[PATCH] libstdc++: Implement C++20 features for <sstream>

[Jonathan Wakely]

On 09/10/20 16:28 -0700, Thomas Rodgers via Libstdc++ wrote:
>
>Jonathan Wakely writes:
>
>> On 07/10/20 18:15 -0700, Thomas Rodgers wrote:
>>>@@ -500,6 +576,40 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>>>       }
>>> #endif
>>>
>>>+#if __cplusplus > 201703L && _GLIBCXX_USE_CXX11_ABI
>>>+      basic_istringstream(ios_base::openmode __mode, const allocator_type& __a)
>>>+      : __istream_type(), _M_stringbuf(__mode | ios_base::in, __a)
>>>+      { this->init(&_M_stringbuf); }
>>
>> All these & operators need to be std::__addressof(_M_stringbuf)
>> instead. _M_stringbuf potentially depends on program-defined types
>> (the traits and allocator classes) which means user namespaces are
>> considered for ADL and they could define a operator& that gets used.
>>
>>
>>>+
>>>+      explicit basic_istringstream(__string_type&& __str,
>>>+	                           ios_base::openmode __mode = ios_base::in )
>>>+      : __istream_type(), _M_stringbuf(std::move(__str), __mode | ios_base::in)
>>>+      { this->init(&_M_stringbuf); }
>>>+
>>>+      template<typename _SAlloc>
>>>+	basic_istringstream(const basic_string<_CharT, _Traits, _SAlloc>& __str,
>>>+			    const allocator_type& __a)
>>>+	: basic_istringstream(__str, ios_base::in, __a)
>>>+	{ }
>>>+
>>>+      using __sv_type = basic_string_view<char_type, traits_type>;
>>
>> This typedef seems to only be used once. Might as well just use
>> basic_string_view<char_type, traits_type> directly in the return type
>> of view().
>>
>> Similarly in basic_ostringstream and basic_stringstream.
>>
>>>diff --git a/libstdc++-v3/src/c++20/Makefile.in b/libstdc++-v3/src/c++20/Makefile.in
>>>new file mode 100644
>>>index 00000000000..0e2de19ae59
>>>diff --git a/libstdc++-v3/src/c++20/sstream-inst.cc b/libstdc++-v3/src/c++20/sstream-inst.cc
>>>new file mode 100644
>>>index 00000000000..c419176ae8e
>>>--- /dev/null
>>>+++ b/libstdc++-v3/src/c++20/sstream-inst.cc
>>>@@ -0,0 +1,111 @@
>>>+// Explicit instantiation file.
>>>+
>>>+// Copyright (C) 1997-2020 Free Software Foundation, Inc.
>>
>> Just 2020 here.
>>
>>>+//
>>>+// This file is part of the GNU ISO C++ Library.  This library is free
>>>+// software; you can redistribute it and/or modify it under the
>>>+// terms of the GNU General Public License as published by the
>>>+// Free Software Foundation; either version 3, or (at your option)
>>>+// any later version.
>>>+
>>>+// This library is distributed in the hope that it will be useful,
>>>+// but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>>+// GNU General Public License for more details.
>>>+
>>>+// Under Section 7 of GPL version 3, you are granted additional
>>>+// permissions described in the GCC Runtime Library Exception, version
>>>+// 3.1, as published by the Free Software Foundation.
>>>+
>>>+// You should have received a copy of the GNU General Public License and
>>>+// a copy of the GCC Runtime Library Exception along with this program;
>>>+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>>>+// <http://www.gnu.org/licenses/>.
>>>+
>>>+//
>>>+// ISO C++ 14882:
>>>+//
>>>+
>>>+#ifndef _GLIBCXX_USE_CXX11_ABI
>>>+// Instantiations in this file use the new SSO std::string ABI unless included
>>>+// by another file which defines _GLIBCXX_USE_CXX11_ABI=0.
>>
>> This copy&pasted comment is misleading now if we're not actually going
>> to include it from another file to generate the old ABI symbols.
>>
>> I think just define it unconditionally and add a comment saying that
>> these new symbols are only defines for the SSO string ABI.
>>
>>>+# define _GLIBCXX_USE_CXX11_ABI 1
>>>+#endif
>>>+#include <sstream>
>>>+
>>>+namespace std _GLIBCXX_VISIBILITY(default)
>>>+{
>>>+_GLIBCXX_BEGIN_NAMESPACE_VERSION
>>>+
>>>+template basic_stringbuf<char>::basic_stringbuf(const allocator_type&);
>>>+template basic_stringbuf<char>::basic_stringbuf(ios_base::openmode,
>>>+						 const allocator_type&);
>>>+template basic_stringbuf<char>::basic_stringbuf(__string_type&&,
>>>+						 ios_base::openmode);
>>>+template basic_stringbuf<char>::basic_stringbuf(basic_stringbuf&&,
>>>+						 const allocator_type&);
>>>+template basic_stringbuf<char>::allocator_type
>>>+basic_stringbuf<char>::get_allocator() const noexcept;
>>>+template basic_stringbuf<char>::__sv_type
>>
>> Looks like this would be a bit simpler if it just used string_view
>> here, not basic_stringbuf<char>::__sv_type, and wstring_view below
>> for the wchar_t specializations.
>>
>> And you could use allocator<char> instead of
>> basic_stringbuf<char>::allocator_type.
>>
>> That looks a little cleaner to me, but it's a matter of opinion.
>>
>> That would be necessary anyway for the basic_*stringstream types if
>> they don't have the __sv_type any more.
>>
>>
>>>diff --git a/libstdc++-v3/testsuite/27_io/basic_istringstream/cons/char/1.cc b/libstdc++-v3/testsuite/27_io/basic_istringstream/cons/char/1.cc
>>>new file mode 100644
>>>index 00000000000..d93141fc232
>>>--- /dev/null
>>>+++ b/libstdc++-v3/testsuite/27_io/basic_istringstream/cons/char/1.cc
>>>@@ -0,0 +1,85 @@
>>>+// Copyright (C) 2020 Free Software Foundation, Inc.
>>>+//
>>>+// This file is part of the GNU ISO C++ Library.  This library is free
>>>+// software; you can redistribute it and/or modify it under the
>>>+// terms of the GNU General Public License as published by the
>>>+// Free Software Foundation; either version 3, or (at your option)
>>>+// any later version.
>>>+
>>>+// This library is distributed in the hope that it will be useful,
>>>+// but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>>+// GNU General Public License for more details.
>>>+
>>>+// You should have received a copy of the GNU General Public License along
>>>+// with this library; see the file COPYING3.  If not see
>>>+// <http://www.gnu.org/licenses/>.
>>>+
>>>+// 27.7.1.1  basic_stringbuf constructors  [lib.stringbuf.cons]
>>
>> These references are to the C++03 standard, but the functions being
>> tested are not in that standard, or that section.
>>
>> It's 29.8.2.2 [stringbuf.cons] in the C++20 standard. To make these
>> references unambiguous I've switched to saying which standard (or
>> which Nxxxx working draft) the reference comes from, i.e.
>>
>> // C++20 29.8.2.2  basic_stringbuf constructors  [stringbuf.cons]
>>
>> One day I might write a script to add "C++03" or "C++11" to all the
>> old comments in old tests. When I drain my TODO list of other things.
>>
>> I was expecting this patch to also add the five new overloads of
>> basic_stringbuf::str, but that can be added later. I don't think
>> adding those will change anything done in this patch.
>
>I'm happy to do that, follow up patch?

Yes, keep it separate.