[libc++][string] P3044R2: sub-string_view from string

libcxx/docs/FeatureTestMacroTable.rst

@@ -404,6 +404,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_string_resize_and_overwrite``                  ``202110L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_string_subview``                               ``202506L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_to_underlying``                                ``202102L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_tuple_like``                                   *unimplemented*

libcxx/docs/ReleaseNotes/22.rst

@@ -39,6 +39,7 @@ Implemented Papers
 ------------------
 
 - P2321R2: ``zip`` (`Github <https://github.com/llvm/llvm-project/issues/105169>`__) (The paper is partially implemented. ``zip_transform_view`` is implemented in this release)
+- P3044R2: sub-``string_view`` from ``string`` (`Github <https://github.com/llvm/llvm-project/issues/148140>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx2cPapers.csv

@@ -129,7 +129,7 @@
 "`P3179R9 <https://wg21.link/P3179R9>`__","Parallel Range Algorithms","2025-06 (Sofia)","","",""
 "`P3709R2 <https://wg21.link/P3709R2>`__","Reconsider parallel ``ranges::rotate_copy`` and ``ranges::reverse_copy``","2025-06 (Sofia)","","",""
 "`P3641R0 <https://wg21.link/P3641R0>`__","Rename ``std::observable`` to ``std::observable_checkpoint``, and add a feature-test macro","2025-06 (Sofia)","","",""
-"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","","",""
+"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","|Complete|","22",""
 "`P2876R3 <https://wg21.link/P2876R3>`__","Proposal to extend ``std::simd`` with more constructors and accessors","2025-06 (Sofia)","","",""
 "`P3480R6 <https://wg21.link/P3480R6>`__","``std::simd`` is a range","2025-06 (Sofia)","","",""
 "`P2664R11 <https://wg21.link/P2664R11>`__","Extend ``std::simd`` with permutation API","2025-06 (Sofia)","","",""

libcxx/include/string

@@ -280,6 +280,8 @@ public:
     basic_string substr(size_type pos = 0, size_type n = npos) const;                           // constexpr in C++20, removed in C++23
     basic_string substr(size_type pos = 0, size_type n = npos) const&;                          // since C++23
     constexpr basic_string substr(size_type pos = 0, size_type n = npos) &&;                    // since C++23
+    constexpr basic_string_view<charT, traits> subview(size_type pos = 0,
+                                                       size_type n = npos) const;               // since C++26
     void swap(basic_string& str)
         noexcept(allocator_traits<allocator_type>::propagate_on_container_swap::value ||
                  allocator_traits<allocator_type>::is_always_equal::value);                     // C++17, constexpr since C++20
@@ -1752,6 +1754,13 @@ public:
   }
 #  endif
 
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_string_view<_CharT, _Traits>
+  subview(size_type __pos = 0, size_type __n = npos) const {
+    return basic_string_view<_CharT, _Traits>(*this).subview(__pos, __n);
+  }
+#  endif
+
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void swap(basic_string& __str)
 #  if _LIBCPP_STD_VER >= 14
       _NOEXCEPT;

libcxx/include/string_view

@@ -130,6 +130,8 @@ namespace std {
       size_type copy(charT* s, size_type n, size_type pos = 0) const;  // constexpr in C++20
 
       constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;
+      constexpr basic_string_view subview(size_type pos = 0,
+                                          size_type n = npos) const;      // freestanding-deleted, since C++26
       constexpr int compare(basic_string_view s) const noexcept;
       constexpr int compare(size_type pos1, size_type n1, basic_string_view s) const;
       constexpr int compare(size_type pos1, size_type n1,
@@ -465,6 +467,12 @@ public:
                           : basic_string_view(__assume_valid(), data() + __pos, std::min(__n, size() - __pos));
   }
 
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_string_view subview(size_type __pos = 0, size_type __n = npos) const {
+    return this->substr(__pos, __n);
+  }
+#  endif
+
   _LIBCPP_CONSTEXPR_SINCE_CXX14 int compare(basic_string_view __sv) const _NOEXCEPT {
     size_type __rlen = std::min(size(), __sv.size());
     int __retval     = _Traits::compare(data(), __sv.data(), __rlen);

libcxx/include/version

@@ -245,6 +245,7 @@ __cpp_lib_starts_ends_with                              201711L <string> <string
 __cpp_lib_stdatomic_h                                   202011L <stdatomic.h>
 __cpp_lib_string_contains                               202011L <string> <string_view>
 __cpp_lib_string_resize_and_overwrite                   202110L <string>
+__cpp_lib_string_subview                                202506L <string> <string_view>
 __cpp_lib_string_udls                                   201304L <string>
 __cpp_lib_string_view                                   202403L <string> <string_view>
                                                         201803L // C++20
@@ -535,6 +536,7 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_stdatomic_h                          202011L
 # define __cpp_lib_string_contains                      202011L
 # define __cpp_lib_string_resize_and_overwrite          202110L
+# define __cpp_lib_string_subview                       202506L
 # define __cpp_lib_to_underlying                        202102L
 // # define __cpp_lib_tuple_like                           202207L
 # define __cpp_lib_unreachable                          202202L

libcxx/test/std/language.support/support.limits/support.limits.general/string.version.compile.pass.cpp

