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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#usage">Usage</a></li>
<li class="level1"><a href="#writing-make-rules-for-invoking-moc">Writing Make Rules for Invoking <tt>moc</tt></a></li>
<li class="level1"><a href="#command-line-options">Command-Line Options</a></li>
<li class="level1"><a href="#diagnostics">Diagnostics</a></li>
<li class="level1"><a href="#limitations">Limitations</a></li>
<li class="level2"><a href="#multiple-inheritance-requires-qobject-to-be-first">Multiple Inheritance Requires QObject to Be First</a></li>
<li class="level2"><a href="#function-pointers-cannot-be-signal-or-slot-parameters">Function Pointers Cannot Be Signal or Slot Parameters</a></li>
<li class="level2"><a href="#enums-and-typedefs-must-be-fully-qualified-for-signal-and-slot-parameters">Enums and Typedefs Must Be Fully Qualified for Signal and Slot Parameters</a></li>
<li class="level2"><a href="#nested-classes-cannot-have-signals-or-slots">Nested Classes Cannot Have Signals or Slots</a></li>
<li class="level2"><a href="#signal-slot-return-types-cannot-be-references">Signal/Slot return types cannot be references</a></li>
<li class="level2"><a href="#only-signals-and-slots-may-appear-in-the-keyword-signals-keyword-and-keyword-slots-keyword-sections-of-a-class">Only Signals and Slots May Appear in the <tt>signals</tt> and <tt>slots</tt> Sections of a Class</a></li>
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<h1 class="title">Using the Meta-Object Compiler (moc)</h1>
<span class="subtitle"></span>
<!-- $$$moc.html-description -->
<div class="descr"> <a name="details"></a>
<a name="moc"></a><p>The Meta-Object Compiler, <tt>moc</tt>, is the program that handles <a href="../qtcore/metaobjects.html#meta-object-system">Qt's C++ extensions</a>.</p>
<p>The <tt>moc</tt> tool reads a C++ header file. If it finds one or more class declarations that contain the <a href="../qtcore/qobject.html#Q_OBJECT">Q_OBJECT</a> macro, it produces a C++ source file containing the meta-object code for those classes. Among other things, meta-object code is required for the signals and slots mechanism, the run-time type information, and the dynamic property system.</p>
<p>The C++ source file generated by <tt>moc</tt> must be compiled and linked with the implementation of the class.</p>
<p>If you use <a href="../qmake/qmake-manual.html#qmake">qmake</a> to create your makefiles, build rules will be included that call the moc when required, so you will not need to use the moc directly. For more background information on <tt>moc</tt>, see <a href="why-moc.html">Why Does Qt Use Moc for Signals and Slots?</a></p>
<a name="usage"></a>
<h2>Usage</h2>
<p><tt>moc</tt> is typically used with an input file containing class declarations like this:</p>
<pre class="cpp"><span class="keyword">class</span> MyClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
MyClass(<span class="type"><a href="../qtcore/qobject.html">QObject</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="operator">~</span>MyClass();
<span class="keyword">signals</span>:
<span class="type">void</span> mySignal();
<span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> mySlot();
};</pre>
<p>In addition to the signals and slots shown above, <tt>moc</tt> also implements object properties as in the next example. The <a href="../qtcore/qobject.html#Q_PROPERTY">Q_PROPERTY</a>() macro declares an object property, while <a href="../qtcore/qobject.html#Q_ENUMS">Q_ENUMS</a>() declares a list of enumeration types within the class to be usable inside the <a href="../qtcore/properties.html#qt-s-property-system">property system</a>.</p>
<p>In the following example, we declare a property of the enumeration type <tt>Priority</tt> that is also called <tt>priority</tt> and has a get function <tt>priority()</tt> and a set function <tt>setPriority()</tt>.</p>
<pre class="cpp"><span class="keyword">class</span> MyClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span>
{
Q_OBJECT
Q_PROPERTY(Priority priority READ priority WRITE setPriority)
Q_ENUMS(Priority)
<span class="keyword">public</span>:
<span class="keyword">enum</span> Priority { High<span class="operator">,</span> Low<span class="operator">,</span> VeryHigh<span class="operator">,</span> VeryLow };
MyClass(<span class="type"><a href="../qtcore/qobject.html">QObject</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="operator">~</span>MyClass();
<span class="type">void</span> setPriority(Priority priority) { m_priority <span class="operator">=</span> priority; }
Priority priority() <span class="keyword">const</span> { <span class="keyword">return</span> m_priority; }
<span class="keyword">private</span>:
Priority m_priority;
