2931. Missed optimization opportunity with single-argument std::next

Section: 25.4.3 [iterator.operations] Status: Open Submitter: Morwenn Opened: 2017-02-04 Last modified: 2018-12-03 11:11:36 UTC

Priority: 3

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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+=.

An equivalent optimization could be allowed by adding a single-argument overload to 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: