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forward_list::resize(size_type, const value_type&)
effects incorrectSection: 24.3.10.5 [forward.list.modifiers] Status: C++17 Submitter: Tim Song Opened: 2016-01-14 Last modified: 2023-02-07
Priority: 0
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Discussion:
[forwardlist.modifiers]/29 says that the effects of forward_list::resize(size_type sz, const value_type& c)
are:
Effects: If
sz < distance(begin(), end())
, erases the lastdistance(begin(), end()) - sz
elements from the list. Otherwise, insertssz - distance(begin(), end())
elements at the end of the list such that each new element,e
, is initialized by a method equivalent to callingallocator_traits<allocator_type>::construct(get_allocator(), std::addressof(e), c)
.
In light of LWG 2218(i), the use of allocator_traits<allocator_type>::construct
is incorrect,
as a rebound allocator may be used. There's no need to repeat this information, in any event — no other specification
of resize()
does it.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
[2016-02-11, Alisdair requests reopening]
I believe the standard is correct as written, and that by removing the clear direction to make the copy with uses-allocator-construction, we open ourselves to disputing this very point again at some point in the future.
The issue seems to be complaining that a rebound allocator may be used instead of the allocator returned byget_allocator()
call, and nailing us down to exactly which
instantiation of allocator_traits
is used. Given the requirements on allocators being
constructible from within the same template "family" though, and specifically that
copies compare equal and can allocate/deallocate on each other's behalf, this should
clearly fall under existing as-if freedom. The construct call is even more clear, as
there is no requirement that the allocator to construct be of a kind that can allocate
the specific type being constructed — a freedom granted precisely so this kind of code
can be written, and be correct, regardless of internal node type of any container and
the actual rebound allocator used internally.
I think the new wording is less clear than the current wording, and would prefer to
resolve as NAD.
Proposed resolution:
This wording is relative to N4567.
Edit [forwardlist.modifiers]/29 as indicated:
[Drafting note: "copies of
c
" is the phrase used byvector::resize
anddeque::resize
.]
void resize(size_type sz, const value_type& c);-29- Effects: If
sz < distance(begin(), end())
, erases the lastdistance(begin(), end()) - sz
elements from the list. Otherwise, insertssz - distance(begin(), end())
elementscopies ofc
at the end of the listsuch that each new element,.e
, is initialized by a method equivalent to callingallocator_traits<allocator_type>::construct(get_allocator(), std::addressof(e), c)