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rotate()
's return value is incorrect when middle == first
Section: 27.7.11 [alg.rotate] Status: NAD Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2014-06-17
Priority: Not Prioritized
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Discussion:
When LWG 488(i) was resolved, the intention was to return "where the subrange [first, middle)
starts after
the rotate is performed". However, this wasn't achieved in one case.
middle == last
, rotate()
does nothing and returns first
. This is good.
When middle == first
, rotate()
does nothing and returns last
. This is bad. In addition to being
inconsistent with the other do-nothing case, it prevents rotate()
from providing the useful guarantee that LWG
488(i) wanted: when [first, last)
is non-empty, rotate()
's return value should always be
dereferenceable to get the originally-first element.
Howard Hinnant:
As the author of LWG 488(i) I can assure everyone that the edge cases the proposed resolution specifies were
not specified by accident. rotate(i, i, j)
should return j
and rotate(i, j, j)
should return i
.
Doing otherwise will break working code. These return values were motivated by the uses of rotate
in the implementation
of algorithms such as stable_partition
and inplace_merge
. The results of these edge cases were not chosen lightly.
Also a good read:
notes-on-programming-2006-10-13
Summary: NAD.
[2014-06-16 Rapperswill]
Closed as NAD.
Proposed resolution:
This wording is relative to N3936.
Change 27.7.11 [alg.rotate] p2 as indicated:
-2- Returns: If
middle == first
, returnsfirst
. Otherwise, returnsfirst + (last - middle)
.