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<td> <div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&amp;</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> Key<span class="sy3">&amp;</span> key <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td> (1) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep">
<td></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&amp;</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> Key<span class="sy3">&amp;&amp;</span> key <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td> (2) </td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl-sep">
<td></td>
<td></td>
<td></td>
</tr>
</tbody></table>
<p>Returns a reference to the value that is mapped to a key equivalent to <code>key</code>, performing an insertion if such key does not already exist. 
</p>
<p>If an insertion is performed, the mapped value is <a href="../../language/value_initialization.html" title="cpp/language/value initialization">value-initialized</a> (default-constructed for class types, zero-initialized otherwise) and a reference to it is returned.
</p>
<div class="t-li1">
<span class="t-li">1)</span> Inserts <span class="t-c"><span class="mw-geshi cpp source-cpp">value_type<span class="br0">(</span>key, T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 
<table class="t-par-begin">


<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div>
<code>key_type</code> must meet the requirements of <a href="../../concept/CopyConstructible.html" title="cpp/concept/CopyConstructible"><code>CopyConstructible</code></a>.
</td>
</tr>
<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div>
<code>mapped_type</code> must meet the requirements of <a href="../../concept/DefaultConstructible.html" title="cpp/concept/DefaultConstructible"><code>DefaultConstructible</code></a>. <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td>
</tr>
</table>
</div>
<div class="t-li1">
<span class="t-li">2)</span> Inserts <span class="t-c"><span class="mw-geshi cpp source-cpp">value_type<span class="br0">(</span><a href="../../utility/move.html"><span class="kw914">std::<span class="me2">move</span></span></a><span class="br0">(</span>key<span class="br0">)</span>, T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span>
<table class="t-par-begin">


<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div>
<code>key_type</code> must meet the requirements of <a href="../../concept/MoveConstructible.html" title="cpp/concept/MoveConstructible"><code>MoveConstructible</code></a>. <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td>
</tr>
<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div>
<code>mapped_type</code> must meet the requirements of <a href="../../concept/DefaultConstructible.html" title="cpp/concept/DefaultConstructible"><code>DefaultConstructible</code></a>. <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td>
</tr>
</table>
</div>
<p>No iterators or references are invalidated.
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1"><a href="#Parameters"><span class="tocnumber">1</span> <span class="toctext">Parameters</span></a></li>
<li class="toclevel-1"><a href="#Return_value"><span class="tocnumber">2</span> <span class="toctext">Return value</span></a></li>
<li class="toclevel-1"><a href="#Exceptions"><span class="tocnumber">3</span> <span class="toctext">Exceptions</span></a></li>
<li class="toclevel-1"><a href="#Complexity"><span class="tocnumber">4</span> <span class="toctext">Complexity</span></a></li>
<li class="toclevel-1"><a href="#Notes"><span class="tocnumber">5</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1"><a href="#Example"><span class="tocnumber">6</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1"><a href="#See_also"><span class="tocnumber">7</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Parameters">Parameters</span>
</h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  key
</td>
<td> -
</td>
<td>  the key of the element to find
</td>
</tr>
</table>
<h3>
<span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Return_value">Return value</span>
</h3>
<p>Reference to the mapped value of the new element if no element with key <code>key</code> existed. Otherwise a reference to the mapped value of the existing element whose key is equivalent to <code>key</code>.
</p>
<h3>
<span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Exceptions">Exceptions</span>
</h3>
<p>If an exception is thrown by any operation, the insertion has no effect
</p>
<h3>
<span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Complexity">Complexity</span>
</h3>
<p>Logarithmic in the size of the container.
</p>
<h3> <span class="mw-headline" id="Notes">Notes</span>
</h3>
<p>Until C++11, the overload (1) was specified to be equivalent to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>insert<span class="br0">(</span><a href="../../utility/pair/make_pair.html"><span class="kw902">std::<span class="me2">make_pair</span></span></a><span class="br0">(</span>key, T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span>.<span class="me1">first</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>second</span></span>, which required T to be CopyConstructible.
</p>
<h3>
<span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Example">Example</span>
</h3>
<div class="t-example">
<p> This example demonstrates how to modify existing values and insert new values using <code>operator[]</code>:
 </p>
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;map&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../map.html"><span class="kw1073">std::<span class="me2">map</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">&gt;</span> letter_counts <span class="br0">{</span><span class="br0">{</span><span class="st0">'a'</span>, <span class="nu0">27</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'b'</span>, <span class="nu0">3</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'c'</span>, <span class="nu0">1</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span>
 
