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std::nextSection: 24.4.3 [iterator.operations] Status: Open Submitter: Morwenn Opened: 2017-02-04 Last modified: 2018-12-03
Priority: 3
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Discussion:
It seems that std::next is missing an optimization opportunity when taking a single parameter. The standard
mandates that std::next shall call std::advance on the passed iterator and return it. For random-access
iterators, it means that operator+= will be called on the iterator. However, if a single-argument overload was
added to std::next, it could call ++it directly instead of std::advance(it, 1), which means
that operator++ would be called instead of operator+=. This might make a small performance difference
for complicated iterators such as std::deque's ones, where operator++ has a simpler logic than
operator+=.
std::prev too.
[2017-03-04, Kona]
Set priority to 3. Alisdair to provide wording.
[2018-11-30, Jonathan comments, recommending NAD]
Jonathan suggested NAD, because the proposed "just use increment when n==1" optimization can be done in
std::next (and/or std::advance, and/or complicated iterators like deque::iterator) without
adding an overload. Billy said the overload would avoid metaprogramming costs for dispatching to the right
std::advance, and help in non-optimized builds. Zhihao said the overload would make it clear to users that
the n==1 case is optimized (Jonathan thinks this is irrelevant as there's no requirement that we tell users
what we optimize).
Proposed resolution: