[libc++][string] P3044R2: sub-`string_view` from `string` #147095

H-G-Hristov · 2025-07-04T18:36:14Z

Implements P3044R2

Note: substr.pass.cpp is refactored to accommodate the test of basic_string_view's subview which is an alias of substr without changing the test cases.

Closes #148140

References

llvmbot · 2025-07-05T00:17:13Z

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

Implements P3044R2

Note: substr.pass.cpp is refactored to accommodate the test of basic_string_view's subview which is an alias of substr without changing the test cases.

References

P3044R2 sub-string_view from string cplusplus/draft#7975

Full diff: https://github.com/llvm/llvm-project/pull/147095.diff

5 Files Affected:

(modified) libcxx/include/string (+9)
(modified) libcxx/include/string_view (+8)
(added) libcxx/test/std/strings/basic.string/string.ops/string_substr/subview.pass.cpp (+33)
(modified) libcxx/test/std/strings/string.view/string.view.ops/substr.pass.cpp (+92-60)
(added) libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp (+20)

diff --git a/libcxx/include/string b/libcxx/include/string
index c450ddcb41914..9476c3df3b8e7 100644
--- a/libcxx/include/string
+++ b/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
@@ -1745,6 +1747,13 @@ 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;
diff --git a/libcxx/include/string_view b/libcxx/include/string_view
index 861187c0640e1..ad036882e1630 100644
--- a/libcxx/include/string_view
+++ b/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);
diff --git a/libcxx/test/std/strings/basic.string/string.ops/string_substr/subview.pass.cpp b/libcxx/test/std/strings/basic.string/string.ops/string_substr/subview.pass.cpp
new file mode 100644
index 0000000000000..65418dcb4d7be
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.ops/string_substr/subview.pass.cpp
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+
+constexpr bool test() {
+  std::string s{"Hello cruel world!"};
+  auto sub = s.subview(6);
+  assert(sub == "cruel world!");
+  auto subsub = sub.subview(0, 5);
+  assert(subsub == "cruel");
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/strings/string.view/string.view.ops/substr.pass.cpp b/libcxx/test/std/strings/string.view/string.view.ops/substr.pass.cpp
index 62b0259c175f8..02755985ead70 100644
--- a/libcxx/test/std/strings/string.view/string.view.ops/substr.pass.cpp
+++ b/libcxx/test/std/strings/string.view/string.view.ops/substr.pass.cpp
@@ -11,6 +11,10 @@
 // <string_view>
 
 // 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
+
+// subview is alternative name of substr
 
 // Throws: out_of_range if pos > size().
 // Effects: Determines the effective length rlen of the string to reference as the smaller of n and size() - pos.
@@ -25,15 +29,21 @@
 #include "test_macros.h"
 
 template <typename CharT>
-void test1(std::basic_string_view<CharT> sv, std::size_t n, size_t pos) {
+struct Test {
+  typedef std::basic_string_view<CharT> (std::basic_string_view<CharT>::*Sub)(
+      typename std::basic_string_view<CharT>::size_type, typename std::basic_string_view<CharT>::size_type) const;
+};
+
+template <typename CharT, typename Test<CharT>::Sub TestSub>
+void testDetail(std::basic_string_view<CharT> sv, std::size_t n, size_t pos) {
   std::basic_string_view<CharT> sv1;
 #ifdef TEST_HAS_NO_EXCEPTIONS
   if (pos > sv.size())
     return; // would throw if exceptions were enabled
-  sv1 = sv.substr(pos, n);
+  sv1 = (sv.*TestSub)(pos, n);
 #else
   try {
-    sv1 = sv.substr(pos, n);
+    sv1 = (sv.*TestSub)(pos, n);
     assert(pos <= sv.size());
   } catch (const std::out_of_range&) {
     assert(pos > sv.size());
@@ -46,79 +56,101 @@ void test1(std::basic_string_view<CharT> sv, std::size_t n, size_t pos) {
     assert(sv[pos + i] == sv1[i]);
 }
 
