changes to variant haven't been implemented yet.
+   .. [#note-P0408] P0408: Only `view()` members implemented.
 
 .. _issues-status-cxx20:
 
 Library Working Group Issues Status
 ====================================
 
 .. csv-table::
    :file: Cxx20Issues.csv
    :header-rows: 1
    :widths: auto

libcxx/docs/Status/Cxx20Papers.csv

 "`P1357R1 <https://wg21.link/P1357R1>`__","LWG","Traits for [Un]bounded Arrays","Kona","|Complete|","9.0"
 "`P1458R1 <https://wg21.link/P1458R1>`__","LWG","Mandating the Standard Library: Clause 16 - Language support library","Kona","|Complete|","9.0"
 "`P1459R1 <https://wg21.link/P1459R1>`__","LWG","Mandating the Standard Library: Clause 18 - Diagnostics library","Kona","|Complete|","9.0"
 "`P1462R1 <https://wg21.link/P1462R1>`__","LWG","Mandating the Standard Library: Clause 20 - Strings library","Kona","|Complete|","9.0"
 "`P1463R1 <https://wg21.link/P1463R1>`__","LWG","Mandating the Standard Library: Clause 21 - Containers library","Kona","",""
 "`P1464R1 <https://wg21.link/P1464R1>`__","LWG","Mandating the Standard Library: Clause 22 - Iterators library","Kona","|Complete|","9.0"
 "","","","","","",""
 "`P0325R4 <https://wg21.link/P0325R4>`__","LWG","to_array from LFTS with updates","Cologne","|Complete|","10.0"
-"`P0408R7 <https://wg21.link/P0408R7>`__","LWG","Efficient Access to basic_stringbuf's Buffer","Cologne","",""
+"`P0408R7 <https://wg21.link/P0408R7>`__","LWG","Efficient Access to basic_stringbuf's Buffer","Cologne","|In Progress| [#note-P0408]_",""
 "`P0466R5 <https://wg21.link/P0466R5>`__","LWG","Layout-compatibility and Pointer-interconvertibility Traits","Cologne","",""
 "`P0553R4 <https://wg21.link/P0553R4>`__","LWG","Bit operations","Cologne","|Complete|","9.0"
 "`P0631R8 <https://wg21.link/P0631R8>`__","LWG","Math Constants","Cologne","|Complete|","11.0"
 "`P0645R10 <https://wg21.link/P0645R10>`__","LWG","Text Formatting","Cologne","|Complete| [#note-P0645]_","14.0"
 "`P0660R10 <https://wg21.link/P0660R10>`__","LWG","Stop Token and Joining Thread, Rev 10","Cologne","",""
 "`P0784R7 <https://wg21.link/P0784R7>`__","CWG","More constexpr containers","Cologne","|Complete|","12.0"
 "`P0980R1 <https://wg21.link/P0980R1>`__","LWG","Making std::string constexpr","Cologne","|Complete|","15.0"
 "`P1004R2 <https://wg21.link/P1004R2>`__","LWG","Making std::vector constexpr","Cologne","|Complete|","15.0"

