Here is what I eventually would like to commit.
I was not able to remove the _Safe_iterator_base branch in ptr_traits.h.
When adding the _Safe_iterator overload in C++20 and removing the branch
the 20_util/to_address/debug.cc test started to fail because it was not
calling my overload. I tried to declare the overload in ptr_traits.h
directly so it is known at the time it is used in std::to_address but
then it failed to match it with the implementation in safe_iterator.h.
The declaration was not easy to do and I guess I had it wrong.
But it does not matter cause I think this version is the simplest one
(as it does not change a lot of code).
libstdc++: Overload std::__to_address for __gnu_cxx::__normal_iterator.
Prefer to overload __to_address to partially specialize
std::pointer_traits because
std::pointer_traits would be mostly useless. Moreover partial
specialization of
pointer_traits<__normal_iterator<P, C>> fails to rebind C, so you
get incorrect types
like __normal_iterator<long*, vector<int>>. In the case of
__gnu_debug::_Safe_iterator
the to_pointer method is impossible to implement correctly because
we are missing
the parent container to associate the iterator to.
libstdc++-v3/ChangeLog:
* include/bits/stl_iterator.h
(std::pointer_traits<__gnu_cxx::__normal_iterator<>>): Remove.
(std::__to_address(const __gnu_cxx::__normal_iterator<>&)):
New for C++11 to C++17.
* include/debug/safe_iterator.h
(std::__to_address(const
__gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<>, _Sequence>&)):
New for C++11 to C++17.
* testsuite/24_iterators/normal_iterator/to_address.cc: Add
check on std::vector::iterator
to validate both __gnu_cxx::__normal_iterator<>
__to_address overload in normal mode and
__gnu_debug::_Safe_iterator in _GLIBCXX_DEBUG mode.
Tested under Linux x86_64 normal and _GLIBCXX_DEBUG modes for
C++11/C++14/C++17/C++20.
Ok to commit ?
François
On 14/12/21 2:12 pm, Jonathan Wakely wrote:
On Tue, 14 Dec 2021 at 06:53, François Dumont wrote:
Hi
Any conclusion regarding this thread ?
François
On 06/10/21 7:25 pm, François Dumont wrote:
> I forgot to ask if with this patch this overload:
>
> template<typename _Ptr, typename... _None>
> constexpr auto
> __to_address(const _Ptr& __ptr, _None...) noexcept
> {
> if constexpr
(is_base_of_v<__gnu_debug::_Safe_iterator_base, _Ptr>)
> return std::__to_address(__ptr.base().operator->());
> else
> return std::__to_address(__ptr.operator->());
> }
>
> should be removed ?
No, definitely not.
That is the default overload for types that do not have a
pointer_traits::to_address specialization. If you remove it,
__to_address won't work for fancy pointers or any other pointer-like
types. That would completely break it.
The purpose of C++20's std::to_address is to get a real pointer from a
pointer-like type. Using it with iterators is not the primary use
case, but it needs to work with contiguous iterators because those are
pointer-like. I made it work correctly with __normal_iterator because
that was necessary to support the uses of std::__to_address in <regex>
and <filesystem>, but I removed those uses in:
https://gcc.gnu.org/g:247bac507e63b32d4dc23ef1c55f300aafea24c6
<https://gcc.gnu.org/g:247bac507e63b32d4dc23ef1c55f300aafea24c6>
https://gcc.gnu.org/g:b83b810ac440f72e7551b6496539e60ac30c0d8a
<https://gcc.gnu.org/g:b83b810ac440f72e7551b6496539e60ac30c0d8a>
So now we don't really need the C++17 version of std::__to_address to
work with __normal_iterator at all.
I think it's OK to add the overload for __normal_iterator though, but
only for C++11/14/17, because the default std::__to_address handles
__normal_iterator correctly in C++20.
> Or perhaps just the _Safe_iterator_base branch in it ?
Yes, you can just remove that branch, because your new overload
handles it.
>
> On 06/10/21 7:18 pm, François Dumont wrote:
>> Here is another proposal with the __to_address overload.
>>
>> I preferred to let it open to any kind of __normal_iterator
>> instantiation cause afaics std::vector supports fancy pointer
types.
>> It is better if __to_address works fine also in this case, no ?
If we intend to support that, then we should verify it in the
testsuite, using __gnu_test::CustomPointerAlloc.
>> libstdc++: Overload std::__to_address for
>> __gnu_cxx::__normal_iterator.
>>
>> Prefer to overload __to_address to partially specialize
>> std::pointer_traits because
>> std::pointer_traits would be mostly useless. In the case of
>> __gnu_debug::_Safe_iterator
>> the to_pointer method is even impossible to implement
correctly
>> because we are missing
>> the parent container to associate the iterator to.
To record additional rationale in the git history, please add that the
partial specialization of pointer_traits<__normal_iterator<P, C>>
fails to rebind C, so you get incorrect types like
__normal_iterator<long*, vector<int>>.
>>
>> libstdc++-v3/ChangeLog:
>>
>> * include/bits/stl_iterator.h
>> (std::pointer_traits<__gnu_cxx::__normal_iterator<>>): Remove.