};</pre>
<p>The <a href="../qtcore/qobject.html#Q_FLAGS">Q_FLAGS</a>() macro declares enums that are to be used as flags, i.e. OR'd together. Another macro, <a href="../qtcore/qobject.html#Q_CLASSINFO">Q_CLASSINFO</a>(), allows you to attach additional name/value pairs to the class's meta-object:</p>
<pre class="cpp"><span class="keyword">class</span> MyClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span>
{
Q_OBJECT
Q_CLASSINFO(<span class="string">"Author"</span><span class="operator">,</span> <span class="string">"Oscar Peterson"</span>)
Q_CLASSINFO(<span class="string">"Status"</span><span class="operator">,</span> <span class="string">"Active"</span>)
<span class="keyword">public</span>:
MyClass(<span class="type"><a href="../qtcore/qobject.html">QObject</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="operator">~</span>MyClass();
};</pre>
<p>The output produced by <tt>moc</tt> must be compiled and linked, just like the other C++ code in your program; otherwise, the build will fail in the final link phase. If you use <tt>qmake</tt>, this is done automatically. Whenever <tt>qmake</tt> is run, it parses the project's header files and generates make rules to invoke <tt>moc</tt> for those files that contain a <a href="../qtcore/qobject.html#Q_OBJECT">Q_OBJECT</a> macro.</p>
<p>If the class declaration is found in the file <tt>myclass.h</tt>, the moc output should be put in a file called <tt>moc_myclass.cpp</tt>. This file should then be compiled as usual, resulting in an object file, e.g., <tt>moc_myclass.obj</tt> on Windows. This object should then be included in the list of object files that are linked together in the final building phase of the program.</p>
<a name="writing-make-rules-for-invoking-moc"></a>
<h2>Writing Make Rules for Invoking <tt>moc</tt></h2>
<p>For anything but the simplest test programs, it is recommended that you automate running the <tt>moc</tt>. By adding some rules to your program's makefile, <tt>make</tt> can take care of running moc when necessary and handling the moc output.</p>
<p>We recommend using the <a href="../qmake/qmake-manual.html#qmake">qmake</a> makefile generation tool for building your makefiles. This tool generates a makefile that does all the necessary <tt>moc</tt> handling.</p>
<p>If you want to create your makefiles yourself, here are some tips on how to include moc handling.</p>
<p>For <a href="../qtcore/qobject.html#Q_OBJECT">Q_OBJECT</a> class declarations in header files, here is a useful makefile rule if you only use GNU make:</p>
<pre class="cpp">moc_<span class="operator">%</span><span class="operator">.</span>cpp: <span class="operator">%</span><span class="operator">.</span>h
moc $(DEFINES) $(INCPATH) $<span class="operator"><</span> <span class="operator">-</span>o $@</pre>
<p>If you want to write portably, you can use individual rules of the following form:</p>
<pre class="cpp">moc_foo<span class="operator">.</span>cpp: foo<span class="operator">.</span>h
moc $(DEFINES) $(INCPATH) $<span class="operator"><</span> <span class="operator">-</span>o $@</pre>
<p>You must also remember to add <tt>moc_foo.cpp</tt> to your <tt>SOURCES</tt> (substitute your favorite name) variable and <tt>moc_foo.o</tt> or <tt>moc_foo.obj</tt> to your <tt>OBJECTS</tt> variable.</p>
<p>Both examples assume that <tt>$(DEFINES)</tt> and <tt>$(INCPATH)</tt> expand to the define and include path options that are passed to the C++ compiler. These are required by <tt>moc</tt> to preprocess the source files.</p>
<p>While we prefer to name our C++ source files <tt>.cpp</tt>, you can use any other extension, such as <tt>.C</tt>, <tt>.cc</tt>, <tt>.CC</tt>, <tt>.cxx</tt>, and <tt>.c++</tt>, if you prefer.</p>
<p>For <a href="../qtcore/qobject.html#Q_OBJECT">Q_OBJECT</a> class declarations in implementation (<tt>.cpp</tt>) files, we suggest a makefile rule like this:</p>
<pre class="cpp">foo<span class="operator">.</span>o: foo<span class="operator">.</span>moc
foo<span class="operator">.</span>moc: foo<span class="operator">.</span>cpp
moc $(DEFINES) $(INCPATH) <span class="operator">-</span>i $<span class="operator"><</span> <span class="operator">-</span>o $@</pre>
<p>This guarantees that make will run the moc before it compiles <tt>foo.cpp</tt>. You can then put</p>
<pre class="cpp"><span class="preprocessor">#include "foo.moc"</span></pre>
<p>at the end of <tt>foo.cpp</tt>, where all the classes declared in that file are fully known.</p>
<a name="command-line-options"></a>
<h2>Command-Line Options</h2>
<p>Here are the command-line options supported by the moc:</p>
<table class="generic">
<thead><tr class="qt-style"><th >Option</th><th >Description</th></tr></thead>
