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<td >Qt 5.9</td><td ><a href="qtcore-index.html">Qt Core</a></td><td ><a href="qtcore-module.html">C++ Classes</a></td><td >key_iterator</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
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<h3><a name="toc">Contents</a></h3>
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<li class="level1"><a href="#public-functions">Public Functions</a></li>
<li class="level1"><a href="#details">Detailed Description</a></li>
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<h1 class="title">key_iterator Class</h1>
<span class="small-subtitle">(<a href="qhash-key-iterator.html">QHash::key_iterator</a>)<br/></span>
<!-- $$$key_iterator-brief -->
<p>The <a href="qhash-key-iterator.html">QHash::key_iterator</a> class provides an STL-style const iterator for <a href="qhash.html#qhash">QHash</a> and <a href="qmultihash.html">QMultiHash</a> keys. <a href="#details">More...</a></p>
<!-- @@@key_iterator -->
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<tr><td class="memItemLeft rightAlign topAlign"> Header:</td><td class="memItemRight bottomAlign"> <span class="preprocessor">#include <key_iterator></span>
</td></tr><tr><td class="memItemLeft rightAlign topAlign"> qmake:</td><td class="memItemRight bottomAlign"> QT += core</td></tr><tr><td class="memItemLeft rightAlign topAlign"> Since:</td><td class="memItemRight bottomAlign"> Qt 5.6</td></tr></table></div><ul>
<li><a href="qhash-key-iterator-members.html">List of all members, including inherited members</a></li>
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<a name="public-functions"></a>
<h2 id="public-functions">Public Functions</h2>
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<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></b>() = default</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#key_iterator-1">key_iterator</a></b>(const_iterator <i>o</i>)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> const_iterator </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#base">base</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator-not-eq">operator!=</a></b>(key_iterator <i>other</i>) const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> const Key &</td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator-2a">operator*</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> key_iterator &</td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator-2b-2b">operator++</a></b>()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> key_iterator </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator-2b-2b-1">operator++</a></b>(<i>int</i>)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> key_iterator &</td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator--">operator--</a></b>()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> key_iterator </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator---1">operator--</a></b>(<i>int</i>)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> const Key *</td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator--gt">operator-></a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qhash-key-iterator.html#operator-eq-eq">operator==</a></b>(key_iterator <i>other</i>) const</td></tr>
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<a name="details"></a>
<!-- $$$key_iterator-description -->
<div class="descr">
<h2 id="details">Detailed Description</h2>
<p>The <a href="qhash-key-iterator.html">QHash::key_iterator</a> class provides an STL-style const iterator for <a href="qhash.html#qhash">QHash</a> and <a href="qmultihash.html">QMultiHash</a> keys.</p>
<p><a href="qhash-key-iterator.html">QHash::key_iterator</a> is essentially the same as <a href="qhash-const-iterator.html">QHash::const_iterator</a> with the difference that operator*() and operator->() return a key instead of a value.</p>
<p>For most uses <a href="qhash-iterator.html">QHash::iterator</a> and <a href="qhash-const-iterator.html">QHash::const_iterator</a> should be used, you can easily access the key by calling <a href="qhash-iterator.html#key">QHash::iterator::key</a>():</p>
<pre class="cpp">