OK to remove this (it's broken anyway).
>> (std::__to_address(const
>> __gnu_cxx::__normal_iterator<>&)): New.
Please make this only defined for C++11, 14 and 17.
>> * include/debug/safe_iterator.h
>> (std::__to_address(const
>> __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<>,
>> _Sequence>&)):
>> New.
OK to add this (including for C++20), and remove the _Safe_iterator
branch from the C++20 std::__to_address in <bits/ptr_traits.h>.
I think this new overload could return
std::__to_address(__it.base().base()) though. That saves a function
call, by going directly to the value stored in the __normal_iterator.
>> *
testsuite/24_iterators/normal_iterator/to_address.cc:
>> Add check on std::vector::iterator
>> to validate both __gnu_cxx::__normal_iterator<>
>> __to_address overload in normal mode and the
Add similar checks for vector<int, __gnu_test::CustomPointerAlloc<int>>.
OK with those changes, thanks.
diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 6bd860b803e..ac9342112f4 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1349,32 +1349,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus >= 201103L
- // Need to specialize pointer_traits because the primary template will
- // deduce element_type of __normal_iterator<T*, C> as T* rather than T.
+#if __cplusplus <= 201703L
+ // Need to overload __to_address because the pointer_traits primary template
+ // will deduce element_type of __normal_iterator<T*, C> as T* rather than T.
template<typename _Iterator, typename _Container>
- struct pointer_traits<__gnu_cxx::__normal_iterator<_Iterator, _Container>>
- {
- private:
- using _Base = pointer_traits<_Iterator>;
-
- public:
- using element_type = typename _Base::element_type;
- using pointer = __gnu_cxx::__normal_iterator<_Iterator, _Container>;
- using difference_type = typename _Base::difference_type;
-
- template<typename _Tp>
- using rebind = __gnu_cxx::__normal_iterator<_Tp, _Container>;
-
- static pointer
- pointer_to(element_type& __e) noexcept
- { return pointer(_Base::pointer_to(__e)); }
-
-#if __cplusplus >= 202002L
- static element_type*
- to_address(pointer __p) noexcept
- { return __p.base(); }
+ constexpr auto
+ __to_address(const __gnu_cxx::__normal_iterator<_Iterator,
+ _Container>& __it) noexcept
+ -> decltype(std::__to_address(__it.base()))
+ { return std::__to_address(__it.base()); }
#endif
- };
/**
* @addtogroup iterators
diff --git a/libstdc++-v3/include/debug/safe_iterator.h b/libstdc++-v3/include/debug/safe_iterator.h
index 5584d06de5a..9c821c82e17 100644
--- a/libstdc++-v3/include/debug/safe_iterator.h
+++ b/libstdc++-v3/include/debug/safe_iterator.h
@@ -1013,6 +1013,23 @@ namespace __gnu_debug
} // namespace __gnu_debug
+#if __cplusplus >= 201103L && __cplusplus <= 201703L
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+ template<typename _Iterator, typename _Container, typename _Sequence>
+ constexpr auto
+ __to_address(const __gnu_debug::_Safe_iterator<
+ __gnu_cxx::__normal_iterator<_Iterator, _Container>,
+ _Sequence>& __it) noexcept
+ -> decltype(std::__to_address(__it.base().base()))
+ { return std::__to_address(__it.base().base()); }
+
+_GLIBCXX_END_NAMESPACE_VERSION
+}
+#endif
+
#undef _GLIBCXX_DEBUG_VERIFY_DIST_OPERANDS
#undef _GLIBCXX_DEBUG_VERIFY_REL_OPERANDS
#undef _GLIBCXX_DEBUG_VERIFY_EQ_OPERANDS
diff --git a/libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc b/libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc
index 510d627435f..6afc6540609 100644
--- a/libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc
+++ b/libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc
@@ -1,6 +1,19 @@
// { dg-do compile { target { c++11 } } }
#include <string>
+#include <vector>
#include <memory>
-char* p = std::__to_address(std::string("1").begin());
-const char* q = std::__to_address(std::string("2").cbegin());
+#include <testsuite_allocator.h>
+
+char* p __attribute__((unused))
+ = std::__to_address(std::string("1").begin());
+const char* q __attribute__((unused))
+ = std::__to_address(std::string("2").cbegin());
+int* r __attribute__((unused))
+ = std::__to_address(std::vector<int>(1, 1).begin());
+const int* s __attribute__((unused))
+ = std::__to_address(std::vector<int>(1, 1).cbegin());
+int* t __attribute__((unused))
+ = std::__to_address(std::vector<int, __gnu_test::CustomPointerAlloc<int>>(1, 1).begin());
+const int* u __attribute__((unused))
+ = std::__to_address(std::vector<int, __gnu_test::CustomPointerAlloc<int>>(1, 1).cbegin());
