Re: [PATCH] Add std::string constructor for substring of string_view (LWG 2742)

[Jonathan Wakely]

On 15/11/16 14:33 +, Jonathan Wakely wrote:

This is another issue resolution for C++17 features that was approved
at the recent meeting. I think this resolution is wrong too, but in
this case the fix is obvious so I've gone ahead and done it.

* doc/xml/manual/intro.xml: Document LWG 2742 status.
* doc/html/*: Regenerate.
* include/bits/basic_string.h
(basic_string(const T&, size_type, size_type, const Allocator&)): Add
constructor for substring of basic_string_view, as per LWG 2742 but
with additional constraint to fix ambiguity.
* testsuite/21_strings/basic_string/cons/char/9.cc: New test.
* testsuite/21_strings/basic_string/cons/wchar_t/9.cc: New test.

Tested powerpc64le-linux, comitted to trunk.


I forgot I already added an convenience alias template for checking
the condition in this patch.

Tested powerpc64le-linux, comitted to trunk.
commit f7852de7c77f0d9cc8520d10549da0652e334dc7
Author: Jonathan Wakely 
Date:   Tue Nov 15 18:55:35 2016 +

Use existing helper for new std::string constructor

	* include/bits/basic_string.h: Reuse _If_sv alias template for new
	constructor.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 943e88d..9af7bfb 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -585,6 +585,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	{ _M_construct(__beg, __end); }
 
 #if __cplusplus > 201402L
+  template
+	using _If_sv = enable_if_t<
+	  __and_>::value,
+	  _Res>;
+
   /**
*  @brief  Construct string from a substring of a string_view.
*  @param  __t   Source string view.
@@ -592,9 +598,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
*  @param  __n   The number of characters to copy from __t.
*  @param  __a   Allocator to use.
*/
-  template,
-			__not_>>
+  template>
 	basic_string(const _Tp& __t, size_type __pos, size_type __n,
 		 const _Alloc& __a = _Alloc())
 	: basic_string(__sv_type(__t).substr(__pos, __n), __a) { }
@@ -1252,12 +1256,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   append(__sv_type __sv)
   { return this->append(__sv.data(), __sv.size()); }
 
-  template
-	using _If_sv = enable_if_t<
-	  __and_>::value,
-	  _Res>;
-
   /**
*  @brief  Append a range of characters from a string_view.
*  @param __sv  The string_view to be appended from.

[PATCH] Add std::string constructor for substring of string_view (LWG 2742)

[Jonathan Wakely]

This is another issue resolution for C++17 features that was approved
at the recent meeting. I think this resolution is wrong too, but in
this case the fix is obvious so I've gone ahead and done it.

* doc/xml/manual/intro.xml: Document LWG 2742 status.
* doc/html/*: Regenerate.
* include/bits/basic_string.h
(basic_string(const T&, size_type, size_type, const Allocator&)): Add
constructor for substring of basic_string_view, as per LWG 2742 but
with additional constraint to fix ambiguity.
* testsuite/21_strings/basic_string/cons/char/9.cc: New test.
* testsuite/21_strings/basic_string/cons/wchar_t/9.cc: New test.

Tested powerpc64le-linux, comitted to trunk.

commit d8d7a6fba221a205f28212a8bb6288aa724d5c63
Author: Jonathan Wakely 
Date:   Tue Nov 15 12:52:42 2016 +

Add std::string constructor for substring of string_view (LWG 2742)

	* doc/xml/manual/intro.xml: Document LWG 2742 status.
	* doc/html/*: Regenerate.
	* include/bits/basic_string.h
	(basic_string(const T&, size_type, size_type, const Allocator&)): Add
	constructor for substring of basic_string_view, as per LWG 2742 but
	with additional constraint to fix ambiguity.
	* testsuite/21_strings/basic_string/cons/char/9.cc: New test.
	* testsuite/21_strings/basic_string/cons/wchar_t/9.cc: New test.

diff --git a/libstdc++-v3/doc/xml/manual/intro.xml b/libstdc++-v3/doc/xml/manual/intro.xml
index 0df24bb..7f2586d 100644
--- a/libstdc++-v3/doc/xml/manual/intro.xml
+++ b/libstdc++-v3/doc/xml/manual/intro.xml
@@ -1107,6 +1107,14 @@ requirements of the license of GCC.
 Define the value_compare typedef.
 
 
+http://www.w3.org/1999/xlink; xlink:href="../ext/lwg-defects.html#2742">2742:
+   Inconsistent string interface taking string_view
+   
+
+Add the new constructor and additionally constrain it
+  to avoid ambiguities with non-const charT*.
+
+
 http://www.w3.org/1999/xlink; xlink:href="../ext/lwg-defects.html#2748">2748:
swappable traits for optionals

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index b80e270..943e88d 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -586,12 +586,27 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 #if __cplusplus > 201402L
   /**
+   *  @brief  Construct string from a substring of a string_view.
+   *  @param  __t   Source string view.
+   *  @param  __pos The index of the first character to copy from __t.
+   *  @param  __n   The number of characters to copy from __t.
+   *  @param  __a   Allocator to use.
+   */
+  template,
+			__not_>>
+	basic_string(const _Tp& __t, size_type __pos, size_type __n,
+		 const _Alloc& __a = _Alloc())
+	: basic_string(__sv_type(__t).substr(__pos, __n), __a) { }
+
+  /**
*  @brief  Construct string from a string_view.
*  @param  __sv  Source string view.
*  @param  __a  Allocator to use (default is default allocator).
*/
-  explicit basic_string(__sv_type __sv, const _Alloc& __a = _Alloc())
-	: basic_string(__sv.data(), __sv.size(), __a) {}
+  explicit
+  basic_string(__sv_type __sv, const _Alloc& __a = _Alloc())
+  : basic_string(__sv.data(), __sv.size(), __a) { }
 #endif // C++17
 
   /**
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/9.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/9.cc
new file mode 100644
index 000..0024ffc
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/9.cc
@@ -0,0 +1,46 @@
+// 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
+// .
+
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++1z } }
+
+#include 
+#include 
+
+void
+test01()
+{
+  using C = char;
+  using string_type = std::basic_string;
+  using view_type = std::basic_string_view;
+
+  std::allocator alloc;
+  VERIFY( string_type(view_type("string")) == "string" );
+  VERIFY( string_type(view_type("string"), alloc) == "string" );
+
+  // LWG 2742
+  VERIFY( string_type("substring", 3, 6) == "string" );
+  VERIFY( string_type("substring", 3, 6,