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<title>Issue 1338: LWG 1205 incorrectly applied</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++11">C++11</a> status.</em></p>
<h3 id="1338"><a href="lwg-defects.html#1338">1338</a>. LWG 1205 incorrectly applied</h3>
<p><b>Section:</b> 26.6.15 <a href="https://wg21.link/alg.search">[alg.search]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2010-06-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.search">issues</a> in [alg.search].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
LWG <a href="lwg-defects.html#1205" title="Some algorithms could more clearly document their handling of empty ranges (Status: C++11)">1205</a><sup><a href="https://cplusplus.github.io/LWG/issue1205" title="Latest snapshot">(i)</a></sup> (currently in WP) clarified the return value of several
algorithms when dealing with empty ranges.  In particular it recommended for
26.6.15 <a href="https://wg21.link/alg.search">[alg.search]</a>:
</p>

<blockquote><pre>
template&lt;class ForwardIterator1, class ForwardIterator2&gt;
  ForwardIterator1
    search(ForwardIterator1 first1, ForwardIterator1 last1,
           ForwardIterator2 first2, ForwardIterator2 last2);

template&lt;class ForwardIterator1, class ForwardIterator2,
         class BinaryPredicate&gt;
  ForwardIterator1
    search(ForwardIterator1 first1, ForwardIterator1 last1,
           ForwardIterator2 first2, ForwardIterator2 last2,
           BinaryPredicate pred);
</pre>
<blockquote>
<p>
1 <i>Effects:</i> ...
</p>
<p>
2 <i>Returns:</i> ... Returns <code>last1</code> if no such iterator is found.
</p>
<p><ins>
3 <i>Remarks:</i> Returns <code>first1</code> if <code>[first2,last2)</code> is empty.
</ins></p>
</blockquote>
</blockquote>

<p>
Unfortunately this got translated to an incorrect specification for what gets
returned when no such iterator is found (N3092):
</p>

<blockquote><pre>
template&lt;class ForwardIterator1, class ForwardIterator2&gt;
  ForwardIterator1
    search(ForwardIterator1 first1, ForwardIterator1 last1,
           ForwardIterator2 first2, ForwardIterator2 last2);

template&lt;class ForwardIterator1, class ForwardIterator2,
         class BinaryPredicate&gt;
  ForwardIterator1
    search(ForwardIterator1 first1, ForwardIterator1 last1,
           ForwardIterator2 first2, ForwardIterator2 last2,
           BinaryPredicate pred);
</pre>
<blockquote>
<p>
1 <i>Effects:</i> ...
</p>
<p>
2 <i>Returns:</i> ... Returns <code>first1</code> if <code>[first2,last2)</code> is
empty or if no such iterator is found.
</p>
</blockquote>
</blockquote>

<p>
LWG <a href="lwg-defects.html#1205" title="Some algorithms could more clearly document their handling of empty ranges (Status: C++11)">1205</a><sup><a href="https://cplusplus.github.io/LWG/issue1205" title="Latest snapshot">(i)</a></sup> is correct and N3092 is not equivalent nor correct.
</p>

<p>
I have not reviewed the other 10 recommendations of <a href="lwg-defects.html#1205" title="Some algorithms could more clearly document their handling of empty ranges (Status: C++11)">1205</a><sup><a href="https://cplusplus.github.io/LWG/issue1205" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[
Post-Rapperswil
]</i></p>


<blockquote><p>
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.
</p></blockquote>

<blockquote><p>
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
</p></blockquote>

<p><i>[
Adopted at 2010-11 Batavia
]</i></p>




<p id="res-1338"><b>Proposed resolution:</b></p>
<ol>
<li>Change [alg.search] p.2 as indicated:
<blockquote><pre>
template&lt;class ForwardIterator1, class ForwardIterator2&gt;
  ForwardIterator1
    search(ForwardIterator1 first1, ForwardIterator1 last1,
           ForwardIterator2 first2, ForwardIterator2 last2);

template&lt;class ForwardIterator1, class ForwardIterator2,
            class BinaryPredicate&gt;
  ForwardIterator1
    search(ForwardIterator1 first1, ForwardIterator1 last1,
           ForwardIterator2 first2, ForwardIterator2 last2,
           BinaryPredicate pred);
</pre>
<blockquote><p>
1 - [...]
<p/>
2 - <em>Returns</em>: The first iterator <code>i</code> in the range <code>[first1,last1 - (last2-first2))</code> such that for any nonnegative
integer <code>n</code> less than <code>last2 - first2</code> the following corresponding conditions hold: <code>*(i + n) == *(first2 + n)</code>, 
<code>pred(*(i + n), *(first2 + n)) != false</code>. Returns <code>first1</code> if <code>[first2,last2)</code> is empty <del>or</del><ins>, otherwise
	returns <code>last1</code></ins> if no such iterator is found.
</p></blockquote></blockquote>
</li>
</ol>





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