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<title>Issue 290: Requirements to for_each and its function object</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#NAD">NAD</a> status.</em></p>
<h3 id="290"><a href="lwg-closed.html#290">290</a>. Requirements to for_each and its function object</h3>
<p><b>Section:</b> 26.6.5 <a href="https://wg21.link/alg.foreach">[alg.foreach]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Angelika Langer <b>Opened:</b> 2001-01-03 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#alg.foreach">active issues</a> in [alg.foreach].</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.foreach">issues</a> in [alg.foreach].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>The specification of the for_each algorithm does not have a
&quot;Requires&quot; section, which means that there are no
restrictions imposed on the function object whatsoever. In essence it
means that I can provide any function object with arbitrary side
effects and I can still expect a predictable result. In particular I
can expect that the function object is applied exactly last - first
times, which is promised in the &quot;Complexity&quot; section.
</p>

<p>I don't see how any implementation can give such a guarantee
without imposing requirements on the function object.
</p>

<p>Just as an example: consider a function object that removes
elements from the input sequence.  In that case, what does the
complexity guarantee (applies f exactly last - first times) mean?
</p>

<p>One can argue that this is obviously a nonsensical application and
a theoretical case, which unfortunately it isn't.  I have seen
programmers shooting themselves in the foot this way, and they did not
understand that there are restrictions even if the description of the
algorithm does not say so.
</p>
<p><i>[Lillehammer: This is more general than for_each.  We don't want
  the function object in transform invalidiating iterators
  either. There should be a note somewhere in clause 17 (17, not 25)
  saying that user code operating on a range may not invalidate
  iterators unless otherwise specified.  Bill will provide wording.]</i></p>


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


<blockquote>
<p>
Moved to NAD.
</p>
<p>
It was felt that the current description is adequate, and that there are
limits to what the standard can reasonably say to prohibit perverse uses
of the library.
</p>
</blockquote>



<p id="res-290"><b>Proposed resolution:</b></p>






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