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<title>Issue 365: Lack of const-qualification in clause 27</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#CD1">CD1</a> status.</em></p>
<h3 id="365"><a href="lwg-defects.html#365">365</a>. Lack of const-qualification in clause 27</h3>
<p><b>Section:</b> 31 <a href="https://wg21.link/input.output">[input.output]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Walter Brown, Marc Paterno <b>Opened:</b> 2002-05-10 <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#input.output">issues</a> in [input.output].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Some stream and streambuf member functions are declared non-const,
even thought they appear only to report information rather than to
change an object's logical state.  They should be declared const.  See
document N1360 for details and rationale.
</p>

<p>The list of member functions under discussion: <code>in_avail</code>,
<code>showmanyc</code>, <code>tellg</code>, <code>tellp</code>, <code>is_open</code>.</p>

<p>Related issue: <a href="lwg-closed.html#73" title="is_open should be const (Status: NAD)">73</a><sup><a href="https://cplusplus.github.io/LWG/issue73" title="Latest snapshot">(i)</a></sup></p>



<p id="res-365"><b>Proposed resolution:</b></p>
<p>In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13</p>
<p>Replace</p>
<pre>
  bool is_open();
</pre>
<p>with</p>
<pre>
  bool is_open() const;
</pre>


<p><b>Rationale:</b></p>
<p>Of the changes proposed in N1360, the only one that is safe is
changing the filestreams' is_open to const.  The LWG believed that
this was NAD the first time it considered this issue (issue <a href="lwg-closed.html#73" title="is_open should be const (Status: NAD)">73</a><sup><a href="https://cplusplus.github.io/LWG/issue73" title="Latest snapshot">(i)</a></sup>), but now thinks otherwise.  The corresponding streambuf
member function, after all,is already const.</p>

<p>The other proposed changes are less safe, because some streambuf
functions that appear merely to report a value do actually perform
mutating operations.  It's not even clear that they should be
considered "logically const", because streambuf has two interfaces, a
public one and a protected one.  These functions may, and often do,
change the state as exposed by the protected interface, even if the
state exposed by the public interface is unchanged.</p>

<p>Note that implementers can make this change in a binary compatible
way by providing both overloads; this would be a conforming extension.</p>






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