<span class="keyword">for</span> (<span class="type"><a href="qhash.html#QHash">QHash</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">,</span> <span class="type"><a href="qstring.html">QString</a></span><span class="operator">></span><span class="operator">::</span>const_iterator it <span class="operator">=</span> hash<span class="operator">.</span>cbegin()<span class="operator">,</span> end <span class="operator">=</span> hash<span class="operator">.</span>cend(); it <span class="operator">!</span><span class="operator">=</span> end; <span class="operator">+</span><span class="operator">+</span>it) {
cout <span class="operator"><</span><span class="operator"><</span> <span class="string">"The key: "</span> <span class="operator"><</span><span class="operator"><</span> it<span class="operator">.</span>key() <span class="operator"><</span><span class="operator"><</span> endl
cout <span class="operator"><</span><span class="operator"><</span> <span class="string">"The value: "</span> <span class="operator"><</span><span class="operator"><</span> it<span class="operator">.</span>value() <span class="operator"><</span><span class="operator"><</span> endl;
cout <span class="operator"><</span><span class="operator"><</span> <span class="string">"Also the value: "</span> <span class="operator"><</span><span class="operator"><</span> (<span class="operator">*</span>it) <span class="operator"><</span><span class="operator"><</span> endl;
}
</pre>
<p>However, to have interoperability between <a href="qhash.html#qhash">QHash</a>'s keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With <a href="qhash-key-iterator.html">QHash::key_iterator</a> we can apply an algorithm to a range of keys without having to call <a href="qhash.html#keys">QHash::keys</a>(), which is inefficient as it costs one <a href="qhash.html#qhash">QHash</a> iteration and memory allocation to create a temporary <a href="qlist.html">QList</a>.</p>
<pre class="cpp">
<span class="comment">// Inefficient, keys() is expensive</span>
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">></span> keys <span class="operator">=</span> hash<span class="operator">.</span>keys();
<span class="type">int</span> numPrimes <span class="operator">=</span> std<span class="operator">::</span>count_if(keys<span class="operator">.</span>cbegin()<span class="operator">,</span> keys<span class="operator">.</span>cend()<span class="operator">,</span> isPrimeNumber);
<a href="qtalgorithms.html#qDeleteAll">qDeleteAll</a>(hash2<span class="operator">.</span>keys());
<span class="comment">// Efficient, no memory allocation needed</span>
<span class="type">int</span> numPrimes <span class="operator">=</span> std<span class="operator">::</span>count_if(hash<span class="operator">.</span>keyBegin()<span class="operator">,</span> hash<span class="operator">.</span>keyEnd()<span class="operator">,</span> isPrimeNumber);
<a href="qtalgorithms.html#qDeleteAll">qDeleteAll</a>(hash2<span class="operator">.</span>keyBegin()<span class="operator">,</span> hash2<span class="operator">.</span>keyEnd());
</pre>
<p><a href="qhash-key-iterator.html">QHash::key_iterator</a> is const, it's not possible to modify the key.</p>
<p>The default <a href="qhash-key-iterator.html">QHash::key_iterator</a> constructor creates an uninitialized iterator. You must initialize it using a <a href="qhash.html#qhash">QHash</a> function like <a href="qhash.html#keyBegin">QHash::keyBegin</a>() or <a href="qhash.html#keyEnd">QHash::keyEnd</a>().</p>
<p><b>Warning:</b> Iterators on implicitly shared containers do not work exactly like STL-iterators. You should avoid copying a container while iterators are active on that container. For more information, read <a href="containers.html#implicit-sharing-iterator-problem">Implicit sharing iterator problem</a>.</p>
</div>
<p><b>See also </b><a href="qhash-const-iterator.html">QHash::const_iterator</a> and <a href="qhash-iterator.html">QHash::iterator</a>.</p>
<!-- @@@key_iterator -->
<div class="func">
<h2>Member Function Documentation</h2>
<!-- $$$key_iterator[overload1]$$$key_iterator -->
<h3 class="fn" id="key_iterator"><a name="key_iterator"></a><code>[default] </code>key_iterator::<span class="name">key_iterator</span>()</h3>
<p>Default constructs an instance of key_iterator.</p><!-- @@@key_iterator -->
<!-- $$$key_iterator$$$key_iteratorconst_iterator -->
<h3 class="fn" id="key_iterator-1"><a name="key_iterator-1"></a>key_iterator::<span class="name">key_iterator</span>(<span class="type"><a href="qhash-const-iterator.html">const_iterator</a></span> <i>o</i>)</h3>