<tr valign="top" class="odd"><td ><tt>-o<file></tt></td><td >Write output to <tt><file></tt> rather than to standard output.</td></tr>
<tr valign="top" class="even"><td ><tt>-f[<file>]</tt></td><td >Force the generation of an <tt>#include</tt> statement in the output. This is the default for header files whose extension starts with <tt>H</tt> or <tt>h</tt>. This option is useful if you have header files that do not follow the standard naming conventions. The <tt><file></tt> part is optional.</td></tr>
<tr valign="top" class="odd"><td ><tt>-i</tt></td><td >Do not generate an <tt>#include</tt> statement in the output. This may be used to run the moc on on a C++ file containing one or more class declarations. You should then <tt>#include</tt> the meta-object code in the <tt>.cpp</tt> file.</td></tr>
<tr valign="top" class="even"><td ><tt>-nw</tt></td><td >Do not generate any warnings. (Not recommended.)</td></tr>
<tr valign="top" class="odd"><td ><tt>-p<path></tt></td><td >Makes the moc prepend <tt><path>/</tt> to the file name in the generated <tt>#include</tt> statement.</td></tr>
<tr valign="top" class="even"><td ><tt>-I<dir></tt></td><td >Add dir to the include path for header files.</td></tr>
<tr valign="top" class="odd"><td ><tt>-E</tt></td><td >Preprocess only; do not generate meta-object code.</td></tr>
<tr valign="top" class="even"><td ><tt>-D<macro>[=<def>]</tt></td><td >Define macro, with optional definition.</td></tr>
<tr valign="top" class="odd"><td ><tt>-U<macro></tt></td><td >Undefine macro.</td></tr>
<tr valign="top" class="even"><td ><tt>-M<key=value></tt></td><td >Append additional meta data to plugins. If a class has Q_PLUGIN_METADATA specified, the key-value pair will be added to its meta data. This will end up in the Json object that gets resolved for the plugin at run time (accessible from <a href="../qtcore/qpluginloader.html">QPluginLoader</a>). This argument is typically used for tagging static plugins with information resolved by the build system.</td></tr>
<tr valign="top" class="odd"><td ><tt>@<file></tt></td><td >Read additional command-line options from <tt><file></tt>. Each line of the file is treated as a single option. Empty lines are ignored. Note that this option is not supported within the options file itself (i.e. an options file can't "include" another file).</td></tr>
<tr valign="top" class="even"><td ><tt>-h</tt></td><td >Display the usage and the list of options.</td></tr>
<tr valign="top" class="odd"><td ><tt>-v</tt></td><td >Display <tt>moc</tt>'s version number.</td></tr>
<tr valign="top" class="even"><td ><tt>-Fdir</tt></td><td >Mac OS X. Add the framework directory <tt>dir</tt> to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order (see the manpage for gcc). Normally, use -F /Library/Frameworks/</td></tr>
</table>
<p>You can explicitly tell the moc not to parse parts of a header file. <tt>moc</tt> defines the preprocessor symbol <tt>Q_MOC_RUN</tt>. Any code surrounded by</p>
<pre class="cpp"><span class="preprocessor">#ifndef Q_MOC_RUN</span>
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="preprocessor">#endif</span></pre>
<p>is skipped by the <tt>moc</tt>.</p>
<a name="diagnostics"></a>
<h2>Diagnostics</h2>
<p><tt>moc</tt> will warn you about a number of dangerous or illegal constructs in the <a href="../qtcore/qobject.html#Q_OBJECT">Q_OBJECT</a> class declarations.</p>
<p>If you get linkage errors in the final building phase of your program, saying that <tt>YourClass::className()</tt> is undefined or that <tt>YourClass</tt> lacks a vtable, something has been done wrong. Most often, you have forgotten to compile or <tt>#include</tt> the moc-generated C++ code, or (in the former case) include that object file in the link command. If you use <tt>qmake</tt>, try rerunning it to update your makefile. This should do the trick.</p>
<a name="limitations"></a>
<h2>Limitations</h2>
<p><tt>moc</tt> does not handle all of C++. The main problem is that class templates cannot have signals or slots. Here is an example:</p>
<pre class="cpp"><span class="keyword">class</span> SomeTemplate<span class="operator"><</span><span class="type">int</span><span class="operator">></span> : <span class="keyword">public</span> <span class="type"><a href="../qtwidgets/qframe.html">QFrame</a></span>
{
Q_OBJECT
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="keyword">signals</span>:
<span class="type">void</span> mySignal(<span class="type">int</span>);
};</pre>
<p>Less importantly, the following constructs are illegal. All of them have alternatives which we think are usually better, so removing these limitations is not a high priority for us.</p>
<a name="multiple-inheritance-requires-qobject-to-be-first"></a>
<h3>Multiple Inheritance Requires QObject to Be First</h3>
