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<td >Qt 5.12</td><td ><a href="qtqml-index.html">Qt QML</a></td><td ><a href="qmlextendingexamples.html">Qt QML Examples</a></td><td >Extending QML - Inheritance and Coercion Example</td></tr></table><table class="buildversion"><tr>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#declare-boy-and-girl">Declare Boy and Girl</a></li>
<li class="level2"><a href="#define-people-as-a-base-class">Define People as a Base Class</a></li>
<li class="level2"><a href="#define-boy-and-girl">Define Boy and Girl</a></li>
<li class="level1"><a href="#running-the-example">Running the Example</a></li>
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<h1 class="title">Extending QML - Inheritance and Coercion Example</h1>
<span class="subtitle"></span>
<!-- $$$referenceexamples/coercion-brief -->
<p>C++ Inheritance and Coercion.</p>
<!-- @@@referenceexamples/coercion -->
<!-- $$$referenceexamples/coercion-description -->
<div class="descr"> <a name="details"></a>
<p>This example builds on:</p>
<ul>
<li><a href="qtqml-referenceexamples-properties-example.html">Extending QML - Object and List Property Types Example</a></li>
<li><a href="qtqml-referenceexamples-adding-example.html">Extending QML - Adding Types Example</a></li>
</ul>
<p>The Inheritance and Coercion Example shows how to use base classes to assign types of more than one type to a property. It specializes the Person type developed in the previous examples into two types - a <code>Boy</code> and a <code>Girl</code>.</p>
<pre class="qml">
BirthdayParty {
host: Boy {
name: "Bob Jones"
shoeSize: 12
}
guests: [
Boy { name: "Leo Hodges" },
Boy { name: "Jack Smith" },
Girl { name: "Anne Brown" }
]
}
</pre>
<a name="declare-boy-and-girl"></a>
<h2 id="declare-boy-and-girl">Declare Boy and Girl</h2>
<pre class="cpp">
<span class="keyword">class</span> Boy : <span class="keyword">public</span> Person
{
Q_OBJECT
<span class="keyword">public</span>:
Boy(<span class="type">QObject</span> <span class="operator">*</span> parent <span class="operator">=</span> <span class="number">0</span>);
};
<span class="keyword">class</span> Girl : <span class="keyword">public</span> Person
{
Q_OBJECT
<span class="keyword">public</span>:
Girl(<span class="type">QObject</span> <span class="operator">*</span> parent <span class="operator">=</span> <span class="number">0</span>);
};
</pre>
<p>The Person class remains unaltered in this example and the Boy and Girl C++ classes are trivial extensions of it. As an example, the inheritance used here is a little contrived, but in real applications it is likely that the two extensions would add additional properties or modify the Person classes behavior.</p>
<a name="define-people-as-a-base-class"></a>
<h3 id="define-people-as-a-base-class">Define People as a Base Class</h3>
<p>The implementation of the People class itself has not changed since the previous example. However, as we have repurposed the People class as a common base for Boy and Girl, we want to prevent it from being instantiated from QML directly - an explicit Boy or Girl should be instantiated instead.</p>
<pre class="cpp">
qmlRegisterType<span class="operator"><</span>Person<span class="operator">></span>();
</pre>
<p>While we want to disallow instantiating Person from within QML, it still needs to be registered with the QML engine, so that it can be used as a property type and other types can be coerced to it.</p>
<a name="define-boy-and-girl"></a>
<h3 id="define-boy-and-girl">Define Boy and Girl</h3>
<p>The implementation of Boy and Girl is trivial.</p>
<pre class="cpp">
Boy<span class="operator">::</span>Boy(<span class="type">QObject</span> <span class="operator">*</span> parent)
: Person(parent)
{
}
Girl<span class="operator">::</span>Girl(<span class="type">QObject</span> <span class="operator">*</span> parent)
: Person(parent)
{
}
</pre>
<p>All that is necessary is to implement the constructor, and to register the types and their QML name with the QML engine.</p>
<a name="running-the-example"></a>
<h2 id="running-the-example">Running the Example</h2>
<p>The BirthdayParty type has not changed since the previous example. The celebrant and guests property still use the People type.</p>
<pre class="cpp">
Q_PROPERTY(Person <span class="operator">*</span>host READ host WRITE setHost)
Q_PROPERTY(<span class="type"><a href="qqmllistproperty.html">QQmlListProperty</a></span><span class="operator"><</span>Person<span class="operator">></span> guests READ guests)
</pre>
<p>However, as all three types, Person, Boy and Girl, have been registered with the QML system, on assignment QML automatically (and type-safely) converts the Boy and Girl objects into a Person.</p>
<p>The main.cpp file in the example includes a simple shell application that loads and runs the QML snippet shown at the beginning of this page.</p>
<p>Files:</p>
<ul>
<li><a href="qtqml-referenceexamples-coercion-birthdayparty-cpp.html">referenceexamples/coercion/birthdayparty.cpp</a></li>
<li><a href="qtqml-referenceexamples-coercion-birthdayparty-h.html">referenceexamples/coercion/birthdayparty.h</a></li>
<li><a href="qtqml-referenceexamples-coercion-coercion-pro.html">referenceexamples/coercion/coercion.pro</a></li>
<li><a href="qtqml-referenceexamples-coercion-coercion-qrc.html">referenceexamples/coercion/coercion.qrc</a></li>
<li><a href="qtqml-referenceexamples-coercion-example-qml.html">referenceexamples/coercion/example.qml</a></li>
<li><a href="qtqml-referenceexamples-coercion-main-cpp.html">referenceexamples/coercion/main.cpp</a></li>
<li><a href="qtqml-referenceexamples-coercion-person-cpp.html">referenceexamples/coercion/person.cpp</a></li>
<li><a href="qtqml-referenceexamples-coercion-person-h.html">referenceexamples/coercion/person.h</a></li>
</ul>
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<!-- @@@referenceexamples/coercion -->
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