[libc++] Implement ostringstream members of P0408R7 (Efficient Access to basic_stringbuf's Buffer)

[pfusik]

This is symmetric to https://reviews.llvm.org/D154454

[Mordante]

In general LGTM, modulo one nit. I like to have a quick look before the final approval.

libcxx/include/sstream

967

This overload should be constrained is_same_v<SAlloc,Allocator> is false

[pfusik]

libcxx/include/sstream

967

Also in istringstream and stringstream.

It seems to work fine without the explicit constraint, selecting the non-template constructor, e.g. ostringstream.cons/string.pass.cpp.
I think this is because https://timsong-cpp.github.io/cppwp/over.match.best#general-2.4

Or do we need to have "requires" every time the standard says "Constraints:" ?

[Mordante]

libcxx/include/sstream

967

The overload resolution indeed selects the expected function. However the function is a viable function, which it should not be per http://eel.is/c++draft/description#structure.specifications-3.1

Note I think that page of the standard contains a lot of information regarding the standard library. You might want to read a bit more on that page.

[pfusik]

Added a constructor constraint, also in istringstream.

[Mordante]

LGTM after removing the unneeded constraint.

libcxx/include/sstream

846

Interestingly this one does not seem to be constrained. http://eel.is/c++draft/string.streams#istringstream.cons-6

The same in the paper. I can't find a reason in the WG21 review either. I assume it's a bug in the Standard. For now let's remove this one and maybe we should file an LWG issue to remove them from the constructors when not needed.

[pfusik]

Removed the istringstream constructor constraint.

[leonardchan]

It looks like this change causes an out of range exception to be thrown for this snippet:

// ./bin/clang++ -std=c++20 /tmp/test.cc -g
#include <iostream>
#include <sstream>

int main() {
  std::ostringstream oss;
  std::cerr << std::move(oss).str();
  return 0;
}

This fails with libc++abi: terminating due to uncaught exception of type std::out_of_range: basic_string but doesn't prior to this patch. It looks like the failure comes from this bit here:

    _LIBCPP_HIDE_FROM_ABI_SSTREAM string_type str() && {
        const basic_string_view<_CharT, _Traits> __view = view();
        string_type __result(std::move(__str_), __view.data() - __str_.data(), __view.size());
        __str_.clear();
        __init_buf_ptrs();
        return __result;
    }

Where __view.data() - __str_.data() is an absurdly large value for the pos argument in the string ctor:

#11 0x00005555555a960d in std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >::basic_string (this=0x7fffffffd5f8, __str=..., __pos=18446603336221206952, __n=0, __a=...)

Perhaps the pos is actually a negative number or view.data() is actually nullptr? Would you be able to send out a fix or revert? Thanks.

[pfusik]

@leonardchan Thank you for your report and analysis!
__view.data() was nullptr. I did not realize that the position must be valid even when the count is zero.
Fix: https://reviews.llvm.org/D156783

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: All `stringbuf` and `istringstream` members plus `view()` in all classes implemented.
+   .. [#note-P0408] P0408: Implemented all `stringbuf`/`istringstream`/`ostringstream` members and `view()` in `stringstream`.
    .. [#note-P0660] P0660: Section 32.3 Stop Tokens is complete. ``jthread`` hasn't been implemented yet.
 
 .. _issues-status-cxx20:
 
 Library Working Group Issues Status
 ====================================
 
 .. csv-table::
    :file: Cxx20Issues.csv
    :header-rows: 1
    :widths: auto

libcxx/include/sstream

public:
     typedef charT                          char_type;
     typedef traits                         traits_type;
     typedef typename traits_type::int_type int_type;
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
     typedef Allocator                      allocator_type;
 
     // [ostringstream.cons] Constructors:
-    explicit basic_ostringstream(ios_base::openmode which = ios_base::out); // before C++20
-    basic_ostringstream() : basic_ostringstream(ios_base::out) {}           // C++20
-    explicit basic_ostringstream(ios_base::openmode which);                 // C++20
-
-    explicit basic_ostringstream(const basic_string<char_type, traits_type, allocator_type>& str,
+    explicit basic_ostringstream(ios_base::openmode which = ios_base::out);            // before C++20
+    basic_ostringstream() : basic_ostringstream(ios_base::out) {}                      // C++20
+    explicit basic_ostringstream(ios_base::openmode which);                            // C++20
+    explicit basic_ostringstream(const basic_string<char_type, traits_type, allocator_type>& s,
                                  ios_base::openmode which = ios_base::out);
+    basic_ostringstream(ios_base::openmode which, const allocator_type& a);            // C++20
+    explicit basic_ostringstream(basic_string<char_type, traits_type, allocator_type>&& s,
+                                 ios_base::openmode which = ios_base::out);            // C++20
+    template <class SAlloc>
+    basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, const allocator_type& a)
+        : basic_ostringstream(s, ios_base::out, a) {}                                  // C++20
+    template <class SAlloc>
+    basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s,
+                        ios_base::openmode which, const allocator_type& a);            // C++20
+    template <class SAlloc>
+    explicit basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s,
+                                 ios_base::openmode which = ios_base::out);            // C++20
     basic_ostringstream(basic_ostringstream&& rhs);
 
