[libc++] Eliminate extra allocations from `std::move(oss).str()`

[ldionne]

Summary

As an extension, support string(std::move(s), pos, n) in C++20
as well as C++23. Otherwise ,it's difficult to implement the
behavior for std::move(oss).str() in C++

Add test coverage for the new behaviors, especially to verify
the returned string uses the correct allocator.

Fixes the issue https://github.com/llvm/llvm-project/issues/64644

[pfusik]

LGTM, modulo a lambda that I think could be simplified and some unclear test comments.

libcxx/include/sstream

402–409

410

I think we don't need this __str_.clear() anymore.

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

13

What do you mean by "not copy the string" ? It returns a string by value.

14

Why "should" ?

[philnik]

Given that there is no requirement in the standard for this, all the tests should be libc++ specific. No implementation is required to have this behaviour.

libcxx/include/sstream

410

We do if the allocator is not equal after copying it. Then the old string isn't cleared.

libcxx/include/string

973–975

Let's not add this as an extension in C++20. It's easy enough to add a private constructor for this.

[AMP999]

libcxx/include/sstream

402–409

Your change looks OK to me.

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

13

Yes, but the string it returns is not created by copying the stringbuf's associated string. The copy constructor would use allocator_traits::select_on_container_copy_construction.

14

Because that's consistent both with .str() const& (which preserves the allocator) and with the ordinary move-constructor of string (which preserves the allocator). I don't think the user would ever expect .str() && to do anything but preserve the allocator.

[AMP999]

Add @pfusik 's change.

[AMP999]

libcxx/include/sstream

410

Copies are equal by definition. But if the old string fits in the small-string buffer, the old string won't be cleared, either.

libcxx/include/string

973–975

Do we have a preferred style for adding "private constructors"? For example, could I change this overload set to:

#if _LIBCPP_STD_VER >= 23
  _LIBCPP_HIDE_FROM_ABI constexpr
  basic_string(basic_string&& __str, size_type __pos, const _Allocator& __alloc = _Allocator())
      : basic_string(__libcpp_extension_t(), std::move(__str), __pos, npos, __alloc) {}
  _LIBCPP_HIDE_FROM_ABI constexpr
  basic_string(basic_string&& __str, size_type __pos, size_type __n, const _Allocator& __alloc = _Allocator())
      : basic_string(__libcpp_extension_t(), std::move(__str), __pos, __n, __alloc) {}
#endif

  _LIBCPP_HIDE_FROM_ABI constexpr
  basic_string(__libcpp_extension_t, basic_string&& __str, size_type __pos, size_type __n, const _Allocator& __alloc = _Allocator())
      : __r_(__default_init_tag(), __alloc) {
[...]

But I'm also interested in hearing others' opinions. It seems to me that if std::string(std::move(s), 0) copies in C++20 and moves in C++23, anyone actually relying on that fact won't be able to upgrade safely from C++20 to C++23. (Anyone not relying on that fact won't care... except for performance, where move is better.) I think we should make this overload do the same thing in all versions where it exists (i.e. C++20 and later). What are libstdc++ and MSVC planning to do?

[AMP999]

I appreciate it if someone helps me with the test failures. I'm not sure why the "Apple back-deployment" targets are all failing.

[Mordante]

Mainly glossed over the patch, I didn't do a real review.

libcxx/include/sstream

408–409

auto seems wrong here; there's no guarantee same_as<int, ptrdiff_t> is true.

libcxx/include/string

975

Extensions should be documented in our user documentation.

Can you please elaborate why it's hard? That makes it easier to understand the motivation for the reader (and the reviewer).

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

62

Please write our what COCCC'ed is.

97

pmr is not available on Apple backdepoyment targets. Best add // UNSUPPORTED: availability-pmr-missing to these tests.

[AMP999]

Addressed review comments, removed string(string&&, pos, n, alloc) constructor from C++20 mode again.

