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<title>Issue 865: More algorithms that throw away information</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="865"><a href="lwg-defects.html#865">865</a>. More algorithms that throw away information</h3>
<p><b>Section:</b> 26.7.6 <a href="https://wg21.link/alg.fill">[alg.fill]</a>, 26.7.7 <a href="https://wg21.link/alg.generate">[alg.generate]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2008-07-13 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</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>
In regard to library defect <a href="lwg-defects.html#488" title="rotate throws away useful information (Status: CD1)">488</a><sup><a href="https://cplusplus.github.io/LWG/issue488" title="Latest snapshot">(i)</a></sup> I found some more algorithms which
unnecessarily throw away information. These are typically algorithms,
which sequentially write into an <code>OutputIterator</code>, but do not return the
final value of this output iterator. These cases are:
</p>

<ol>
<li>
<pre>template&lt;class OutputIterator, class Size, class T&gt;
void fill_n(OutputIterator first, Size n, const T&amp; value);</pre></li>

<li>
<pre>template&lt;class OutputIterator, class Size, class Generator&gt;
void generate_n(OutputIterator first, Size n, Generator gen);</pre></li>
</ol>
<p>
In both cases the minimum requirements on the iterator are
<code>OutputIterator</code>, which means according to the requirements of
24.3.5.4 <a href="https://wg21.link/output.iterators">[output.iterators]</a> p. 2 that only single-pass iterations are guaranteed.
So, if users of <code>fill_n</code> and <code>generate_n</code> have <em>only</em> an <code>OutputIterator</code>
available, they have no chance to continue pushing further values
into it, which seems to be a severe limitation to me.
</p>

<p><i>[
Post Summit Daniel "conceptualized" the wording.
]</i></p>


<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
Alisdair likes the idea, but has concerns about the specific wording
about the returns clauses.
</p>
<p>
Alan notes this is a feature request.
</p>
<p>
Bill notes we have made similar changes to other algorithms.
</p>
<p>
Move to Open.
</p>
</blockquote>

<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote><p>
We have a consensus for moving forward on this issue, but Daniel needs
to deconceptify it.
</p></blockquote>

<p><i>[
2009-07-25 Daniel provided non-concepts wording.
]</i></p>


<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
Moved to Ready.
</p></blockquote>



<p id="res-865"><b>Proposed resolution:</b></p>

<ol>
<li>
<p>
Replace the current declaration of <code>fill_n</code> in 26 <a href="https://wg21.link/algorithms">[algorithms]</a>/2, header
<code>&lt;algorithm&gt;</code> synopsis and in 26.7.6 <a href="https://wg21.link/alg.fill">[alg.fill]</a> by
</p>

<blockquote><pre>
template&lt;class OutputIterator, class Size, class T&gt;
  <del>void</del><ins>OutputIterator</ins> fill_n(OutputIterator first, Size n, const T&amp; value);
</pre></blockquote>

<p>
Just after the effects clause add a new returns clause saying:
</p>

<blockquote><p>
<ins><i>Returns:</i> For <code>fill_n</code> and positive <code>n</code>, returns <code>first + n</code>. Otherwise
returns <code>first</code> for <code>fill_n</code>.</ins>
</p></blockquote>
</li>

<li>
<p>
Replace the current declaration of <code>generate_n</code> in 26 <a href="https://wg21.link/algorithms">[algorithms]</a>/2,
header <code>&lt;algorithm&gt;</code> synopsis and in 26.7.7 <a href="https://wg21.link/alg.generate">[alg.generate]</a> by
</p>

<blockquote><pre>
template&lt;class OutputIterator, class Size, class Generator&gt;
  <del>void</del><ins>OutputIterator</ins> generate_n(OutputIterator first, Size n, Generator gen);
</pre></blockquote>

<p>
Just after the effects clause add a new returns clause saying:
</p>

<blockquote><p>
<ins>For <code>generate_n</code> and positive <code>n</code>, returns <code>first + n</code>. Otherwise
returns <code>first</code> for <code>generate_n</code>.</ins>
</p></blockquote>
</li>
</ol>







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