 "`P2045R1 <https://wg21.link/P2045R1>`__","LWG","Missing Mandates for the standard library","Prague","* *",""
 "`P2085R0 <https://wg21.link/P2085R0>`__","CWG","Consistent defaulted comparisons","Prague","* *",""
 "`P2091R0 <https://wg21.link/P2091R0>`__","LWG","Issues with range access CPOs","Prague","|Complete|","15.0","|ranges|"
 "`P2101R0 <https://wg21.link/P2101R0>`__","LWG","'Models' subsumes 'satisfies' (Wording for US298 and US300)","Prague","* *",""
 "`P2102R0 <https://wg21.link/P2102R0>`__","LWG","Make 'implicit expression variations' more explicit (Wording for US185)","Prague","* *",""
 "`P2106R0 <https://wg21.link/P2106R0>`__","LWG","Alternative wording for GB315 and GB316","Prague","|Complete|","15.0","|ranges|"
 "`P2116R0 <https://wg21.link/P2116R0>`__","LWG","Remove tuple-like protocol support from fixed-extent span","Prague","|Complete|","11.0"
 "","","","","","",""
 "`P2231R1 <https://wg21.link/P2231R1>`__","LWG","Missing constexpr in std::optional and std::variant","June 2021","|Partial| [#note-P2231]_","13.0"
 "`P2325R3 <https://wg21.link/P2325R3>`__","LWG","Views should not be required to be default constructible","June 2021","|Complete|","16.0","|ranges|"
 "`P2210R2 <https://wg21.link/P2210R2>`__","LWG","Superior String Splitting","June 2021","|Complete|","16.0","|ranges|"
 "`P2216R3 <https://wg21.link/P2216R3>`__","LWG","std::format improvements","June 2021","|Complete|","15.0"
 "`P2281R1 <https://wg21.link/P2281R1>`__","LWG","Clarifying range adaptor objects","June 2021","|Complete|","14.0","|ranges|"
 "`P2328R1 <https://wg21.link/P2328R1>`__","LWG","join_view should join all views of ranges","June 2021","|Complete|","15.0","|ranges|"
 "`P2367R0 <https://wg21.link/P2367R0>`__","LWG","Remove misuses of list-initialization from Clause 24","June 2021","|Complete|","15.0","|ranges|"
 "","","","","","",""
 "`P2372R3 <https://wg21.link/P2372R3>`__","LWG","Fixing locale handling in chrono formatters","October 2021","|In Progress|",""
 "`P2415R2 <https://wg21.link/P2415R2>`__","LWG","What is a ``view``","October 2021","|Complete|","14.0","|ranges|"
 "`P2418R2 <https://wg21.link/P2418R2>`__","LWG","Add support for ``std::generator``-like types to ``std::format``","October 2021","|Complete|","15.0"
 "`P2432R1 <https://wg21.link/P2432R1>`__","LWG","Fix ``istream_view``","October 2021","|Complete|","16.0","|ranges|"

libcxx/include/sstream

public:
 
     // [stringbuf.cons] constructors:
     explicit basic_stringbuf(ios_base::openmode which = ios_base::in | ios_base::out); // before C++20
     basic_stringbuf() : basic_stringbuf(ios_base::in | ios_base::out) {}               // C++20
     explicit basic_stringbuf(ios_base::openmode which);                                // C++20
     explicit basic_stringbuf(const basic_string<char_type, traits_type, allocator_type>& str,
                              ios_base::openmode which = ios_base::in | ios_base::out);
     basic_stringbuf(basic_stringbuf&& rhs);
 
     // [stringbuf.assign] Assign and swap:
     basic_stringbuf& operator=(basic_stringbuf&& rhs);
     void swap(basic_stringbuf& rhs);
 
     // [stringbuf.members] Member functions:
     basic_string<char_type, traits_type, allocator_type> str() const;
     void str(const basic_string<char_type, traits_type, allocator_type>& s);
+    basic_string_view<char_type, traits_type> view() const noexcept; // C++20
 
 protected:
     // [stringbuf.virtuals] Overridden virtual functions:
     virtual int_type underflow();
     virtual int_type pbackfail(int_type c = traits_type::eof());
     virtual int_type overflow (int_type c = traits_type::eof());
     virtual basic_streambuf<char_type, traits_type>* setbuf(char_type*, streamsize);
     virtual pos_type seekoff(off_type off, ios_base::seekdir way,
public:
     // [istringstream.assign] Assign and swap:
     basic_istringstream& operator=(basic_istringstream&& rhs);
     void swap(basic_istringstream& rhs);
 
