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<title>Issue 245: Which operations on istream_iterator trigger input operations?</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="245"><a href="lwg-closed.html#245">245</a>. Which operations on <code>istream_iterator</code> trigger input operations?</h3>
<p><b>Section:</b> 24.6.2 <a href="https://wg21.link/istream.iterator">[istream.iterator]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Andrew Koenig <b>Opened:</b> 2000-05-02 <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#istream.iterator">issues</a> in [istream.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>I do not think the standard specifies what operation(s) on istream
iterators trigger input operations.  So, for example:</p>

<pre>
        istream_iterator&lt;int&gt; i(cin);

        int n = *i++;
</pre>

<p>I do not think it is specified how many integers have been read
from cin.  The number must be at least 1, of course, but can it be 2?
More?</p>


<p><b>Rationale:</b></p>
<p>The standard is clear as written: the stream is read every time
operator++ is called, and it is also read either when the iterator is
constructed or when operator* is called for the first time.  In the
example above, exactly two integers are read from cin.</p>

<p>There may be a problem with the interaction between istream_iterator
and some STL algorithms, such as find.  There are no guarantees about
how many times find may invoke operator++.</p>




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