-template <typename CharT>
-void test(const CharT* s) {
-  typedef std::basic_string_view<CharT> string_view_t;
+template <typename CharT, typename Test<CharT>::Sub TestSub>
+void testCases(const CharT* s) {
+  std::basic_string_view<CharT> sv(s);
 
-  string_view_t sv1(s);
+  testDetail<CharT, TestSub>(sv, 0, 0);
+  testDetail<CharT, TestSub>(sv, 1, 0);
+  testDetail<CharT, TestSub>(sv, 20, 0);
+  testDetail<CharT, TestSub>(sv, sv.size(), 0);
 
-  test1(sv1, 0, 0);
-  test1(sv1, 1, 0);
-  test1(sv1, 20, 0);
-  test1(sv1, sv1.size(), 0);
+  testDetail<CharT, TestSub>(sv, 100, 3);
 
-  test1(sv1, 0, 3);
-  test1(sv1, 2, 3);
-  test1(sv1, 100, 3);
+  testDetail<CharT, TestSub>(sv, 0, std::basic_string_view<CharT>::npos);
+  testDetail<CharT, TestSub>(sv, 2, std::basic_string_view<CharT>::npos);
+  testDetail<CharT, TestSub>(sv, sv.size(), std::basic_string_view<CharT>::npos);
 
-  test1(sv1, 0, string_view_t::npos);
-  test1(sv1, 2, string_view_t::npos);
-  test1(sv1, sv1.size(), string_view_t::npos);
+  testDetail<CharT, TestSub>(sv, sv.size() + 1, 0);
+  testDetail<CharT, TestSub>(sv, sv.size() + 1, 1);
+  testDetail<CharT, TestSub>(sv, sv.size() + 1, std::basic_string_view<CharT>::npos);
+}
 
-  test1(sv1, sv1.size() + 1, 0);
-  test1(sv1, sv1.size() + 1, 1);
-  test1(sv1, sv1.size() + 1, string_view_t::npos);
+template <typename CharT>
+void testSubs(const CharT* s) {
+  testCases<CharT, &std::basic_string_view<CharT>::substr>(s);
+#if TEST_STD_VER >= 26
+  testCases<CharT, &std::basic_string_view<CharT>::subview>(s);
+#endif // TEST_STD_VER >= 26
 }
 
-int main(int, char**) {
-  test("ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
-  test("ABCDE");
-  test("a");
-  test("");
+void test() {
+  testSubs("ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
+  testSubs("ABCDE");
+  testSubs("a");
+  testSubs("");
 
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  test(L"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
-  test(L"ABCDE");
-  test(L"a");
-  test(L"");
+  testSubs(
+      L"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
+  testSubs(L"ABCDE");
+  testSubs(L"a");
+  testSubs(L"");
 #endif
 
 #if TEST_STD_VER >= 11
-  test(u"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
-  test(u"ABCDE");
-  test(u"a");
-  test(u"");
-
-  test(U"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
-  test(U"ABCDE");
-  test(U"a");
-  test(U"");
+  testSubs(
+      u"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
+  testSubs(u"ABCDE");
+  testSubs(u"a");
+  testSubs(u"");
+
+  testSubs(
+      U"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE");
+  testSubs(U"ABCDE");
+  testSubs(U"a");
+  testSubs(U"");
 #endif
+}
+
+#if TEST_STD_VER >= 14
+template <typename Test<char>::Sub TestSub>
+constexpr void testConstexprDetail() {
+  constexpr std::string_view sv{"ABCDE", 5};
+  {
+    constexpr std::string_view sv2 = (sv.*TestSub)(0, 3);
+
+    static_assert(sv2.size() == 3, "");
+    static_assert(sv2[0] == 'A', "");
+    static_assert(sv2[1] == 'B', "");
+    static_assert(sv2[2] == 'C', "");
+  }
 
