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1338. LWG 1205 incorrectly applied

Section: 26.6.15[alg.search]Status:C++11Submitter: Howard HinnantOpened: 2010-06-25Last modified: 2016-01-28

Priority:Not Prioritized

View all otherissues in [alg.search].

View all issues withC++11 status.

Discussion:

LWG1205⁽ⁱ⁾ (currently in WP) clarified the return value of severalalgorithms when dealing with empty ranges. In particular it recommended for26.6.15[alg.search]:

template<class ForwardIterator1, class ForwardIterator2>  ForwardIterator1    search(ForwardIterator1 first1, ForwardIterator1 last1,           ForwardIterator2 first2, ForwardIterator2 last2);template<class ForwardIterator1, class ForwardIterator2,         class BinaryPredicate>  ForwardIterator1    search(ForwardIterator1 first1, ForwardIterator1 last1,           ForwardIterator2 first2, ForwardIterator2 last2,           BinaryPredicate pred);

1Effects: ...

2Returns: ... Returnslast1 if no such iterator is found.

3Remarks: Returnsfirst1 if[first2,last2) is empty.

Unfortunately this got translated to an incorrect specification for what getsreturned when no such iterator is found (N3092):

template<class ForwardIterator1, class ForwardIterator2>  ForwardIterator1    search(ForwardIterator1 first1, ForwardIterator1 last1,           ForwardIterator2 first2, ForwardIterator2 last2);template<class ForwardIterator1, class ForwardIterator2,         class BinaryPredicate>  ForwardIterator1    search(ForwardIterator1 first1, ForwardIterator1 last1,           ForwardIterator2 first2, ForwardIterator2 last2,           BinaryPredicate pred);

1Effects: ...

2Returns: ... Returnsfirst1 if[first2,last2) isempty or if no such iterator is found.

LWG1205⁽ⁱ⁾ is correct and N3092 is not equivalent nor correct.

I have not reviewed the other 10 recommendations of1205⁽ⁱ⁾.

[Post-Rapperswil]

It was verified that none of the remaining possibly affected algorithms does have any similar problems and a concrete P/R was added that used a similar style as has been applied to the other cases.

Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[Adopted at 2010-11 Batavia]

Proposed resolution:

1. Change [alg.search] p.2 as indicated:

   template<class ForwardIterator1, class ForwardIterator2>  ForwardIterator1    search(ForwardIterator1 first1, ForwardIterator1 last1,           ForwardIterator2 first2, ForwardIterator2 last2);template<class ForwardIterator1, class ForwardIterator2,            class BinaryPredicate>  ForwardIterator1    search(ForwardIterator1 first1, ForwardIterator1 last1,           ForwardIterator2 first2, ForwardIterator2 last2,           BinaryPredicate pred);

   1 - [...]

   2 -Returns: The first iteratori in the range[first1,last1 - (last2-first2)) such that for any nonnegativeintegern less thanlast2 - first2 the following corresponding conditions hold:*(i + n) == *(first2 + n),pred(*(i + n), *(first2 + n)) != false. Returnsfirst1 if[first2,last2) is emptyor, otherwisereturnslast1 if no such iterator is found.