[frederick-vs-ja]

libcxx/include/string

973–975

anyone actually relying on that fact won't be able to upgrade safely from C++20 to C++23

It's extremely weird to write std::move while relying that it doesn't work.

What are libstdc++ and MSVC planning to do?

I've implemented P2438R2 in MSVC STL in C++23 mode only. MSVC STL's basic_stringbuf can't directly reuse basic_string at this moment due to ABI compatibility (see here).

I don't know what is libstdc++ planning to do. Its rvalue str() overload looks simple enough, so I guess it won't get P2438R2 implemented in C++20 mode just for convenience.

[AMP999]

@philnik gentle ping!

Re "there is no requirement in the standard," I think the use of the term "move constructed" in https://eel.is/c++draft/stringbuf#members-10.sentence-1 is that requirement. The original paper P0408 also contains this test case https://github.com/PeterSommerlad/SC22WG21_Papers/blob/master/workspace/Test_basic_stringbuf_efficient/src/Testp0407-p0408-efficientStringstreams.cpp#L105-L112 which won't succeed unless we transfer ownership here. Transfer-of-ownership is clearly P0408's intent.

Re supporting the new basic_string constructor in C++20 mode: Okay, I have removed it from this patch.

[philnik]

In D157776#4622524, @AMP999 wrote:

@philnik gentle ping!

Re "there is no requirement in the standard," I think the use of the term "move constructed" in https://eel.is/c++draft/stringbuf#members-10.sentence-1 is that requirement. The original paper P0408 also contains this test case https://github.com/PeterSommerlad/SC22WG21_Papers/blob/master/workspace/Test_basic_stringbuf_efficient/src/Testp0407-p0408-efficientStringstreams.cpp#L105-L112 which won't succeed unless we transfer ownership here. Transfer-of-ownership is clearly P0408's intent.

A moved-from string has an unspecified state, so there is no way to check what actually happened.

libcxx/include/sstream

410

Right. In that case the clear() is indeed redundant. The old string will be cleared, but even if that wasn't the case, there is no requirement that the string is cleared.

libcxx/include/string

973–976

The extension is still here.

1340

This is a way more canonical name.

[pfusik]

libcxx/include/sstream

410

There is one:

basic_string<charT, traits, Allocator> str() &&;
Postconditions: The underlying character sequence buf is empty (...)

[AMP999]

The extension is removed.

[AMP999]

Migrated to https://github.com/llvm/llvm-project/pull/67294 .

[ldionne]

Abandoning to clean up the review queue.

libcxx/include/sstream

 #endif
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_BUILDING_LIBRARY)
     string_type str() const;
 #else
     _LIBCPP_HIDE_FROM_ABI_SSTREAM string_type str() const & { return str(__str_.get_allocator()); }
 
     _LIBCPP_HIDE_FROM_ABI_SSTREAM string_type str() && {
-        string_type __result;
+        string_type __result = [&]() {
             const basic_string_view<_CharT, _Traits> __view = view();
-        if (!__view.empty()) {
-            auto __pos = __view.data() - __str_.data();
-            __result.assign(std::move(__str_), __pos, __view.size());
+            if (__view.empty()) {
+                return string_type(__str_.get_allocator());
             }
+            auto __pos = __view.data() - __str_.data();
+            return string_type(std::move(__str_), __pos, __view.size(), __str_.get_allocator());
+        }();
         __str_.clear();
         __init_buf_ptrs();
         return __result;
     }
 #endif // _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_BUILDING_LIBRARY)
 
 #if _LIBCPP_STD_VER >= 20
     template <class _SAlloc>
       requires __is_allocator<_SAlloc>::value

libcxx/include/string

basic_string(const _CharT* __s, size_type __n, const _Allocator& __a)
     __init(__s, __n);
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string(size_type __n, _CharT __c)
       : __r_(__default_init_tag(), __default_init_tag()) {
     __init(__n, __c);
   }
 
