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<title>Issue 761: unordered_map needs an at() member function</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="761"><a href="lwg-defects.html#761">761</a>. <code>unordered_map</code> needs an <code>at()</code> member function</h3>
<p><b>Section:</b> 23.5.3.3 <a href="https://wg21.link/unord.map.elem">[unord.map.elem]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Joe Gottman <b>Opened:</b> 2007-11-15 <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#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The new member function <code>at()</code> was recently added to <code>std::map()</code>.  It acts 
like <code>operator[]()</code>, except it throws an exception when the input key is 
not found.  It is useful when the <code>map</code> is <code>const</code>, the <code>value_type</code> of the 
key doesn't have  a default constructor, it is an error if the key is 
not found, or the user wants to avoid accidentally adding an element to 
the map.  For exactly these same reasons, <code>at()</code> would be equally useful 
in <code>std::unordered_map</code>.
</p>


<p id="res-761"><b>Proposed resolution:</b></p>
<p>
Add the following functions to the definition of <code>unordered_map</code> under "lookup" (23.5.3 <a href="https://wg21.link/unord.map">[unord.map]</a>):
</p>

<blockquote><pre>
mapped_type&amp; at(const key_type&amp; k);
const mapped_type &amp;at(const key_type &amp;k) const;
</pre></blockquote>

<p>
Add the following definitions to 23.5.3.3 <a href="https://wg21.link/unord.map.elem">[unord.map.elem]</a>:
</p>

<blockquote>
<pre>
mapped_type&amp; at(const key_type&amp; k);
const mapped_type &amp;at(const key_type &amp;k) const;
</pre>
<blockquote>
<p>
<i>Returns:</i> A reference to <code>x.second</code>, where <code>x</code> is the (unique) element 
whose key is equivalent to <code>k</code>.
</p>
<p>
<i>Throws:</i> An exception object of type <code>out_of_range</code> if no such element 
is present.
</p>
</blockquote>
</blockquote>

<p><i>[
Bellevue:  Editorial note: the "(unique)" differs from map.
]</i></p>







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