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

[Jonathan Wakely]

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.