-#if _LIBCPP_STD_VER >= 23
+#if _LIBCPP_STD_VER >= 20
+  // Extension: Support these overloads in C++20 rather than C++23.
+  // Otherwise C++20's `basic_stringbuf::str() &&` is hard to implement.
   _LIBCPP_HIDE_FROM_ABI constexpr
   basic_string(basic_string&& __str, size_type __pos, const _Allocator& __alloc = _Allocator())
       : basic_string(std::move(__str), __pos, npos, __alloc) {}
 
   _LIBCPP_HIDE_FROM_ABI constexpr
   basic_string(basic_string&& __str, size_type __pos, size_type __n, const _Allocator& __alloc = _Allocator())
       : __r_(__default_init_tag(), __alloc) {
     if (__pos > __str.size())
       __throw_out_of_range();
#endif // _LIBCPP_CXX03_LANG
   _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string&
   assign(const _Tp& __t) {
     __self_view __sv = __t;
     return assign(__sv.data(), __sv.size());
   }
 
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     basic_string& assign(const basic_string& __str) { return *this = __str; }
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     basic_string& assign(basic_string&& __str)
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value))
         {*this = std::move(__str); return *this;}
 #endif
     _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& assign(const basic_string& __str, size_type __pos, size_type __n=npos);
 
     template <class _Tp,

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/str.allocator_propagation.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
+
+// This test ensures that we properly propagate allocators from istringstream's
+// inner string object to the new string returned from .str().
+// `str() const&` is specified to preserve the allocator (not copy the string).
+// `str() &&` isn't specified, but should preserve the allocator (move the string).
+
+#include <cassert>
+#include <memory>
+#include <memory_resource>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+template <class T>
+struct SocccAllocator {
+  using value_type = T;
+
+  int count_ = 0;
+  explicit SocccAllocator(int i) : count_(i) {}
+
+  template<class U>
+  SocccAllocator(const SocccAllocator<U>& a) : count_(a.count_) {}
+
+  T* allocate(std::size_t n) { return std::allocator<T>().allocate(n); }
+  void deallocate(T* p, std::size_t n) { std::allocator<T>().deallocate(p, n); }
+
+  SocccAllocator select_on_container_copy_construction() const {
+    return SocccAllocator(count_ + 1);
+  }
+
+  bool operator==(const SocccAllocator&) const { return true; }
+
+  using propagate_on_container_copy_assignment = std::false_type;
+  using propagate_on_container_move_assignment = std::false_type;
+  using propagate_on_container_swap = std::false_type;
+};
+
+template <class CharT>
+void test_soccc_behavior()
+{
+  using Alloc = SocccAllocator<CharT>;
+  using SS = std::basic_istringstream<CharT, std::char_traits<CharT>, Alloc>;
+  using S = std::basic_string<CharT, std::char_traits<CharT>, Alloc>;
+  {
+    SS ss = SS(std::ios_base::in, Alloc(10));
+
+    // [stringbuf.members]/6 specifies that the allocator is copied,
+    // not SOCCC'ed.
+    //
+    S copied = ss.str();
+    assert(copied.get_allocator().count_ == 10);
+    assert(ss.rdbuf()->get_allocator().count_ == 10);
+    assert(copied.empty());
+
+    assert(S(copied).get_allocator().count_ == 11);
+      // sanity-check that SOCCC does in fact work
+
+    // [stringbuf.members]/10 doesn't specify the allocator to use,
+    // but copying the allocator as-if-by moving the string makes sense.
+    //
+    S moved = std::move(ss).str();
+    assert(moved.get_allocator().count_ == 10);
+    assert(ss.rdbuf()->get_allocator().count_ == 10);
+    assert(moved.empty());
+  }
+}
+
