Add non-const std::basic_string::data() for C++17

	* include/bits/basic_string.h (data() const): Update comment.
	(data()): Add non-const overload for C++17.
	* testsuite/21_strings/basic_string/operations/data/char/2.cc: New.
	* testsuite/21_strings/basic_string/operations/data/wchar_t/2.cc: New.

From-SVN: r238963

libstdc++-v3/ChangeLog

 2016-08-01  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/basic_string.h (data() const): Update comment.
+	(data()): Add non-const overload for C++17.
+	* testsuite/21_strings/basic_string/operations/data/char/2.cc: New.
+	* testsuite/21_strings/basic_string/operations/data/wchar_t/2.cc: New.
+
 	* include/bits/basic_string.tcc: Disable explicit instantiation
 	declarations for C++17.
 

libstdc++-v3/include/bits/basic_string.h

@@ -2159,13 +2159,27 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       /**
        *  @brief  Return const pointer to contents.
        *
-       *  This is a handle to internal data.  Do not modify or dire things may
-       *  happen.
+       *  This is a pointer to internal data.  It is undefined to modify
+       *  the contents through the returned pointer. To get a pointer that
+       *  allows modifying the contents use @c &str[0] instead,
+       *  (or in C++17 the non-const @c str.data() overload).
       */
       const _CharT*
       data() const _GLIBCXX_NOEXCEPT
       { return _M_data(); }
 
+#if __cplusplus > 201402L
+      /**
+       *  @brief  Return non-const pointer to contents.
+       *
+       *  This is a pointer to the character sequence held by the string.
+       *  Modifying the characters in the sequence is allowed.
+      */
+      _CharT*
+      data() noexcept
+      { return _M_data(); }
+#endif
+
       /**
        *  @brief  Return copy of allocator used to construct this string.
       */
@@ -4658,13 +4672,27 @@ _GLIBCXX_END_NAMESPACE_CXX11
       /**
        *  @brief  Return const pointer to contents.
        *
-       *  This is a handle to internal data.  Do not modify or dire things may
-       *  happen.
+       *  This is a pointer to internal data.  It is undefined to modify
+       *  the contents through the returned pointer. To get a pointer that
+       *  allows modifying the contents use @c &str[0] instead,
+       *  (or in C++17 the non-const @c str.data() overload).
       */
       const _CharT*
       data() const _GLIBCXX_NOEXCEPT
       { return _M_data(); }
 
+#if __cplusplus > 201402L
+      /**
+       *  @brief  Return non-const pointer to contents.
+       *
+       *  This is a pointer to the character sequence held by the string.
+       *  Modifying the characters in the sequence is allowed.
+      */
+      _CharT*
+      data() noexcept
+      { return _M_data(); }
+#endif
+
       /**
        *  @brief  Return copy of allocator used to construct this string.
       */

libstdc++-v3/testsuite/21_strings/basic_string/operations/data/char/2.cc

+// Copyright (C) 2016 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/>.
+
+// { dg-options "-std=gnu++17" }
+
+// C++17 21.3.1.7 [string.ops] string operations
+
+#include <string>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::string s;
+  char* p = s.data();
+  VERIFY( *p == '\0' );
+  s = "a string that is longer than a short string";
+  p = s.data();
+  VERIFY( p == &s.front() );
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/21_strings/basic_string/operations/data/wchar_t/2.cc

+// Copyright (C) 2016 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/>.
+
+// { dg-options "-std=gnu++17" }
+
+// C++17 21.3.1.7 [string.ops] string operations
+
+#include <string>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::wstring s;
+  wchar_t* p = s.data();
+  VERIFY( *p == L'\0' );
+  s = L"a string that is longer than a short string";
+  p = s.data();
+  VERIFY( p == &s.front() );
+}
+
+int
+main()
+{
+  test01();
+}