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Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-05-14 Last modified: 2017-07-30
Priority: 3
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Discussion:
For an allocator A<T> which defines A<T>::pointer to a class type, i.e. not T*, I see no requirement that A<T>::pointer is convertible to A<U>::pointer, even if T* is convertible to U*. Such conversions are needed in containers to convert from e.g. ListNodeBase* to ListNode<T>*.
The obvious way to do such conversions appears to be pointer_traits::pointer_to(), but that's ill-formed if the static member function A<T>::pointer::pointer_to() doesn't exist and the allocator requirements don't mention that function, so you need to cast A<T>::pointer to A<T>::void_pointer then cast that to A<U>::pointer.
Is converting via void_pointer really intended, or are we missing a requirement that pointer_traits<A<T>::pointer>::pointer_to() be well-formed?
Proposed resolution:
Add to the Allocator requirements table the following requirement:
The expression pointer_traits<XX::pointer>::pointer_to(r) is well-defined.
[2013-09 Chicago]
Pablo to come back with proposed wording
[2015-07 Telecon]
Marshall to ping Pablo for proposed wording and disable current wording.
Previous resolution [SUPERSEDED]:
Edit Table 28 as indicated:
Table 28 — Allocator requirements (continued) Expression Return type Assertion/note pre-/post-condition Default … static_cast<X::const_pointer>(z) X::const_pointer static_cast<X::const_pointer>(z) == q pointer_traits<X::pointer>::pointer_to(r) X::pointer …
[2016-11-12, Issaquah]
This is related to 1521.
Sat PM: Restore original P/R and move to tentatively ready.
Proposed resolution:
Edit Table 28 as indicated:
Table 28 — Allocator requirements (continued) Expression Return type Assertion/note pre-/post-condition Default … static_cast<X::const_pointer>(z) X::const_pointer static_cast<X::const_pointer>(z) == q pointer_traits<X::pointer>::pointer_to(r) X::pointer …