+template<class CharT, class Base = std::basic_stringbuf<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>>
+struct StringBuf : Base {
+  explicit StringBuf(std::pmr::memory_resource *mr) : Base(std::ios_base::in, mr) {}
+  void public_setg(int a, int b, int c) {
+    CharT *p = this->eback();
+    assert(this->view().data() == p);
+    this->setg(p + a, p + b, p + c);
+    assert(this->eback() == p + a);
+    assert(this->view().data() == p + a);
+  }
+};
+
+template <class CharT>
+void test_allocation_is_pilfered() {
+  using SS = std::basic_istringstream<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>;
+  using S = std::pmr::basic_string<CharT>;
+  alignas(void*) char buf[80 * sizeof(CharT)];
+  const CharT *initial = MAKE_CSTRING(CharT, "a very long string that exceeds the small string optimization buffer length");
+  {
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(buf, sizeof(buf), std::pmr::null_memory_resource());
+    SS ss = SS(S(initial, &mr1));
+    S s = std::move(ss).str();
+    assert(s == initial);
+  }
+  {
+    // Try moving-out-of a stringbuf whose view() is not the entire string.
+    // This is libc++'s behavior; libstdc++ doesn't allow such stringbufs to be created.
+    //
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(buf, sizeof(buf), std::pmr::null_memory_resource());
+    auto src = StringBuf<CharT>(&mr1);
+    src.str(S(initial, &mr1));
+    src.public_setg(2, 6, 40);
+    SS ss(std::ios_base::in, &mr1);
+    *ss.rdbuf() = std::move(src);
+    LIBCPP_ASSERT(ss.view() == std::basic_string_view<CharT>(initial).substr(2, 38));
+    S s = std::move(ss).str();
+    LIBCPP_ASSERT(s == std::basic_string_view<CharT>(initial).substr(2, 38));
+  }
+}
+
+template <class CharT>
+void test_no_foreign_allocations() {
+  using SS = std::basic_istringstream<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>;
+  using S = std::pmr::basic_string<CharT>;
+  const CharT *initial = MAKE_CSTRING(CharT, "a very long string that exceeds the small string optimization buffer length");
+  {
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(std::pmr::new_delete_resource());
+    auto ss = SS(S(initial, &mr1));
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+
+    // [stringbuf.members]/6 specifies that the result of `str() const &`
+    // does NOT use the default allocator; it uses the original allocator.
+    //
+    S copied = ss.str();
+    assert(copied.get_allocator().resource() == &mr1);
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+    assert(copied == initial);
+
+    // [stringbuf.members]/10 doesn't specify the allocator to use,
+    // but copying the allocator as-if-by moving the string makes sense.
+    //
+    S moved = std::move(ss).str();
+    assert(moved.get_allocator().resource() == &mr1);
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+    assert(moved == initial);
+  }
+}
+
+int main(int, char**) {
+  test_soccc_behavior<char>();
+  test_allocation_is_pilfered<char>();
+  test_no_foreign_allocations<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test_soccc_behavior<wchar_t>();
+  test_allocation_is_pilfered<wchar_t>();
+  test_no_foreign_allocations<wchar_t>();
+#endif
+
+  return 0;
+}

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

static void test() {
     assert(ss.view().empty());
   }
   {
     std::basic_istringstream<CharT> ss;
     std::basic_string<CharT> s = std::move(ss).str();
     assert(s.empty());
     assert(ss.view().empty());
   }
+  {
+    std::basic_istringstream<CharT> ss(STR("a very long string that exceeds the small string optimization buffer length"));
+    const CharT *p = ss.view().data();
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s.data() == p); // the allocation was pilfered
+    assert(ss.view().empty());
+  }
 }
 