     // [istringstream.members] Member functions:
     basic_stringbuf<char_type, traits_type, allocator_type>* rdbuf() const;
     basic_string<char_type, traits_type, allocator_type> str() const;
     void str(const basic_string<char_type, traits_type, allocator_type>& s);
+    basic_string_view<char_type, traits_type> view() const noexcept; // C++20
 };
 
 template <class charT, class traits, class Allocator>
   void swap(basic_istringstream<charT, traits, Allocator>& x,
             basic_istringstream<charT, traits, Allocator>& y);
 
 typedef basic_istringstream<char>    istringstream;
 typedef basic_istringstream<wchar_t> wistringstream;
public:
     // [ostringstream.assign] Assign and swap:
     basic_ostringstream& operator=(basic_ostringstream&& rhs);
     void swap(basic_ostringstream& rhs);
 
     // [ostringstream.members] Member functions:
     basic_stringbuf<char_type, traits_type, allocator_type>* rdbuf() const;
     basic_string<char_type, traits_type, allocator_type> str() const;
     void str(const basic_string<char_type, traits_type, allocator_type>& s);
+    basic_string_view<char_type, traits_type> view() const noexcept; // C++20
 };
 
 template <class charT, class traits, class Allocator>
   void swap(basic_ostringstream<charT, traits, Allocator>& x,
             basic_ostringstream<charT, traits, Allocator>& y);
 
 typedef basic_ostringstream<char>    ostringstream;
 typedef basic_ostringstream<wchar_t> wostringstream;
public:
     // [stringstream.assign] Assign and swap:
     basic_stringstream& operator=(basic_stringstream&& rhs);
     void swap(basic_stringstream& rhs);
 
     // [stringstream.members] Member functions:
     basic_stringbuf<char_type, traits_type, allocator_type>* rdbuf() const;
     basic_string<char_type, traits_type, allocator_type> str() const;
     void str(const basic_string<char_type, traits_type, allocator_type>& str);
+    basic_string_view<char_type, traits_type> view() const noexcept; // C++20
 };
 
 template <class charT, class traits, class Allocator>
   void swap(basic_stringstream<charT, traits, Allocator>& x,
             basic_stringstream<charT, traits, Allocator>& y);
 
 typedef basic_stringstream<char>    stringstream;
 typedef basic_stringstream<wchar_t> wstringstream;
public:
 
     // [stringbuf.assign] Assign and swap:
     basic_stringbuf& operator=(basic_stringbuf&& __rhs);
     void swap(basic_stringbuf& __rhs);
 
     // [stringbuf.members] Member functions:
     string_type str() const;
     void str(const string_type& __s);
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI
+    basic_string_view<char_type, traits_type> view() const noexcept;
+#endif
 
 protected:
     // [stringbuf.virtuals] Overridden virtual functions:
     int_type underflow() override;
     int_type pbackfail(int_type __c = traits_type::eof()) override;
     int_type overflow (int_type __c = traits_type::eof()) override;
     pos_type seekoff(off_type __off, ios_base::seekdir __way,
                      ios_base::openmode __wch = ios_base::in | ios_base::out) override;

 {
     __x.swap(__y);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>
 basic_stringbuf<_CharT, _Traits, _Allocator>::str() const
 {
+#if _LIBCPP_STD_VER >= 20
+    return string_type(view(), __str_.get_allocator());
+#else // _LIBCPP_STD_VER >= 20
     if (__mode_ & ios_base::out)
     {
         if (__hm_ < this->pptr())
             __hm_ = this->pptr();
         return string_type(this->pbase(), __hm_, __str_.get_allocator());
     }
     else if (__mode_ & ios_base::in)
         return string_type(this->eback(), this->egptr(), __str_.get_allocator());
     return string_type(__str_.get_allocator());
+#endif // _LIBCPP_STD_VER >= 20
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 void
 basic_stringbuf<_CharT, _Traits, _Allocator>::str(const string_type& __s)
 {
     __str_ = __s;
     __hm_ = nullptr;
if (__mode_ & ios_base::out)
                 __sz -= INT_MAX;
             }
             if (__sz > 0)
                 this->pbump(__sz);
         }
     }
 }
 
