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<title>Issue 941: Ref-qualifiers for assignment operators</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="941"><a href="lwg-closed.html#941">941</a>. Ref-qualifiers for assignment operators</h3>
<p><b>Section:</b> 16 <a href="https://wg21.link/library">[library]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Niels Dekker <b>Opened:</b> 2008-12-18 <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#library">active issues</a> in [library].</p>
<p><b>View all other</b> <a href="lwg-index.html#library">issues</a> in [library].</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 assignment and equality operators <code>=</code> and <code>==</code> are easily confused, just
because of their visual similarity, and in this case a simple typo can cause
a serious bug. When the left side of an <code>operator=</code> is an rvalue, it's
highly unlikely that the assignment was intended by the programmer:
</p>
<blockquote><pre>
if ( func() = value )  // Typical typo: == intended!
</pre></blockquote>
<p>
Built-in types don't support assignment to an rvalue, but unfortunately,
a lot of types provided by the Standard Library do.
</p>
<p>
Fortunately the language now offers a syntax to prevent a certain member
function from having an rvalue as <code>*this</code>: by adding a ref-qualifier (<code>&amp;</code>)
to the member function declaration.  Assignment operators are explicitly
mentioned as a use case of ref-qualifiers, in "Extending Move Semantics
To <code>*this</code> (Revision 1)",
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1821.htm">N1821</a> by Daveed
Vandevoorde and Bronek Kozicki
</p>
<p>
Hereby I would like to propose adding ref-qualifiers to all appropriate
assignment operators in the library.
</p>

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

<blockquote><p>
Move to Open.
We recommend this be deferred until after the next Committee Draft.
</p></blockquote>

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


<blockquote>
<p>
The LWG declined to move forward with
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2819.html">N2819</a>.
</p>
<p>
Moved to NAD.
</p>
</blockquote>


<p id="res-941"><b>Proposed resolution:</b></p>
<p>
A proposed resolution is provided by the paper on this subject,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2819.html">N2819</a>,
<i>Ref-qualifiers for assignment operators of the Standard Library</i>
</p>





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