 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/str.allocator_propagation.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
+
+// This test ensures that we properly propagate allocators from ostringstream's
+// inner string object to the new string returned from .str().
+// `str() const&` is specified to preserve the allocator (not copy the string).
+// `str() &&` isn't specified, but should preserve the allocator (move the string).
+
+#include <cassert>
+#include <memory>
+#include <memory_resource>
+#include <sstream>
+#include <string>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+template <class T>
+struct SocccAllocator {
+  using value_type = T;
+
+  int count_ = 0;
+  explicit SocccAllocator(int i) : count_(i) {}
+
+  template<class U>
+  SocccAllocator(const SocccAllocator<U>& a) : count_(a.count_) {}
+
+  T* allocate(std::size_t n) { return std::allocator<T>().allocate(n); }
+  void deallocate(T* p, std::size_t n) { std::allocator<T>().deallocate(p, n); }
+
+  SocccAllocator select_on_container_copy_construction() const {
+    return SocccAllocator(count_ + 1);
+  }
+
+  bool operator==(const SocccAllocator&) const { return true; }
+
+  using propagate_on_container_copy_assignment = std::false_type;
+  using propagate_on_container_move_assignment = std::false_type;
+  using propagate_on_container_swap = std::false_type;
+};
+
+template <class CharT>
+void test_soccc_behavior()
+{
+  using Alloc = SocccAllocator<CharT>;
+  using SS = std::basic_ostringstream<CharT, std::char_traits<CharT>, Alloc>;
+  using S = std::basic_string<CharT, std::char_traits<CharT>, Alloc>;
+  {
+    SS ss = SS(std::ios_base::out, Alloc(10));
+
+    // [stringbuf.members]/6 specifies that the allocator is copied,
+    // not SOCCC'ed.
+    //
+    S copied = ss.str();
+    assert(copied.get_allocator().count_ == 10);
+    assert(ss.rdbuf()->get_allocator().count_ == 10);
+    assert(copied.empty());
+
+    assert(S(copied).get_allocator().count_ == 11);
+      // sanity-check that SOCCC does in fact work
+
+    // [stringbuf.members]/10 doesn't specify the allocator to use,
+    // but copying the allocator as-if-by moving the string makes sense.
+    //
+    S moved = std::move(ss).str();
+    assert(moved.get_allocator().count_ == 10);
+    assert(ss.rdbuf()->get_allocator().count_ == 10);
+    assert(moved.empty());
+  }
+}
+
+template <class CharT>
+void test_allocation_is_pilfered() {
+  using SS = std::basic_ostringstream<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>;
+  using S = std::pmr::basic_string<CharT>;
+  alignas(void*) char buf[80 * sizeof(CharT)];
+  const CharT *initial = MAKE_CSTRING(CharT, "a very long string that exceeds the small string optimization buffer length");
+  {
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(buf, sizeof(buf), std::pmr::null_memory_resource());
+    SS ss = SS(S(initial, &mr1));
+    S s = std::move(ss).str();
+    assert(s == initial);
+  }
+}
+
+template <class CharT>
+void test_no_foreign_allocations() {
+  using SS = std::basic_ostringstream<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>;
+  using S = std::pmr::basic_string<CharT>;
+  const CharT *initial = MAKE_CSTRING(CharT, "a very long string that exceeds the small string optimization buffer length");
+  {
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(std::pmr::new_delete_resource());
+    auto ss = SS(S(initial, &mr1));
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+
+    // [stringbuf.members]/6 specifies that the result of `str() const &`
+    // does NOT use the default allocator; it uses the original allocator.
+    //
+    S copied = ss.str();
+    assert(copied.get_allocator().resource() == &mr1);
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+    assert(copied == initial);
+
+    // [stringbuf.members]/10 doesn't specify the allocator to use,
+    // but copying the allocator as-if-by moving the string makes sense.
+    //
+    S moved = std::move(ss).str();
+    assert(moved.get_allocator().resource() == &mr1);
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+    assert(moved == initial);
+  }
+}
+
+int main(int, char**) {
+  test_soccc_behavior<char>();
+  test_allocation_is_pilfered<char>();
+  test_no_foreign_allocations<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test_soccc_behavior<wchar_t>();
+  test_allocation_is_pilfered<wchar_t>();
+  test_no_foreign_allocations<wchar_t>();
+#endif
+
+  return 0;
+}