     // [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
+    basic_string<char_type, traits_type, allocator_type> str() const;                  // before C++20
+    basic_string<char_type, traits_type, allocator_type> str() const &;                // C++20
+    template <class SAlloc>
+    basic_string<char_type, traits_type, SAlloc> str(const SAlloc& sa) const;          // C++20
+    basic_string<char_type, traits_type, allocator_type> str() &&;                     // C++20
+    basic_string_view<char_type, traits_type> view() const noexcept;                   // C++20
+    void str(const basic_string<char_type, traits_type, allocator_type>& s);
+    template <class SAlloc>
+    void str(const basic_string<char_type, traits_type, SAlloc>& s);                   // C++20
+    void str(basic_string<char_type, traits_type, allocator_type>&& s);                // 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;
_LIBCPP_HIDE_FROM_ABI basic_istringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s, const _Allocator& __a)
         : basic_istringstream(__s, ios_base::in, __a) {}
 
     template <class _SAlloc>
     _LIBCPP_HIDE_FROM_ABI basic_istringstream(
         const basic_string<_CharT, _Traits, _SAlloc>& __s, ios_base::openmode __wch, const _Allocator& __a)
         : basic_istream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::in, __a) {}
 
     template <class _SAlloc>
     _LIBCPP_HIDE_FROM_ABI explicit basic_istringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s,
                                                        ios_base::openmode __wch = ios_base::in)
         : basic_istream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::in) {}
 #endif // _LIBCPP_STD_VER >= 20
 
     _LIBCPP_INLINE_VISIBILITY
     basic_istringstream(basic_istringstream&& __rhs)
         : basic_istream<_CharT, _Traits>(_VSTD::move(__rhs))
         , __sb_(_VSTD::move(__rhs.__sb_))
public:
 
     _LIBCPP_INLINE_VISIBILITY
     explicit basic_ostringstream(const string_type& __s,
                                  ios_base::openmode __wch = ios_base::out)
         : basic_ostream<_CharT, _Traits>(&__sb_)
         , __sb_(__s, __wch | ios_base::out)
     { }
 
+#if _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI basic_ostringstream(ios_base::openmode __wch, const _Allocator& __a)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__wch | ios_base::out, __a) {}
+
+    _LIBCPP_HIDE_FROM_ABI explicit basic_ostringstream(string_type&& __s, ios_base::openmode __wch = ios_base::out)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(std::move(__s), __wch | ios_base::out) {}
+
+    template <class _SAlloc>
+    _LIBCPP_HIDE_FROM_ABI basic_ostringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s, const _Allocator& __a)
+        : basic_ostringstream(__s, ios_base::out, __a) {}
+
+    template <class _SAlloc>
+    _LIBCPP_HIDE_FROM_ABI basic_ostringstream(
+        const basic_string<_CharT, _Traits, _SAlloc>& __s, ios_base::openmode __wch, const _Allocator& __a)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::out, __a) {}
+
+    template <class _SAlloc>
+      requires (!is_same_v<_SAlloc, allocator_type>)
+    _LIBCPP_HIDE_FROM_ABI explicit basic_ostringstream(const basic_string<_CharT, _Traits, _SAlloc>& __s,
+                                                       ios_base::openmode __wch = ios_base::out)
+        : basic_ostream<_CharT, _Traits>(std::addressof(__sb_)), __sb_(__s, __wch | ios_base::out) {}
+#endif // _LIBCPP_STD_VER >= 20
+
     _LIBCPP_INLINE_VISIBILITY
     basic_ostringstream(basic_ostringstream&& __rhs)
         : basic_ostream<_CharT, _Traits>(_VSTD::move(__rhs))
         , __sb_(_VSTD::move(__rhs.__sb_))
     {
         basic_ostream<_CharT, _Traits>::set_rdbuf(&__sb_);
     }
 
