Issue 1206: Incorrect requires for move_backward and copy_backward (original) (raw)
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1206. Incorrect requires for move_backward and copy_backward
Section: 26.7.2 [alg.move] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
26.7.2 [alg.move], p6 says:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);
...
Requires:
resultshall not be in the range[first,last).
This is essentially an "off-by-one" error.
When result == last, which_is_ allowed by this specification, then the range [first, last)is being move assigned into the range [first, last). The move(forward) algorithm doesn't allow self move assignment, and neither shouldmove_backward. So last should be included in the range whichresult can not be in.
Conversely, when result == first, which is not allowed by this specification, then the range [first, last)is being move assigned into the range [first - (last-first), first). I.e. into a non-overlapping range. Therefore first should not be included in the range which result can not be in.
The same argument applies to copy_backward though copy assigning elements to themselves (result == last) should be harmless (though is disallowed by copy).
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Change 26.7.2 [alg.move], p6:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);
...
Requires:
resultshall not be in the range~~[~~(first,last]~~)~~.
Change 26.7.1 [alg.copy], p13:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 copy_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);
...
Requires:
resultshall not be in the range~~[~~(first,last]~~)~~.