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

static void test() {
     assert(ss.view().empty());
   }
   {
     std::basic_ostringstream<CharT> ss;
     std::basic_string<CharT> s = std::move(ss).str();
     assert(s.empty());
     assert(ss.view().empty());
   }
+  {
+    std::basic_ostringstream<CharT> ss(STR("a very long string that exceeds the small string optimization buffer length"));
+    const CharT *p = ss.view().data();
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s.data() == p); // the allocation was pilfered
+    assert(ss.view().empty());
+  }
 }
 
 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/str.move.pass.cpp

static void test() {
     assert(buf.view().empty());
   }
   {
     std::basic_stringbuf<CharT> buf;
     std::basic_string<CharT> s = std::move(buf).str();
     assert(s.empty());
     assert(buf.view().empty());
   }
+  {
+    std::basic_stringbuf<CharT> buf(STR("a very long string that exceeds the small string optimization buffer length"));
+    const CharT *p = buf.view().data();
+    std::basic_string<CharT> s = std::move(buf).str();
+    assert(s.data() == p); // the allocation was pilfered
+    assert(buf.view().empty());
+  }
+}
+
+struct StringBuf : std::stringbuf {
+  using basic_stringbuf::basic_stringbuf;
+  void public_setg(int a, int b, int c) {
+    char *p = eback();
+    this->setg(p + a, p + b, p + c);
+  }
+};
+
+static void test_altered_sequence_pointers() {
+  {
+    auto src = StringBuf("hello world", std::ios_base::in);
+    src.public_setg(4, 6, 9);
+    std::stringbuf dest;
+    dest = std::move(src);
+    std::string view = std::string(dest.view());
+    std::string str = std::move(dest).str();
+    assert(view == str);
+    LIBCPP_ASSERT(str == "o wor");
+  }
+  {
+    auto src = StringBuf("hello world", std::ios_base::in);
+    src.public_setg(4, 6, 9);
+    std::stringbuf dest;
+    dest.swap(src);
+    std::string view = std::string(dest.view());
+    std::string str = std::move(dest).str();
+    assert(view == str);
+    LIBCPP_ASSERT(str == "o wor");
+  }
 }
 
 int main(int, char**) {
   test<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test<wchar_t>();
 #endif
+  test_altered_sequence_pointers();
   return 0;
 }

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.members/str.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
 //
 //===----------------------------------------------------------------------===//
 
 // <sstream>
 
 // template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
 // class basic_stringbuf
 
 // void str(const basic_string<charT,traits,Allocator>& s);
 
 #include <sstream>
+#include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
+struct StringBuf : std::stringbuf {
+  explicit StringBuf(const char *s, std::ios_base::openmode mode) :
+    basic_stringbuf(s, mode) { }
+  void public_setg(int a, int b, int c) {
+    char *p = eback();
+    this->setg(p + a, p + b, p + c);
+  }
+};
+
+static void test_altered_sequence_pointers() {
+  {
+    StringBuf src("hello world", std::ios_base::in);
+    src.public_setg(4, 6, 9);
+    std::stringbuf dest;
+    dest = std::move(src);
+    std::string str = dest.str();
+    assert(5 <= str.size() && str.size() <= 11);
+    LIBCPP_ASSERT(str == "o wor");
+  }
+  {
+    StringBuf src("hello world", std::ios_base::in);
+    src.public_setg(4, 6, 9);
+    std::stringbuf dest;
+    dest.swap(src);
+    std::string str = dest.str();
+    assert(5 <= str.size() && str.size() <= 11);
+    LIBCPP_ASSERT(str == "o wor");
+  }
+}
+
 int main(int, char**)
 {
+    test_altered_sequence_pointers();
     {
         std::stringbuf buf("testing");
         assert(buf.str() == "testing");
         buf.str("another test");
         assert(buf.str() == "another test");
     }
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     {

