Section: 27.4.3 [iterator.operations] Status: Resolved Submitter: Thomas Opened: 2008-12-14 Last modified: 2016-02-10
Priority: Not Prioritized
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Addresses UK 270
Regarding the std::distance - function, 27.4.3 [iterator.operations] p.4 says:
Returns the number of increments or decrements needed to get from first to last.
This sentence is completely silent about the sign of the return value. 27.4.3 [iterator.operations] p.1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.
Practically speaking, my implementations did what common sense and knowledge based on pointer arithmetic forecasts, namely a positive sign for increments (that is, going from first to last by operator++), and a negative sign for decrements (going from first to last by operator--).
Here are my two questions:
First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.
Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?
[ Daniel adds: ]
My first thought was that resolution 204 would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:
Referring to N2798, 27.4.3 [iterator.operations]/ p.4 says:
Effects: Returns the number of increments or decrements needed to get from first to last.
IMO the part " or decrements" is in contradiction to p. 5 which says
Requires: last shall be reachable from first.
because "reachable" is defined in 99 [iterator.concepts]/7 as
An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j.[..]
Here is wording that would be consistent with this definition of "reachable":
Change 27.4.3 [iterator.operations] p4 as follows:
Effects: Returns the number of increments
or decrementsneeded to get from first to last.
Thomas adds more discussion and an alternative view point here.
[ Summit: ]
The proposed wording below was verbally agreed to. Howard provided.
[ Batavia (2009-05): ]
Pete reports that a recent similar change has been made for the advance() function.
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-07 Frankfurt: ]
Leave Open pending arrival of a post-Concepts WD.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".
[ 2010 Pittsburgh: ]
NAD Editorial. Rationale added below.
Solved by N3066.
Change 27.2.7 [random.access.iterators], Table 105 as indicated [This change is not essential but it simplifies the specification] for the row with expression "b - a" and the column Operational semantics:
(a < b) ?distance(a,b) : -distance(b,a)
Change 27.4.3 [iterator.operations]/4+5 as indicated:
template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);
Returns the number of increments or decrementsneeded to get from first to last.
5 Requires: last shall be reachable from first.