<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <!-- customtype.qdoc --> <title>Custom Type Example | Qt Core 5.12.6</title> <link rel="stylesheet" type="text/css" href="style/offline-simple.css" /> <script type="text/javascript"> document.getElementsByTagName("link").item(0).setAttribute("href", "style/offline.css"); // loading style sheet breaks anchors that were jumped to before // so force jumping to anchor again setTimeout(function() { var anchor = location.hash; // need to jump to different anchor first (e.g. none) location.hash = "#"; setTimeout(function() { location.hash = anchor; }, 0); }, 0); </script> </head> <body> <div class="header" id="qtdocheader"> <div class="main"> <div class="main-rounded"> <div class="navigationbar"> <table><tr> <td >Qt 5.12</td><td ><a href="qtcore-index.html">Qt Core</a></td><td >Custom Type Example</td></tr></table><table class="buildversion"><tr> <td id="buildversion" width="100%" align="right"><a href="qtcore-index.html">Qt 5.12.6 Reference Documentation</a></td> </tr></table> </div> </div> <div class="content"> <div class="line"> <div class="content mainContent"> <div class="sidebar"> <div class="toc"> <h3><a name="toc">Contents</a></h3> <ul> <li class="level1"><a href="#overview">Overview</a></li> <li class="level1"><a href="#the-message-class-definition">The Message Class Definition</a></li> <li class="level1"><a href="#the-message-class-implementation">The Message Class Implementation</a></li> <li class="level1"><a href="#using-the-message">Using the Message</a></li> <li class="level1"><a href="#further-reading">Further Reading</a></li> </ul> </div> <div class="sidebar-content" id="sidebar-content"></div></div> <h1 class="title">Custom Type Example</h1> <span class="subtitle"></span> <!-- $$$tools/customtype-brief --> <p>The Custom Type example shows how to integrate a custom type into Qt's meta-object system.</p> <!-- @@@tools/customtype --> <!-- $$$tools/customtype-description --> <div class="descr"> <a name="details"></a> <p>Contents:</p> <a name="overview"></a> <h2 id="overview">Overview</h2> <p>Qt provides a range of standard value types that are used to provide rich and meaningful APIs. These types are integrated with the meta-object system, enabling them to be stored in <a href="qvariant.html">QVariant</a> objects, written out in debugging information and sent between components in signal-slot communication.</p> <p>Custom types can also be integrated with the meta-object system as long as they are written to conform to some simple guidelines. In this example, we introduce a simple <code>Message</code> class, we describe how we make it work with <a href="qvariant.html">QVariant</a>, and we show how it can be extended to generate a printable representation of itself for use in debugging output.</p> <a name="the-message-class-definition"></a> <h2 id="the-message-class-definition">The Message Class Definition</h2> <p>The <code>Message</code> class is a simple value class that contains two pieces of information (a <a href="qstring.html">QString</a> and a <a href="qstringlist.html">QStringList</a>), each of which can be read using trivial getter functions:</p> <pre class="cpp"> <span class="keyword">class</span> Message { <span class="keyword">public</span>: Message(); Message(<span class="keyword">const</span> Message <span class="operator">&</span>other); <span class="operator">~</span>Message(); Message(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>body<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="qstringlist.html">QStringList</a></span> <span class="operator">&</span>headers); <span class="type"><a href="qstring.html">QString</a></span> body() <span class="keyword">const</span>; <span class="type"><a href="qstringlist.html">QStringList</a></span> headers() <span class="keyword">const</span>; <span class="keyword">private</span>: <span class="type"><a href="qstring.html">QString</a></span> m_body; <span class="type"><a href="qstringlist.html">QStringList</a></span> m_headers; }; </pre> <p>The default constructor, copy constructor and destructor are all required, and must be public, if the type is to be integrated into the meta-object system. Other than this, we are free to implement whatever we need to make the type do what we want, so we also include a constructor that lets us set the type's data members.</p> <p>To enable the type to be used with <a href="qvariant.html">QVariant</a>, we declare it using the <a href="qmetatype.html#Q_DECLARE_METATYPE">Q_DECLARE_METATYPE</a>() macro:</p> <pre class="cpp"> Q_DECLARE_METATYPE(Message); </pre> <p>We do not need to write any additional code to accompany this macro.</p> <p>To allow us to see a readable description of each <code>Message</code> object when it is sent to the debug output stream, we define a streaming operator:</p> <pre class="cpp"> <span class="type"><a href="qdebug.html">QDebug</a></span> <span class="keyword">operator</span><span class="operator"><</span><span class="operator"><</span>(<span class="type"><a href="qdebug.html">QDebug</a></span> dbg<span class="operator">,</span> <span class="keyword">const</span> Message <span class="operator">&</span>message); </pre> <p>This facility is useful if you need to insert tracing statements in your code for debugging purposes.</p> <a name="the-message-class-implementation"></a> <h2 id="the-message-class-implementation">The Message Class Implementation</h2> <p>The implementation of the default constructor, copy constructor and destructor are straightforward for the <code>Message</code> class:</p> <pre class="cpp"> Message<span class="operator">::</span>Message() { } Message<span class="operator">::</span>Message(<span class="keyword">const</span> Message <span class="operator">&</span>other) { m_body <span class="operator">=</span> other<span class="operator">.</span>m_body; m_headers <span class="operator">=</span> other<span class="operator">.</span>m_headers; } Message<span class="operator">::</span><span class="operator">~</span>Message() { } </pre> <p>The streaming operator is implemented in the following way:</p> <pre class="cpp"> <span class="type"><a href="qdebug.html">QDebug</a></span> <span class="keyword">operator</span><span class="operator"><</span><span class="operator"><</span>(<span class="type"><a href="qdebug.html">QDebug</a></span> dbg<span class="operator">,</span> <span class="keyword">const</span> Message <span class="operator">&</span>message) { <span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> body <span class="operator">=</span> message<span class="operator">.</span>body(); <span class="type"><a href="qvector.html">QVector</a></span><span class="operator"><</span><span class="type"><a href="qstringref.html">QStringRef</a></span><span class="operator">></span> pieces <span class="operator">=</span> body<span class="operator">.</span>splitRef(<span class="string">"\r\n"</span><span class="operator">,</span> <span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>SkipEmptyParts); <span class="keyword">if</span> (pieces<span class="operator">.</span>isEmpty()) dbg<span class="operator">.</span>nospace() <span class="operator"><</span><span class="operator"><</span> <span class="string">"Message()"</span>; <span class="keyword">else</span> <span class="keyword">if</span> (pieces<span class="operator">.</span>size() <span class="operator">=</span><span class="operator">=</span> <span class="number">1</span>) dbg<span class="operator">.</span>nospace() <span class="operator"><</span><span class="operator"><</span> <span class="string">"Message("</span> <span class="operator"><</span><span class="operator"><</span> pieces<span class="operator">.</span>first() <span class="operator"><</span><span class="operator"><</span> <span class="string">")"</span>; <span class="keyword">else</span> dbg<span class="operator">.</span>nospace() <span class="operator"><</span><span class="operator"><</span> <span class="string">"Message("</span> <span class="operator"><</span><span class="operator"><</span> pieces<span class="operator">.</span>first() <span class="operator"><</span><span class="operator"><</span> <span class="string">" ...)"</span>; <span class="keyword">return</span> dbg<span class="operator">.</span>maybeSpace(); } </pre> <p>Here, we want to represent each value depending on how many lines are stored in the message body. We stream text to the <a href="qdebug.html">QDebug</a> object passed to the operator and return the <a href="qdebug.html">QDebug</a> object obtained from its maybeSpace() member function; this is described in more detail in the <a href="custom-types.html#making-the-type-printable">Creating Custom Qt Types</a> document.