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

static void test() {
   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>>>);
 }
 
+struct StringBuf : std::stringbuf {
+  using basic_stringbuf::basic_stringbuf;
+  void public_setg(int a, int b, int c) {
+    char *p = eback();
+    this->setg(p + a, p + b, p + c);
+  }
+};
+
+static void test_altered_sequence_pointers() {
+  {
+    auto src = StringBuf("hello world", std::ios_base::in);
+    src.public_setg(4, 6, 9);
+    std::stringbuf dest;
+    dest = std::move(src);
+    assert(dest.view() == dest.str());
+    LIBCPP_ASSERT(dest.view() == "o wor");
+  }
+  {
+    auto src = StringBuf("hello world", std::ios_base::in);
+    src.public_setg(4, 6, 9);
+    std::stringbuf dest;
+    dest.swap(src);
+    assert(dest.view() == dest.str());
+    LIBCPP_ASSERT(dest.view() == "o wor");
+  }
+}
+
 int main(int, char**) {
   test<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test<wchar_t>();
 #endif
+  test_altered_sequence_pointers();
   return 0;
 }

libcxx/test/std/input.output/string.streams/stringstream/stringstream.members/str.allocator_propagation.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
+
+// This test ensures that we properly propagate allocators from stringstream's
+// inner string object to the new string returned from .str().
+// `str() const&` is specified to preserve the allocator (not copy the string).
+// `str() &&` isn't specified, but should preserve the allocator (move the string).
+
+#include <cassert>
+#include <memory>
+#include <memory_resource>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+template <class T>
+struct SocccAllocator {
+  using value_type = T;
+
+  int count_ = 0;
+  explicit SocccAllocator(int i) : count_(i) {}
+
+  template<class U>
+  SocccAllocator(const SocccAllocator<U>& a) : count_(a.count_) {}
+
+  T* allocate(std::size_t n) { return std::allocator<T>().allocate(n); }
+  void deallocate(T* p, std::size_t n) { std::allocator<T>().deallocate(p, n); }
+
+  SocccAllocator select_on_container_copy_construction() const {
+    return SocccAllocator(count_ + 1);
+  }
+
+  bool operator==(const SocccAllocator&) const { return true; }
+
+  using propagate_on_container_copy_assignment = std::false_type;
+  using propagate_on_container_move_assignment = std::false_type;
+  using propagate_on_container_swap = std::false_type;
+};
+
+template <class CharT>
+void test_soccc_behavior()
+{
+  using Alloc = SocccAllocator<CharT>;
+  using SS = std::basic_stringstream<CharT, std::char_traits<CharT>, Alloc>;
+  using S = std::basic_string<CharT, std::char_traits<CharT>, Alloc>;
+  {
+    SS ss = SS(std::ios_base::out, Alloc(10));
+
+    // [stringbuf.members]/6 specifies that the allocator is copied,
+    // not SOCCC'ed.
+    //
+    S copied = ss.str();
+    assert(copied.get_allocator().count_ == 10);
+    assert(ss.rdbuf()->get_allocator().count_ == 10);
+    assert(copied.empty());
+
+    assert(S(copied).get_allocator().count_ == 11);
+      // sanity-check that SOCCC does in fact work
+
+    // [stringbuf.members]/10 doesn't specify the allocator to use,
+    // but copying the allocator as-if-by moving the string makes sense.
+    //
+    S moved = std::move(ss).str();
+    assert(moved.get_allocator().count_ == 10);
+    assert(ss.rdbuf()->get_allocator().count_ == 10);
+    assert(moved.empty());
+  }
+}
+
+template<class CharT, class Base = std::basic_stringbuf<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>>
+struct StringBuf : Base {
+  explicit StringBuf(std::pmr::memory_resource *mr) : Base(std::ios_base::in, mr) {}
+  void public_setg(int a, int b, int c) {
+    CharT *p = this->eback();
+    assert(this->view().data() == p);
