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<td >Qt 5.9</td><td ><a href="qtqml-index.html">Qt QML</a></td><td >The QML Type System</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>
<ul>
<li class="level1"><a href="#basic-types">Basic Types</a></li>
<li class="level1"><a href="#javascript-types">JavaScript Types</a></li>
<li class="level1"><a href="#qml-object-types">QML Object Types</a></li>
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<h1 class="title">The QML Type System</h1>
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<p>The types which may be used in the definition of an object hierarchy in a QML document can come from various sources. They may be:</p>
<ul>
<li>provided natively by the QML language</li>
<li>registered via C++ by QML modules</li>
<li>provided as QML documents by QML modules</li>
</ul>
<p>Furthermore, application developers can provide their own types, either by registering C++ types directly, or by defining reusable components in QML documents which can then be imported.</p>
<p>Wherever the type definitions come from, the engine will enforce type-safety for properties and instances of those types.</p>
<a name="basic-types"></a>
<h2 id="basic-types">Basic Types</h2>
<p>The QML language has built-in support for various primitive types including integers, double-precision floating point numbers, strings, and boolean values. Objects may have properties of these types, and values of these types may be passed as arguments to methods of objects.</p>
<p>See the <a href="qtqml-typesystem-basictypes.html">QML Basic Types</a> documentation for more information about basic types.</p>
<a name="javascript-types"></a>
<h2 id="javascript-types">JavaScript Types</h2>
<p>JavaScript objects and arrays are supported by the QML engine. Any standard JavaScript type can be created and stored using the generic <a href="qml-var.html">var</a> type.</p>
<p>For example, the standard <code>Date</code> and <code>Array</code> types are available, as below:</p>
<pre class="qml">
import QtQuick 2.0
<span class="type">Item</span> {
property <span class="type">var</span> <span class="name">theArray</span>: new <span class="name">Array</span>()
property <span class="type">var</span> <span class="name">theDate</span>: new <span class="name">Date</span>()
<span class="name">Component</span>.onCompleted: {
<span class="keyword">for</span> (<span class="keyword">var</span> <span class="name">i</span> = <span class="number">0</span>; <span class="name">i</span> <span class="operator"><</span> <span class="number">10</span>; i++)
<span class="name">theArray</span>.<span class="name">push</span>(<span class="string">"Item "</span> <span class="operator">+</span> <span class="name">i</span>)
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"There are"</span>, <span class="name">theArray</span>.<span class="name">length</span>, <span class="string">"items in the array"</span>)
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"The time is"</span>, <span class="name">theDate</span>.<span class="name">toUTCString</span>())
}
}
</pre>
<p>See <a href="qtqml-javascript-expressions.html">JavaScript Expressions in QML Documents</a> for more details.</p>
<a name="qml-object-types"></a>
<h2 id="qml-object-types">QML Object Types</h2>
<p>A QML object type is a type from which a QML object can be instantiated. QML object types are derived from <a href="qml-qtqml-qtobject.html">QtObject</a>, and are provided by QML modules. Applications can import these modules to use the object types they provide. The <code>QtQuick</code> module provides the most common object types needed to create user interfaces in QML.</p>
<p>Finally, every QML document implicitly defines a QML object type, which can be re-used in other QML documents. See the documentation about <a href="qtqml-typesystem-objecttypes.html">object types in the QML type system</a> for in-depth information about object types.</p>
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