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std::prev
should not require BidirectionalIterator
Section: 25.4.3 [iterator.operations] Status: New Submitter: Billy O'Neal III Opened: 2019-04-03 Last modified: 2022-04-22
Priority: 3
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Discussion:
MSVC++ (and apparently libc++) have asserts that std::prev
only accepts BidirectionalIterator
s,
because it's declared in the standard as accepting only BidirectionalIterator
. libc++ changed their tests
(in this commit),
apparently from a bug report from Ville and Jonathan, saying that one could theoretically call std::prev
with a negative number.
prev
requires a BidirectionalIterator
,
but I don't see the usual wording that connects template type parameters of that name to the <algorithm>
requirements or similar. So perhaps one could argue that the name Bidirectional
there has no meaning. Even
if that is the case, that's a defect in the other direction.
[2019-06-12 Priority set to 3 after reflector discussion]
[2022-04-22; Jonathan adds a comment]
P2408 changes the requirements for
types substituting BidirectionalIterator
etc. in the Algorithms clause.
We should consider whether that is appropriate here, especially as algorithms
might make use of std::prev
internally.
An algorithm that was changed by P2408 to accept types that model
bidirectional_iterator
instead of requiring
Cpp17BidirectionalIterator
might have to stop using
std::prev
if we don't resolve this issue to allow it.
We should consider whether distance
, advance
and next
need the same treatment.
Proposed resolution:
This wording is relative to N4810.
[Drafting Note: Three mutually exclusive options are prepared, depicted below by Option A, Option B, and Option C, respectively.]
Option A
NAD, the name BidirectionalIterator
actually means that prev
requires bidirectional iterators, in which
case this change to libcxx is incorrect.
Option B
Modify 25.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
// 25.4.3 [iterator.operations], iterator operations […] template<classBidirectionalInputIterator> constexprBidirectionalInputIterator prev(BidirectionalInputIterator x, typename iterator_traits<BidirectionalInputIterator>::difference_type n = 1);
Modify 25.4.3 [iterator.operations] as indicated:
template<classBidirectionalInputIterator> constexprBidirectionalInputIterator prev(BidirectionalInputIterator x, typename iterator_traits<BidirectionalInputIterator>::difference_type n = 1);-7- Effects: Equivalent to:
advance(x, -n); return x;
Option C
The intent of the wording is that the template parameters apply requirements, and the defect is that they do not. We should add a requirement in 25.4.3 [iterator.operations]/1 to the effect that the template parameter names impose said requirements.