<p>Default constructs an instance of key_iterator.</p><!-- @@@key_iterator -->
<!-- $$$base[overload1]$$$base -->
<h3 class="fn" id="base"><a name="base"></a><span class="type"><a href="qhash-const-iterator.html">const_iterator</a></span> key_iterator::<span class="name">base</span>() const</h3>
<p>Returns the underlying <a href="qhash-const-iterator.html">const_iterator</a> this <a href="qhash-key-iterator.html">key_iterator</a> is based on.</p>
<!-- @@@base -->
<!-- $$$operator!=[overload1]$$$operator!=key_iterator -->
<h3 class="fn" id="operator-not-eq"><a name="operator-not-eq"></a><span class="type">bool</span> key_iterator::<span class="name">operator!=</span>(<span class="type"><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></span> <i>other</i>) const</h3>
<p>Returns <code>true</code> if <i>other</i> points to a different item than this iterator; otherwise returns <code>false</code>.</p>
<p><b>See also </b><a href="qhash-key-iterator.html#operator-eq-eq">operator==</a>().</p>
<!-- @@@operator!= -->
<!-- $$$operator*[overload1]$$$operator* -->
<h3 class="fn" id="operator-2a"><a name="operator-2a"></a>const <span class="type">Key</span> &key_iterator::<span class="name">operator*</span>() const</h3>
<p>Returns the current item's key.</p>
<!-- @@@operator* -->
<!-- $$$operator++[overload1]$$$operator++ -->
<h3 class="fn" id="operator-2b-2b"><a name="operator-2b-2b"></a><span class="type"><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></span> &key_iterator::<span class="name">operator++</span>()</h3>
<p>The prefix ++ operator (<code>++i</code>) advances the iterator to the next item in the hash and returns an iterator to the new current item.</p>
<p>Calling this function on <a href="qhash.html#keyEnd">QHash::keyEnd</a>() leads to undefined results.</p>
<p><b>See also </b><a href="qhash-key-iterator.html#operator--">operator--</a>().</p>
<!-- @@@operator++ -->
<!-- $$$operator++$$$operator++int -->
<h3 class="fn" id="operator-2b-2b-1"><a name="operator-2b-2b-1"></a><span class="type"><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></span> key_iterator::<span class="name">operator++</span>(<i>int</i>)</h3>
<p>This is an overloaded function.</p>
<p>The postfix ++ operator (<code>i++</code>) advances the iterator to the next item in the hash and returns an iterator to the previous item.</p>
<!-- @@@operator++ -->
<!-- $$$operator--[overload1]$$$operator-- -->
<h3 class="fn" id="operator--"><a name="operator--"></a><span class="type"><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></span> &key_iterator::<span class="name">operator--</span>()</h3>
<p>The prefix -- operator (<code>--i</code>) makes the preceding item current and returns an iterator pointing to the new current item.</p>
<p>Calling this function on <a href="qhash.html#keyBegin">QHash::keyBegin</a>() leads to undefined results.</p>
<p><b>See also </b><a href="qhash-key-iterator.html#operator-2b-2b">operator++</a>().</p>
<!-- @@@operator-- -->
<!-- $$$operator--$$$operator--int -->
<h3 class="fn" id="operator---1"><a name="operator---1"></a><span class="type"><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></span> key_iterator::<span class="name">operator--</span>(<i>int</i>)</h3>
<p>This is an overloaded function.</p>
<p>The postfix -- operator (<code>i--</code>) makes the preceding item current and returns an iterator pointing to the previous item.</p>
<!-- @@@operator-- -->
<!-- $$$operator->[overload1]$$$operator-> -->
<h3 class="fn" id="operator--gt"><a name="operator--gt"></a>const <span class="type">Key</span> *key_iterator::<span class="name">operator-></span>() const</h3>
<p>Returns a pointer to the current item's key.</p>
<!-- @@@operator-> -->
<!-- $$$operator==[overload1]$$$operator==key_iterator -->
<h3 class="fn" id="operator-eq-eq"><a name="operator-eq-eq"></a><span class="type">bool</span> key_iterator::<span class="name">operator==</span>(<span class="type"><a href="qhash-key-iterator.html#key_iterator">key_iterator</a></span> <i>other</i>) const</h3>
<p>Returns <code>true</code> if <i>other</i> points to the same item as this iterator; otherwise returns <code>false</code>.</p>
<p><b>See also </b><a href="qhash-key-iterator.html#operator-not-eq">operator!=</a>().</p>
<!-- @@@operator== -->
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