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* [Bug libstdc++/110952] New: Allocator::pointer is required to be implicitly convertible from and into a native pointer
@ 2023-08-08 21:58 kamkaz at windowslive dot com
2023-08-09 12:42 ` [Bug libstdc++/110952] " redi at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: kamkaz at windowslive dot com @ 2023-08-08 21:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110952
Bug ID: 110952
Summary: Allocator::pointer is required to be implicitly
convertible from and into a native pointer
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: kamkaz at windowslive dot com
Target Milestone: ---
For std::list and containers based on _Rb_tree (std::(multi_)set,
std::(multi_)map) there are some non-standard requirements currently imposed on
`std::allocator_traits<Allocator>::pointer` type.
In the implementation of these containers, this pointer type (which might be a
fancy_pointer) is required to be implicitly convertible from and into native
pointers, which in this case are equivalent to
`std::pointer_traits<pointer>::element_type *`.
This bug is present in all the GCC versions I managed to test, from 6.2 until
13.2.
The proper way to convert from/into these custom pointer types is to use:
- std::__to_address(__ptr) to obtain the native pointer (which either calls
`__ptr.operator->()` or `std::pointer_traits<pointer>::to_address(__ptr)`)
- std::pointer_traits<pointer>::pointer_to(*__ptr) to get back the potentially
"fancy" pointer.
This proper way of handling allocator pointer is already implemented in
std::forward_list.
To fix this bug, the following changes must be made:
In bits/stl_tree.h:
Current:
protected:
_Link_type
_M_get_node()
{ return _Alloc_traits::allocate(_M_get_Node_allocator(), 1); }
void
_M_put_node(_Link_type __p) _GLIBCXX_NOEXCEPT
{ _Alloc_traits::deallocate(_M_get_Node_allocator(), __p, 1); }
Fixed:
protected:
_Link_type
_M_get_node()
{
auto __ptr = _Alloc_traits::allocate(_M_get_Node_allocator(), 1);
return std::__to_address(__ptr);
}
void
_M_put_node(_Link_type __p) _GLIBCXX_NOEXCEPT
{
typedef typename _Alloc_traits::pointer _Ptr;
auto __ptr = std::pointer_traits<_Ptr>::pointer_to(*__p);
_Alloc_traits::deallocate(_M_get_Node_allocator(), __ptr, 1);
}
In bits/stl_list.h:
Current:
typename _Node_alloc_traits::pointer
_M_get_node()
{ return _Node_alloc_traits::allocate(_M_impl, 1); }
void
_M_put_node(typename _Node_alloc_traits::pointer __p) _GLIBCXX_NOEXCEPT
{ _Node_alloc_traits::deallocate(_M_impl, __p, 1); }
Fixed:
typename _Node_alloc_traits::value_type*
_M_get_node()
{
auto __ptr = _Node_alloc_traits::allocate(_M_impl, 1);
return std::__to_address(__ptr);
}
void
_M_put_node(typename _Node_alloc_traits::value_type* __p)
_GLIBCXX_NOEXCEPT
{
typedef typename _Node_alloc_traits::pointer _Ptr;
auto __ptr = std::pointer_traits<_Ptr>::pointer_to(*__p);
_Node_alloc_traits::deallocate(_M_impl, __ptr, 1);
}
This fix does not goes along with the coding style (81 characters in a line),
it might require some extra typedefs.
It is NOT a duplicate of Bug 57272 - it's not about the internal representation
of the nodes, just handling and requirements imposed on the allocator pointer.
There are no ABI issues here that I can think of.
There is a minuscule possibility it might be a breaking change for someone - if
their Fancy Pointer's implicit conversions behaved differently than its
`pointer_to` and `.operator->()` (or if they didn't provide them and relied on
implicit conversions, which are not part of the standard).
Here there is a small example reproducing the issue:
https://godbolt.org/z/fnno3jGYs
Note, that if implicit construction from `T*` and `operator T*()` are added to
the fancy pointer type, the example compiles.
(Yes, ppointer there doesn't meet the requirement of RandomAccessIterator that
is required for Allocator::pointer. However, since these functionalities are
not used by the mentioned containers, it doesn't matter here).
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug libstdc++/110952] Allocator::pointer is required to be implicitly convertible from and into a native pointer
2023-08-08 21:58 [Bug libstdc++/110952] New: Allocator::pointer is required to be implicitly convertible from and into a native pointer kamkaz at windowslive dot com
@ 2023-08-09 12:42 ` redi at gcc dot gnu.org
0 siblings, 0 replies; 2+ messages in thread
From: redi at gcc dot gnu.org @ 2023-08-09 12:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110952
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-08-09
Status|UNCONFIRMED |NEW
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=57272
Ever confirmed|0 |1
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Kamil Kaznowski from comment #0)
> It is NOT a duplicate of Bug 57272 - it's not about the internal
> representation of the nodes, just handling and requirements imposed on the
> allocator pointer.
It is though, because if we actually used the allocator's pointer then this
wouldn't be an issue in the first place.
The changes need to be conditional on C++11 or later, because pointer_traits
and __to_address aren't available in C++98.
^ permalink raw reply [flat|nested] 2+ messages in thread
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2023-08-08 21:58 [Bug libstdc++/110952] New: Allocator::pointer is required to be implicitly convertible from and into a native pointer kamkaz at windowslive dot com
2023-08-09 12:42 ` [Bug libstdc++/110952] " redi at gcc dot gnu.org
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