<p>If you are using multiple inheritance, <tt>moc</tt> assumes that the first inherited class is a subclass of <a href="../qtcore/qobject.html">QObject</a>. Also, be sure that only the first inherited class is a <a href="../qtcore/qobject.html">QObject</a>.</p>
<pre class="cpp"><span class="comment">// correct</span>
<span class="keyword">class</span> SomeClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span><span class="operator">,</span> <span class="keyword">public</span> OtherClass
{
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
};</pre>
<p>Virtual inheritance with <a href="../qtcore/qobject.html">QObject</a> is <i>not</i> supported.</p>
<a name="function-pointers-cannot-be-signal-or-slot-parameters"></a>
<h3>Function Pointers Cannot Be Signal or Slot Parameters</h3>
<p>In most cases where you would consider using function pointers as signal or slot parameters, we think inheritance is a better alternative. Here is an example of illegal syntax:</p>
<pre class="cpp"><span class="keyword">class</span> SomeClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span>
{
Q_OBJECT
<span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> apply(<span class="type">void</span> (<span class="operator">*</span>apply)(List <span class="operator">*</span><span class="operator">,</span> <span class="type">void</span> <span class="operator">*</span>)<span class="operator">,</span> <span class="type">char</span> <span class="operator">*</span>); <span class="comment">// WRONG</span>
};</pre>
<p>You can work around this restriction like this:</p>
<pre class="cpp"><span class="keyword">typedef</span> <span class="type">void</span> (<span class="operator">*</span>ApplyFunction)(List <span class="operator">*</span><span class="operator">,</span> <span class="type">void</span> <span class="operator">*</span>);
<span class="keyword">class</span> SomeClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span>
{
Q_OBJECT
<span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> apply(ApplyFunction<span class="operator">,</span> <span class="type">char</span> <span class="operator">*</span>);
};</pre>
<p>It may sometimes be even better to replace the function pointer with inheritance and virtual functions.</p>
<a name="enums-and-typedefs-must-be-fully-qualified-for-signal-and-slot-parameters"></a>
<h3>Enums and Typedefs Must Be Fully Qualified for Signal and Slot Parameters</h3>
<p>When checking the signatures of its arguments, <a href="../qtcore/qobject.html#connect">QObject::connect</a>() compares the data types literally. Thus, <a href="../qtcore/qt.html#AlignmentFlag-enum">Alignment</a> and <a href="../qtcore/qt.html#AlignmentFlag-enum">Qt::Alignment</a> are treated as two distinct types. To work around this limitation, make sure to fully qualify the data types when declaring signals and slots, and when establishing connections. For example:</p>
<pre class="cpp"><span class="keyword">class</span> MyClass : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qobject.html">QObject</a></span>
{
Q_OBJECT
<span class="keyword">enum</span> Error {
ConnectionRefused<span class="operator">,</span>
RemoteHostClosed<span class="operator">,</span>
UnknownError
};
<span class="keyword">signals</span>:
<span class="type">void</span> stateChanged(MyClass<span class="operator">::</span>Error error);
};</pre>
<a name="nested-classes-cannot-have-signals-or-slots"></a>
<h3>Nested Classes Cannot Have Signals or Slots</h3>
<p>Here's an example of the offending construct:</p>
<pre class="cpp"><span class="keyword">class</span> A
{
<span class="keyword">public</span>:
<span class="keyword">class</span> B
{
Q_OBJECT
<span class="keyword">public</span> <span class="keyword">slots</span>: <span class="comment">// WRONG</span>
<span class="type">void</span> b();
};
};</pre>
<a name="signal-slot-return-types-cannot-be-references"></a>
<h3>Signal/Slot return types cannot be references</h3>
<p>Signals and slots can have return types, but signals or slots returning references will be treated as returning void.</p>
<a name="only-signals-and-slots-may-appear-in-the-keyword-signals-keyword-and-keyword-slots-keyword-sections-of-a-class"></a>
<h3>Only Signals and Slots May Appear in the <tt>signals</tt> and <tt>slots</tt> Sections of a Class</h3>
<p><tt>moc</tt> will complain if you try to put other constructs in the <tt>signals</tt> or <tt>slots</tt> sections of a class than signals and slots.</p>
</div>
<p><b>See also </b><a href="../qtcore/metaobjects.html#meta-object-system">Meta-Object System</a>, <a href="../qtcore/signalsandslots.html#signals-and-slots">Signals and Slots</a>, and <a href="../qtcore/properties.html#qt-s-property-system">Qt's Property System</a>.</p>
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