-#if TEST_STD_VER > 11
   {
-    constexpr std::string_view sv1{"ABCDE", 5};
-
-    {
-      constexpr std::string_view sv2 = sv1.substr(0, 3);
-      static_assert(sv2.size() == 3, "");
-      static_assert(sv2[0] == 'A', "");
-      static_assert(sv2[1] == 'B', "");
-      static_assert(sv2[2] == 'C', "");
-    }
-
-    {
-      constexpr std::string_view sv2 = sv1.substr(3, 0);
-      static_assert(sv2.size() == 0, "");
-    }
-
-    {
-      constexpr std::string_view sv2 = sv1.substr(3, 3);
-      static_assert(sv2.size() == 2, "");
-      static_assert(sv2[0] == 'D', "");
-      static_assert(sv2[1] == 'E', "");
-    }
+    constexpr std::string_view sv2 = (sv.*TestSub)(3, 0);
+    static_assert(sv2.size() == 0, "");
   }
+
+  {
+    constexpr std::string_view sv2 = (sv.*TestSub)(3, 3);
+    static_assert(sv2.size() == 2, "");
+    static_assert(sv2[0] == 'D', "");
+    static_assert(sv2[1] == 'E', "");
+  }
+}
+
+void test_constexpr() {
+  testConstexprDetail<&std::string_view::substr>();
+#  if TEST_STD_VER >= 26
+  testConstexprDetail<&std::string_view::subview>();
+#  endif
+}
+#endif
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 14
+  test_constexpr();
 #endif
 
   return 0;
diff --git a/libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp b/libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp
new file mode 100644
index 0000000000000..fec258eef4546
--- /dev/null
+++ b/libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp
@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_view>
+
+// constexpr basic_string_view subview(size_type pos = 0,
+//                                     size_type n = npos) const;      // freestanding-deleted
+
+int main(int, char**) {
+  // This test is intentionally empty because subview is an alias for substr
+  // and is tested in substr.pass.cpp.
+  return 0;
+}

libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp

Implements [P3044R2](https://wg21.link/P3044R2) - cplusplus/draft#7975

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

libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp

libcxx/test/std/strings/string.view/string.view.ops/substr.pass.cpp

…s://github.com/H-G-Hristov/llvm-project into hgh/libcxx/P3044R2-sub-string_view-from-string

github-actions · 2025-08-06T15:58:26Z

✅ With the latest revision this PR passed the C/C++ code formatter.

libcxx/test/std/strings/string.view/string.view.ops/subview.pass.cpp

…ss.cpp

Zingam added libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. c++26 labels Jul 5, 2025

frederick-vs-ja reviewed Jul 5, 2025

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H-G-Hristov added 2 commits July 22, 2025 19:27

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

0510c78

Implements [P3044R2](https://wg21.link/P3044R2) - cplusplus/draft#7975

Renamed test

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H-G-Hristov force-pushed the hgh/libcxx/P3044R2-sub-string_view-from-string branch from 582e674 to ad5e410 Compare July 22, 2025 17:08

H-G-Hristov added 2 commits July 22, 2025 22:01

LLVM22

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Updated test

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H-G-Hristov changed the title ~~WIP [libc++][string] P3044R2: sub-string_view from string~~ [libc++][string] P3044R2: sub-string_view from string Jul 23, 2025

H-G-Hristov marked this pull request as ready for review July 23, 2025 03:44

H-G-Hristov requested a review from a team as a code owner July 23, 2025 03:44

Fix the test!!!

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H-G-Hristov added 9 commits August 2, 2025 07:35

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Merge branch 'main' into hgh/libcxx/P3044R2-sub-string_view-from-string

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Merge branch 'hgh/libcxx/P3044R2-sub-string_view-from-string' of http…

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Merge branch 'main' into hgh/libcxx/P3044R2-sub-string_view-from-string

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