+    this->setg(p + a, p + b, p + c);
+    assert(this->eback() == p + a);
+    assert(this->view().data() == p + a);
+  }
+};
+
+template <class CharT>
+void test_allocation_is_pilfered() {
+  using SS = std::basic_stringstream<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>;
+  using S = std::pmr::basic_string<CharT>;
+  alignas(void*) char buf[80 * sizeof(CharT)];
+  const CharT *initial = MAKE_CSTRING(CharT, "a very long string that exceeds the small string optimization buffer length");
+  {
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(buf, sizeof(buf), std::pmr::null_memory_resource());
+    SS ss = SS(S(initial, &mr1));
+    S s = std::move(ss).str();
+    assert(s == initial);
+  }
+  {
+    // Try moving-out-of a stringbuf whose view() is not the entire string.
+    // This is libc++'s behavior; libstdc++ doesn't allow such stringbufs to be created.
+    //
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(buf, sizeof(buf), std::pmr::null_memory_resource());
+    auto src = StringBuf<CharT>(&mr1);
+    src.str(S(initial, &mr1));
+    src.public_setg(2, 6, 40);
+    SS ss(std::ios_base::in, &mr1);
+    *ss.rdbuf() = std::move(src);
+    LIBCPP_ASSERT(ss.view() == std::basic_string_view<CharT>(initial).substr(2, 38));
+    S s = std::move(ss).str();
+    LIBCPP_ASSERT(s == std::basic_string_view<CharT>(initial).substr(2, 38));
+  }
+}
+
+template <class CharT>
+void test_no_foreign_allocations() {
+  using SS = std::basic_stringstream<CharT, std::char_traits<CharT>, std::pmr::polymorphic_allocator<CharT>>;
+  using S = std::pmr::basic_string<CharT>;
+  const CharT *initial = MAKE_CSTRING(CharT, "a very long string that exceeds the small string optimization buffer length");
+  {
+    std::pmr::set_default_resource(std::pmr::null_memory_resource());
+    auto mr1 = std::pmr::monotonic_buffer_resource(std::pmr::new_delete_resource());
+    auto ss = SS(S(initial, &mr1));
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+
+    // [stringbuf.members]/6 specifies that the result of `str() const &`
+    // does NOT use the default allocator; it uses the original allocator.
+    //
+    S copied = ss.str();
+    assert(copied.get_allocator().resource() == &mr1);
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+    assert(copied == initial);
+
+    // [stringbuf.members]/10 doesn't specify the allocator to use,
+    // but copying the allocator as-if-by moving the string makes sense.
+    //
+    S moved = std::move(ss).str();
+    assert(moved.get_allocator().resource() == &mr1);
+    assert(ss.rdbuf()->get_allocator().resource() == &mr1);
+    assert(moved == initial);
+  }
+}
+
+int main(int, char**) {
+  test_soccc_behavior<char>();
+  test_allocation_is_pilfered<char>();
+  test_no_foreign_allocations<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test_soccc_behavior<wchar_t>();
+  test_allocation_is_pilfered<wchar_t>();
+  test_no_foreign_allocations<wchar_t>();
+#endif
+
+  return 0;
+}

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

static void test() {
     assert(ss.view().empty());
   }
   {
     std::basic_stringstream<CharT> ss;
     std::basic_string<CharT> s = std::move(ss).str();
     assert(s.empty());
     assert(ss.view().empty());
   }
+  {
+    std::basic_stringstream<CharT> ss(STR("a very long string that exceeds the small string optimization buffer length"));
+    const CharT *p = ss.view().data();
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s.data() == p); // the allocation was pilfered
+    assert(ss.view().empty());
+  }
 }
 
 int main(int, char**) {
   test<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test<wchar_t>();
 #endif
   return 0;
 }