Re: [PATCH] Implement LWG 2686, hash<error_condition>

[Christophe Lyon <christophe.lyon@linaro.org>]

On Sat, 4 May 2019 at 16:36, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 03/05/19 23:42 +0100, Jonathan Wakely wrote:
> >On 23/03/17 17:49 +0000, Jonathan Wakely wrote:
> >>On 12/03/17 13:16 +0100, Daniel Krügler wrote:
> >>>The following is an *untested* patch suggestion, please verify.
> >>>
> >>>Notes: My interpretation is that hash<error_condition> should be
> >>>defined outside of the _GLIBCXX_COMPATIBILITY_CXX0X block, please
> >>>double-check that course of action.
> >>
> >>That's right.
> >>
> >>>I noticed that the preexisting hash<error_code> did directly refer to
> >>>the private members of error_code albeit those have public access
> >>>functions. For consistency I mimicked that existing style when
> >>>implementing hash<error_condition>.
> >>
> >>I see no reason for that, so I've removed the friend declaration and
> >>used the public member functions.
> >
> >I'm going to do the same for hash<error_code> too. It can also use the
> >public members instead of being a friend.
> >
> >
> >>Although this is a DR, I'm treating it as a new C++17 feature, so I've
> >>adjusted the patch to only add the new specialization for C++17 mode.
> >>We're too close to the GCC 7 release to be adding new things to the
> >>default mode, even minor things like this. After GCC 7 is released we
> >>can revisit it and decide if we want to enable it for all modes.
> >
> >We never revisited that, and it's still only enabled for C++17 and up.
> >I guess that's OK, but we could enabled it for C++11 and 14 on trunk
> >if we want. Anybody care enough to argue for that?
> >
> >>Here's what I've tested and will be committing.
> >>
> >>
> >
> >>commit 90ca0fd91f5c65af370beb20af06bdca257aaf63
> >>Author: Jonathan Wakely <jwakely@redhat.com>
> >>Date:   Thu Mar 23 11:47:39 2017 +0000
> >>
> >>   Implement LWG 2686, std::hash<error_condition>, for C++17
> >>   2017-03-23  Daniel Kruegler  <daniel.kruegler@gmail.com>
> >>      Implement LWG 2686, Why is std::hash specialized for error_code,
> >>      but not error_condition?
> >>      * include/std/system_error (hash<error_condition>): Define for C++17.
> >>      * testsuite/20_util/hash/operators/size_t.cc (hash<error_condition>):
> >>      Instantiate test for error_condition.
> >>      * testsuite/20_util/hash/requirements/explicit_instantiation.cc
> >>      (hash<error_condition>): Instantiate hash<error_condition>.
> >>
> >>diff --git a/libstdc++-v3/include/std/system_error b/libstdc++-v3/include/std/system_error
> >>index 6775a6e..ec7d25f 100644
> >>--- a/libstdc++-v3/include/std/system_error
> >>+++ b/libstdc++-v3/include/std/system_error
> >>@@ -373,14 +373,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>_GLIBCXX_END_NAMESPACE_VERSION
> >>} // namespace
> >>
> >>-#ifndef _GLIBCXX_COMPATIBILITY_CXX0X
> >>-
> >>#include <bits/functional_hash.h>
> >>
> >>namespace std _GLIBCXX_VISIBILITY(default)
> >>{
> >>_GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>
> >>+#ifndef _GLIBCXX_COMPATIBILITY_CXX0X
> >>  // DR 1182.
> >>  /// std::hash specialization for error_code.
> >>  template<>
> >>@@ -394,12 +393,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>      return std::_Hash_impl::__hash_combine(__e._M_cat, __tmp);
> >>      }
> >>    };
> >>+#endif // _GLIBCXX_COMPATIBILITY_CXX0X
> >>+
> >>+#if __cplusplus > 201402L
> >>+  // DR 2686.
> >>+  /// std::hash specialization for error_condition.
> >>+  template<>
> >>+    struct hash<error_condition>
> >>+    : public __hash_base<size_t, error_condition>
> >>+    {
> >>+      size_t
> >>+      operator()(const error_condition& __e) const noexcept
> >>+      {
> >>+     const size_t __tmp = std::_Hash_impl::hash(__e.value());
> >>+     return std::_Hash_impl::__hash_combine(__e.category(), __tmp);
> >
> >When I changed this from using __e._M_cat (as in Daniel's patch) to
> >__e.category() I introduced a bug, because the former is a pointer to
> >the error_category (and error_category objects are unique and so can
> >be identified by their address) and the latter is the object itself,
> >so we hash the bytes of an abstract base class instead of hashing the
> >pointer to it. Oops.
> >
> >Patch coming up to fix that.
>
> Here's the fix. Tested powerpc64le-linux, committed to trunk.
>
> I'll backport this to 7, 8 and 9 as well.
>

Hi Jonathan,

Does the new test lack dg-require-filesystem-ts ?

I'm seeing link failures on arm-eabi (using newlib):
Excess errors:
/libstdc++-v3/src/c++17/fs_ops.cc:806: undefined reference to `chdir'
/libstdc++-v3/src/c++17/fs_ops.cc:583: undefined reference to `mkdir'
/libstdc++-v3/src/c++17/fs_ops.cc:1134: undefined reference to `chmod'
/libstdc++-v3/src/c++17/../filesystem/ops-common.h:439: undefined
reference to `chmod'
/libstdc++-v3/src/c++17/fs_ops.cc:750: undefined reference to `pathconf'
/libstdc++-v3/src/c++17/fs_ops.cc:769: undefined reference to `getcwd'

Christophe