    <a href="../../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"initially:<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&amp;</span>pair <span class="sy4">:</span> letter_counts<span class="br0">)</span> <span class="br0">{</span>
        <a href="../../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> pair.<span class="me1">first</span> <span class="sy1">&lt;&lt;</span> <span class="st0">": "</span> <span class="sy1">&lt;&lt;</span> pair.<span class="me1">second</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    letter_counts<span class="br0">[</span><span class="st0">'b'</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>  <span class="co1">// update an existing value</span>
 
    letter_counts<span class="br0">[</span><span class="st0">'x'</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">9</span><span class="sy4">;</span>  <span class="co1">// insert a new value</span>
 
    <a href="../../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"after modifications:<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&amp;</span>pair <span class="sy4">:</span> letter_counts<span class="br0">)</span> <span class="br0">{</span>
        <a href="../../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> pair.<span class="me1">first</span> <span class="sy1">&lt;&lt;</span> <span class="st0">": "</span> <span class="sy1">&lt;&lt;</span> pair.<span class="me1">second</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">initially:
a: 27
b: 3
c: 1
after modifications:
a: 27
b: 42
c: 1
x: 9</pre></div></div> 
</div>
<div class="t-example">
<p> The following example counts the occurrences of each word in a vector of strings:
 </p>
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;map&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../vector.html"><span class="kw1067">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><a href="../../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span> words <span class="sy1">=</span> <span class="br0">{</span> 
        <span class="st0">"this"</span>, <span class="st0">"sentence"</span>, <span class="st0">"is"</span>, <span class="st0">"not"</span>, <span class="st0">"a"</span>, <span class="st0">"sentence"</span>,
	<span class="st0">"this"</span>, <span class="st0">"sentence"</span>, <span class="st0">"is"</span>, <span class="st0">"a"</span>, <span class="st0">"hoax"</span> 
    <span class="br0">}</span><span class="sy4">;</span>
 
    <a href="../map.html"><span class="kw1073">std::<span class="me2">map</span></span></a><span class="sy1">&lt;</span><a href="../../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a>, size_t<span class="sy1">&gt;</span>  word_map<span class="sy4">;</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&amp;</span>w <span class="sy4">:</span> words<span class="br0">)</span> <span class="br0">{</span>
        <span class="sy2">++</span>word_map<span class="br0">[</span>w<span class="br0">]</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&amp;</span>pair <span class="sy4">:</span> word_map<span class="br0">)</span> <span class="br0">{</span>
        <a href="../../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> pair.<span class="me1">second</span>
	          <span class="sy1">&lt;&lt;</span> <span class="st0">" occurrences of word '"</span>
	          <span class="sy1">&lt;&lt;</span> pair.<span class="me1">first</span> <span class="sy1">&lt;&lt;</span> <span class="st0">"'<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'</pre></div></div> 
</div>
<h3>
<span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">
</td>
</tr>
<tr class="t-dsc">
<td>  <div class="t-dsc-member-div">
<div><a href="at.html" title="cpp/container/map/at"> <span class="t-lines"><span>at</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
</div>
</td>
<td>   access specified element with bounds checking  <br> <span class="t-mark">(public member function)</span> 
</td>
</tr>
</table>

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