void swap(basic_ostringstream& __rhs) {
         __sb_.swap(__rhs.__sb_);
     }
 
     // [ostringstream.members] Member functions:
     _LIBCPP_INLINE_VISIBILITY
     basic_stringbuf<char_type, traits_type, allocator_type>* rdbuf() const {
         return const_cast<basic_stringbuf<char_type, traits_type, allocator_type>*>(&__sb_);
     }
-    _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 string_type str() const & { return __sb_.str(); }
+
+    template <class _SAlloc>
+      requires __is_allocator<_SAlloc>::value
+    _LIBCPP_HIDE_FROM_ABI basic_string<char_type, traits_type, _SAlloc> str(const _SAlloc& __sa) const {
+        return __sb_.str(__sa);
     }
+
+    _LIBCPP_HIDE_FROM_ABI string_type str() && { return std::move(__sb_).str(); }
+
+    _LIBCPP_HIDE_FROM_ABI basic_string_view<char_type, traits_type> view() const noexcept { return __sb_.view(); }
+#else // _LIBCPP_STD_VER >= 20
+    _LIBCPP_HIDE_FROM_ABI string_type str() const { return __sb_.str(); }
+#endif // _LIBCPP_STD_VER >= 20
+
+    _LIBCPP_HIDE_FROM_ABI 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();
+    template <class _SAlloc>
+    _LIBCPP_HIDE_FROM_ABI void str(const basic_string<char_type, traits_type, _SAlloc>& __s) {
+        __sb_.str(__s);
     }
-#endif
+
+    _LIBCPP_HIDE_FROM_ABI void str(string_type&& __s) { __sb_.str(std::move(__s)); }
+#endif // _LIBCPP_STD_VER >= 20
 };
 
 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)
 {

libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.cons/mode.alloc.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_ostringstream(ios_base::openmode which, const Allocator& a);
+
+#include <sstream>
+#include <cassert>
+
+#include "test_allocator.h"
+#include "test_macros.h"
+
+template <class CharT>
+static void test() {
+  const test_allocator<CharT> a(2);
+  const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(std::ios_base::binary, a);
+  assert(ss.rdbuf()->get_allocator() == a);
+  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.cons/string-alloc.mode.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
+
+// template <class SAlloc>
+// explicit basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, ios_base::openmode which = ios_base::out)
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    const std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s);
+    assert(ss.view() == SV("testing"));
+  }
+  {
+    const std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s, std::ios_base::binary);
+    assert(ss.view() == SV("testing"));
+  }
+}
+
+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.cons/string.alloc.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
+
+// template <class SAlloc>
+// basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, const Allocator& a)
+//     : basic_ostringstream(s, ios_base::out, a) {}
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  const std::basic_string<CharT> s(STR("testing"));
+  const test_allocator<CharT> a(2);
+  const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s, a);
+  assert(ss.rdbuf()->get_allocator() == a);
+  assert(ss.view() == SV("testing"));
+}
+
+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.cons/string.mode.alloc.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
+
+// template <class SAlloc>
+// basic_ostringstream(const basic_string<char_type, traits_type, SAlloc>& s, ios_base::openmode which, const Allocator& a)
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  const std::basic_string<CharT> s(STR("testing"));
+  const test_allocator<CharT> a(2);
+  const std::basic_ostringstream<CharT, std::char_traits<CharT>, test_allocator<CharT>> ss(s, std::ios_base::binary, a);
+  assert(ss.rdbuf()->get_allocator() == a);
+  assert(ss.view() == SV("testing"));
+}
+
+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.cons/string.move.mode.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
+
+// explicit basic_ostringstream(basic_string<char_type, traits_type, allocator_type>&& s, ios_base::openmode which = ios_base::out);
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT> ss(std::move(s));
+    assert(ss.str() == STR("testing"));
+  }
+  {
+    std::basic_string<CharT> s(STR("testing"));
+    const std::basic_ostringstream<CharT> ss(std::move(s), std::ios_base::binary);
+    assert(ss.str() == STR("testing"));
+  }
+}
+
+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.alloc.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
+
+// template <class SAlloc>
+// basic_string<charT, traits, SAlloc> str(const SAlloc& sa) const;
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static void test() {
+  const std::basic_ostringstream<CharT> ss(STR("testing"));
+  const test_allocator<CharT> a(1);
+  const std::basic_string<CharT, std::char_traits<CharT>, test_allocator<CharT>> s = ss.str(a);
+  assert(s == SV("testing"));
+  assert(s.get_allocator() == a);
+}
+
+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.move.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<charT, traits, Allocator> str() &&;
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    std::basic_ostringstream<CharT> ss(STR("testing"));
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s == STR("testing"));
+    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.string-alloc.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
+
+// template <class SAlloc>
+// void str(const basic_string<charT, traits, SAlloc>& s);
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    const test_allocator<CharT> a(6);
+    const std::basic_string<CharT, std::char_traits<CharT>, test_allocator<CharT>> s(STR("testing"), a);
+    std::basic_ostringstream<CharT> ss;
+    ss.str(s);
+    assert(ss.str() == STR("testing"));
+  }
+}
+
+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.string.move.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
+
+// void str(basic_string<charT, traits, Allocator>&& s);
+
+#include <sstream>
+#include <cassert>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+#define STR(S) MAKE_STRING(CharT, S)
+
+template <class CharT>
+static void test() {
+  {
+    std::basic_ostringstream<CharT> ss;
+    std::basic_string<CharT> s(STR("testing"));
+    ss.str(std::move(s));
+    assert(ss.str() == STR("testing"));
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+  return 0;
+}