+#if _LIBCPP_STD_VER >= 20
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_HIDE_FROM_ABI basic_string_view<_CharT, _Traits>
+basic_stringbuf<_CharT, _Traits, _Allocator>::view() const noexcept {
+    if (__mode_ & ios_base::out) {
+        if (__hm_ < this->pptr())
+            __hm_ = this->pptr();
+        return basic_string_view<_CharT, _Traits>(this->pbase(), __hm_);
+    } else if (__mode_ & ios_base::in)
+        return basic_string_view<_CharT, _Traits>(this->eback(), this->egptr());
+    return basic_string_view<_CharT, _Traits>();
+}
+#endif // _LIBCPP_STD_VER >= 20
+
 template <class _CharT, class _Traits, class _Allocator>
 typename basic_stringbuf<_CharT, _Traits, _Allocator>::int_type
 basic_stringbuf<_CharT, _Traits, _Allocator>::underflow()
 {
     if (__hm_ < this->pptr())
         __hm_ = this->pptr();
     if (__mode_ & ios_base::in)
     {
public:
     _LIBCPP_INLINE_VISIBILITY
     string_type str() const {
         return __sb_.str();
     }
     _LIBCPP_INLINE_VISIBILITY
     void str(const string_type& __s) {
         __sb_.str(__s);
     }
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI
+    basic_string_view<char_type, traits_type> view() const noexcept {
+        return __sb_.view();
+    }
+#endif
 };
 
 template <class _CharT, class _Traits, class _Allocator>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 swap(basic_istringstream<_CharT, _Traits, _Allocator>& __x,
      basic_istringstream<_CharT, _Traits, _Allocator>& __y)
 {
public:
     _LIBCPP_INLINE_VISIBILITY
     string_type str() const {
         return __sb_.str();
     }
     _LIBCPP_INLINE_VISIBILITY
     void str(const string_type& __s) {
         __sb_.str(__s);
     }
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI
+    basic_string_view<char_type, traits_type> view() const noexcept {
+        return __sb_.view();
+    }
+#endif
 };
 
 template <class _CharT, class _Traits, class _Allocator>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 swap(basic_ostringstream<_CharT, _Traits, _Allocator>& __x,
      basic_ostringstream<_CharT, _Traits, _Allocator>& __y)
 {
public:
     _LIBCPP_INLINE_VISIBILITY
     string_type str() const {
         return __sb_.str();
     }
     _LIBCPP_INLINE_VISIBILITY
     void str(const string_type& __s) {
         __sb_.str(__s);
     }
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI
+    basic_string_view<char_type, traits_type> view() const noexcept {
+        return __sb_.view();
+    }
+#endif
 };
 
 template <class _CharT, class _Traits, class _Allocator>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 swap(basic_stringstream<_CharT, _Traits, _Allocator>& __x,
      basic_stringstream<_CharT, _Traits, _Allocator>& __y)
 {

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/view.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
+// class basic_istringstream
+
+// basic_string_view<charT, traits> view() const noexcept;
+
+#include <sstream>
+#include <cassert>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+struct my_char_traits : public std::char_traits<CharT> {};
+
+template <class CharT>
+static void test() {
+  std::basic_istringstream<CharT> ss(STR(" 123 456"));
+  static_assert(noexcept(ss.view()));
+  assert(ss.view() == SV(" 123 456"));
+  ss.str(STR(" 789"));
+  ss.clear();
+  assert(ss.view() == SV(" 789"));
+
+  const std::basic_istringstream<CharT> css(STR("abc"));
+  static_assert(noexcept(css.view()));
+  assert(css.view() == SV("abc"));
+
+  std::basic_istringstream<CharT, my_char_traits<CharT>> tss;
+  static_assert(std::is_same_v<decltype(tss.view()), std::basic_string_view<CharT, my_char_traits<CharT>>>);
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}

libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/view.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
+// class basic_ostringstream
+
+// basic_string_view<charT, traits> view() const noexcept;
+
+#include <sstream>
+#include <cassert>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+struct my_char_traits : public std::char_traits<CharT> {};
+
+template <class CharT>
+static void test() {
+  std::basic_ostringstream<CharT> ss(STR(" 123 456"));
+  static_assert(noexcept(ss.view()));
+  assert(ss.view() == SV(" 123 456"));
+  int i = 0;
+  ss << i;
+  assert(ss.view() == SV("0123 456"));
+  ss << 456;
+  assert(ss.view() == SV("0456 456"));
+  ss.str(STR(" 789"));
+  assert(ss.view() == SV(" 789"));
+  ss << SV("abc");
+  assert(ss.view() == SV("abc9"));
+
+  const std::basic_ostringstream<CharT> css(STR("abc"));
+  static_assert(noexcept(css.view()));
+  assert(css.view() == SV("abc"));
+
+  std::basic_ostringstream<CharT, my_char_traits<CharT>> tss;
+  static_assert(std::is_same_v<decltype(tss.view()), std::basic_string_view<CharT, my_char_traits<CharT>>>);
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.members/view.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
+// class basic_stringbuf
+
+// basic_string_view<charT, traits> view() const noexcept;
+
+#include <sstream>
+#include <cassert>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+struct my_char_traits : public std::char_traits<CharT> {};
+
+template <class CharT>
+static void test() {
+  std::basic_stringbuf<CharT> buf(STR("testing"));
+  static_assert(noexcept(buf.view()));
+  assert(buf.view() == SV("testing"));
+  buf.str(STR("another test"));
+  assert(buf.view() == SV("another test"));
+
+  std::basic_stringbuf<CharT> robuf(STR("foo"), std::ios_base::in);
+  assert(robuf.view() == SV("foo"));
+
+  std::basic_stringbuf<CharT> nbuf(STR("not used"), 0);
+  assert(nbuf.view() == std::basic_string_view<CharT>());
+
+  const std::basic_stringbuf<CharT> cbuf(STR("abc"));
+  static_assert(noexcept(cbuf.view()));
+  assert(cbuf.view() == SV("abc"));
+
+  std::basic_stringbuf<CharT, my_char_traits<CharT>> tbuf;
+  static_assert(std::is_same_v<decltype(tbuf.view()), std::basic_string_view<CharT, my_char_traits<CharT>>>);
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}

libcxx/test/std/input.output/string.streams/stringstream.members/view.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <sstream>
+
+// template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
+// class basic_stringstream
+
+// basic_string_view<charT, traits> view() const noexcept;
+
+#include <sstream>
+#include <cassert>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+struct my_char_traits : public std::char_traits<CharT> {};
+
+template <class CharT>
+static void test() {
+  std::basic_stringstream<CharT> ss(STR(" 123 456 "));
+  static_assert(noexcept(ss.view()));
+  assert(ss.view() == SV(" 123 456 "));
+  int i = 0;
+  ss >> i;
+  assert(i == 123);
+  ss >> i;
+  assert(i == 456);
+  ss << i << ' ' << 123;
+  assert(ss.view() == SV("456 1236 "));
+  ss.str(STR("5466 89 "));
+  ss >> i;
+  assert(i == 5466);
+  ss >> i;
+  assert(i == 89);
+  ss << i << ' ' << 321;
+  assert(ss.view() == SV("89 3219 "));
+
+  const std::basic_stringstream<CharT> css(STR("abc"));
+  static_assert(noexcept(css.view()));
+  assert(css.view() == SV("abc"));
+
+  std::basic_stringstream<CharT, my_char_traits<CharT>> tss;
+  static_assert(std::is_same_v<decltype(tss.view()), std::basic_string_view<CharT, my_char_traits<CharT>>>);
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  std::stringstream ss;
+  ss.write("\xd1", 1);
+  assert(ss.view().length() == 1);
+  return 0;
+}