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operator +
in the description of the algorithmsSection: 27 [algorithms] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-08-01 Last modified: 2018-06-12
Priority: 4
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Discussion:
According to 27.1 [algorithms.general]/12,
In the description of the algorithms operators
+
and-
are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics ofa+n
is the same as that ofX tmp = a; advance(tmp, n); return tmp;
There are several places where such operator +
is applied to an output iterator — for example, see the
description of std::copy
:
template<class InputIterator, class OutputIterator> OutputIterator copy(InputIterator first, InputIterator last, OutputIterator result);-1- Effects: Copies elements in the range
[first,last)
into the range[result,result + (last - first))
starting fromfirst
and proceeding tolast
. For each non-negative integern < (last - first)
, performs*(result + n) = *(first + n)
.
std::advance
is not supposed to be applicable to output iterators, so we need a different method of description.
[2014-06-07 Daniel comments and provides wording]
The specification for output iterators is somewhat tricky, because here a sequence of increments is required to be combined with intervening assignments to the dereferenced iterator. I tried to respect this fact by using a conceptual assignment operation as part of the specification.
Another problem in the provided as-if-code is the question which requirements are imposed onn
. Unfortunately,
the corresponding function advance
is completely underspecified in this regard, so I couldn't borrow wording
from it. We cannot even assume here that n
is the difference type of the iterator, because for output iterators there is
no requirements for this associated type to be defined. The presented wording attempts to minimize assumptions, but still
can be considered as controversial.
[2018-06 Rapperswil Wednesday issues processing]
Status to Open
Proposed resolution:
This wording is relative to N4606.
Change 27.1 [algorithms.general] around p12 as indicated:
-12- In the description of the algorithms operators
+
and-
are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics ofa+n
is the same as that ofX tmp = a; advance(tmp, n); return tmp;when
X
meets the input iterator requirements (25.3.5.3 [input.iterators]), otherwise it is the same as that ofX tmp = a; for (auto i = n; i; ++tmp, (void) --i) *tmp = Expr(i); return tmp;where
Expr(i)
denotes the(n-i)
th expression that is assigned to for the corresponding algorithm; and that ofb-a
is the same as ofreturn distance(a, b);