</p> <p>We include the code for the getter functions for completeness:</p> <pre class="cpp"> <span class="type"><a href="qstring.html">QString</a></span> Message<span class="operator">::</span>body() <span class="keyword">const</span> { <span class="keyword">return</span> m_body; } <span class="type"><a href="qstringlist.html">QStringList</a></span> Message<span class="operator">::</span>headers() <span class="keyword">const</span> { <span class="keyword">return</span> m_headers; } </pre> <p>With the type fully defined, implemented, and integrated with the meta-object system, we can now use it.</p> <a name="using-the-message"></a> <h2 id="using-the-message">Using the Message</h2> <p>In the example's <code>main()</code> function, we show how a <code>Message</code> object can be printed to the console by sending it to the debug stream:</p> <pre class="cpp"> Message message(body<span class="operator">,</span> headers); <a href="qtglobal.html#qDebug">qDebug</a>() <span class="operator"><</span><span class="operator"><</span> <span class="string">"Original:"</span> <span class="operator"><</span><span class="operator"><</span> message; </pre> <p>You can use the type with <a href="qvariant.html">QVariant</a> in exactly the same way as you would use standard Qt value types. Here's how to store a value using the <a href="qvariant.html#setValue">QVariant::setValue</a>() function:</p> <pre class="cpp"> <span class="type"><a href="qvariant.html">QVariant</a></span> stored; stored<span class="operator">.</span>setValue(message); </pre> <p>Alternatively, the <a href="qvariant.html#fromValue">QVariant::fromValue</a>() and qVariantSetValue() functions can be used if you are using a compiler without support for member template functions.</p> <p>The value can be retrieved using the <a href="qvariant.html#value">QVariant::value</a>() member template function:</p> <pre class="cpp"> Message retrieved <span class="operator">=</span> stored<span class="operator">.</span>value<span class="operator"><</span>Message<span class="operator">></span>(); <a href="qtglobal.html#qDebug">qDebug</a>() <span class="operator"><</span><span class="operator"><</span> <span class="string">"Retrieved:"</span> <span class="operator"><</span><span class="operator"><</span> retrieved; retrieved <span class="operator">=</span> qvariant_cast<span class="operator"><</span>Message<span class="operator">></span>(stored); <a href="qtglobal.html#qDebug">qDebug</a>() <span class="operator"><</span><span class="operator"><</span> <span class="string">"Retrieved:"</span> <span class="operator"><</span><span class="operator"><</span> retrieved; </pre> <p>Alternatively, the qVariantValue() template function can be used if you are using a compiler without support for member template functions.</p> <a name="further-reading"></a> <h2 id="further-reading">Further Reading</h2> <p>The custom <code>Message</code> type can also be used with direct signal-slot connections.</p> <p>To register a custom type for use with queued signals and slots, such as those used in cross-thread communication, see the <a href="qtcore-threads-queuedcustomtype-example.html">Queued Custom Type Example</a>.</p> <p>More information on using custom types with Qt can be found in the <a href="custom-types.html">Creating Custom Qt Types</a> document.</p> <p>Files:</p> <ul> <li><a href="qtcore-tools-customtype-customtype-pro.html">tools/customtype/customtype.pro</a></li> <li><a href="qtcore-tools-customtype-main-cpp.html">tools/customtype/main.cpp</a></li> <li><a href="qtcore-tools-customtype-message-cpp.html">tools/customtype/message.cpp</a></li> <li><a href="qtcore-tools-customtype-message-h.html">tools/customtype/message.h</a></li> </ul> </div> <!-- @@@tools/customtype --> </div> </div> </div> </div> </div> <div class="footer"> <p> <acronym title="Copyright">©</acronym> 2019 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners.<br/> The documentation provided herein is licensed under the terms of the <a href="http://www.gnu.org/licenses/fdl.html">GNU Free Documentation License version 1.3</a> as published by the Free Software Foundation.<br/> Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners. </p> </div> </body> </html>