@@ -60,6 +60,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifdef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should not be defined before c++14"
 #  endif
@@ -114,6 +118,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++14"
 #  endif
@@ -177,6 +185,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++17"
 #  endif
@@ -261,6 +273,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++20"
 #  endif
@@ -354,6 +370,13 @@
 #    error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++23"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++23"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++23"
 #  endif
@@ -456,6 +479,13 @@
 #    error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++26"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++26"
 #  endif

libcxx/test/std/language.support/support.limits/support.limits.general/string_view.version.compile.pass.cpp

@@ -40,6 +40,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifdef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should not be defined before c++17"
 #  endif
@@ -66,6 +70,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifdef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should not be defined before c++17"
 #  endif
@@ -92,6 +100,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++17"
 #  endif
@@ -136,6 +148,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++20"
 #  endif
@@ -183,6 +199,13 @@
 #    error "__cpp_lib_string_contains should have the value 202011L in c++23"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++23"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++23"
 #  endif
@@ -239,6 +262,13 @@
 #    error "__cpp_lib_string_contains should have the value 202011L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++26"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++26"
 #  endif

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

@@ -820,6 +820,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifdef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should not be defined before c++14"
 #  endif
@@ -1775,6 +1779,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++14"
 #  endif
@@ -2916,6 +2924,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++17"
 #  endif
@@ -4330,6 +4342,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++20"
 #  endif
@@ -5978,6 +5994,13 @@
 #    error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++23"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++23"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++23"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++23"
 #  endif
@@ -7947,6 +7970,13 @@
 #    error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++26"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++26"
 #  endif

libcxx/test/std/strings/basic.string/string.ops/string_substr/subview.pass.cpp

@@ -0,0 +1,88 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <string>
+
+// constexpr basic_string_view<_CharT, _Traits> subview(size_type __pos = 0, size_type __n = npos) const;
+
+#include <cassert>
+#include <string>
+
+#include "constexpr_char_traits.h"
+#include "make_string.h"
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define CS(S) MAKE_CSTRING(CharT, S)
+
+template <typename CharT, typename TraitsT, typename AllocT>
+constexpr void test() {
+  std::basic_string<CharT, TraitsT, AllocT> s{CS("Hello cruel world!"), AllocT{}};
+
+  { // With a default position and a character length.
+    std::same_as<std::basic_string_view<CharT, TraitsT>> decltype(auto) sv = s.subview();
+    assert(sv == CS("Hello cruel world!"));
+  }
+
+  { // With a explict position and a character length.
+    std::same_as<std::basic_string_view<CharT, TraitsT>> decltype(auto) sv = s.subview(6, 5);
+    assert(sv == CS("cruel"));
+  }
+
+  { // From the beginning of the string with a explicit character length.
+    std::same_as<std::basic_string_view<CharT, TraitsT>> decltype(auto) sv = s.subview(0, 5);
+    assert(sv == CS("Hello"));
+  }
+
+  { // To the end of string with the default character length.
+    std::same_as<std::basic_string_view<CharT, TraitsT>> decltype(auto) sv = s.subview(12);
+    assert(sv == CS("world!"));
+  }
+
+  { // From the beginning to the end of the string with explicit values.
+    std::same_as<std::basic_string_view<CharT, TraitsT>> decltype(auto) sv = s.subview(0, s.size());
+    assert(sv == CS("Hello cruel world!"));
+  }
+}
+
+template <typename CharT>
+constexpr void test() {
+  test<CharT, std::char_traits<CharT>, std::allocator<CharT>>();
+  test<CharT, std::char_traits<CharT>, min_allocator<CharT>>();
+  test<CharT, std::char_traits<CharT>, safe_allocator<CharT>>();
+  test<CharT, std::char_traits<CharT>, test_allocator<CharT>>();
+
+  test<CharT, constexpr_char_traits<CharT>, std::allocator<CharT>>();
+  test<CharT, constexpr_char_traits<CharT>, min_allocator<CharT>>();
+  test<CharT, constexpr_char_traits<CharT>, safe_allocator<CharT>>();
+  test<CharT, constexpr_char_traits<CharT>, test_allocator<CharT>>();
+}
+
+constexpr bool test() {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+#ifndef TEST_HAS_NO_CHAR8_T
+  test<char8_t>();
+#endif
+  test<char16_t>();
+  test<char32_t>();
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}