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<title>Qt 4.1: Using the Meta-Object Compiler (moc)</title>
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<title>Qt 4.2: Using the Meta-Object Compiler (moc)</title>
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<td width="1"> </td><td class="postheader" valign="center"><a href="index.html"><font color="#004faf">Home</font></a> · <a href="classes.html"><font color="#004faf">All Classes</font></a> · <a href="mainclasses.html"><font color="#004faf">Main Classes</font></a> · <a href="groups.html"><font color="#004faf">Grouped Classes</font></a> · <a href="modules.html"><font color="#004faf">Modules</font></a> · <a href="functions.html"><font color="#004faf">Functions</font></a></td>
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<td align="right" valign="top" width="230"><img src="images/trolltech-logo.png" align="right" width="203" height="32" border="0" /></td></tr></table><h1 align="center">Using the Meta-Object Compiler (moc)<br /><small></small></h1>
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<td align="right" valign="top" width="230"><a href="http://www.trolltech.com"><img src="images/trolltech-logo.png" align="right" width="203" height="32" border="0" /></a></td></tr></table><h1 align="center">Using the Meta-Object Compiler (moc)<br /><small></small></h1>
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<a name="moc"></a><p>The Meta-Object Compiler, <tt>moc</tt>, is the program that handles <a href="metaobjects.html">Qt's C++ extensions</a>.</p>
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<p>The <tt>moc</tt> tool reads a C++ header file. If it finds one or more class declarations that contain the <a href="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>
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<p>The C++ source file generated by <tt>moc</tt> must be compiled and linked with the implementation of the class.</p>
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<a name="usage"></a>
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<p><tt>moc</tt> is typically used with an input file containing class declarations like this:</p>
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<pre> class MyClass : public QObject
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MyClass(QObject *parent = 0);
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<pre> class MyClass : public QObject
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MyClass(QObject *parent = 0);
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<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="qobject.html#Q_PROPERTY">Q_PROPERTY</a>() macro declares an object property, while <a href="qobject.html#Q_ENUMS">Q_ENUMS</a>() declares a list of enumeration types within the class to be usable inside the <a href="properties.html">property system</a>.</p>
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<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>
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<pre> class MyClass : public QObject
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Q_PROPERTY(Priority priority READ priority WRITE setPriority)
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enum Priority { High, Low, VeryHigh, VeryLow };
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MyClass(QObject *parent = 0);
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void setPriority(Priority priority);
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Priority priority() const;
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<pre> class MyClass : public QObject
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Q_PROPERTY(Priority priority READ priority WRITE setPriority)
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enum Priority { High, Low, VeryHigh, VeryLow };
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MyClass(QObject *parent = 0);
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void setPriority(Priority priority);
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Priority priority() const;
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<p>The <a href="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="qobject.html#Q_CLASSINFO">Q_CLASSINFO</a>(), allows you to attach additional name/value pairs to the class's meta-object:</p>
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<pre> class MyClass : public QObject
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Q_CLASSINFO("Author", "Oscar Peterson")
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Q_CLASSINFO("Status", "Active")
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<pre> class MyClass : public QObject
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Q_CLASSINFO("Author", "Oscar Peterson")
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Q_CLASSINFO("Status", "Active")
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MyClass(QObject *parent = 0);
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MyClass(QObject *parent = 0);
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<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="qobject.html#Q_OBJECT">Q_OBJECT</a> macro.</p>
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<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>
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<a name="writing-make-rules-for-invoking"></a>
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<p>We recommend using Trolltech's makefile generation tool, <a href="qmake-manual.html#qmake">qmake</a>, for building your makefiles. This tool generates a makefile that does all the necessary <tt>moc</tt> handling.</p>
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<p>If you want to create your makefiles yourself, here are some tips on how to include moc handling.</p>
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<p>For <a href="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>
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<pre> moc_%.cpp: %.h
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moc $(DEFINES) $(INCPATH) $< -o $@</pre>
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moc $(DEFINES) $(INCPATH) $< -o $@</pre>
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<p>If you want to write portably, you can use individual rules of the following form:</p>
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<pre> moc_foo.cpp: foo.h
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moc $(DEFINES) $(INCPATH) $< -o $@</pre>
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<pre> moc_foo.cpp: foo.h
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moc $(DEFINES) $(INCPATH) $< -o $@</pre>
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<p>You must also remember to add <tt>moc_foo.cpp</tt> to your <tt>SOURCES</tt> (substitute your favorite name) variable and <tt>moc_foo.o</tt> or <tt>moc_foo.obj</tt> to your <tt>OBJECTS</tt> variable.</p>
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<p>Both examples assume that <tt>$(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>
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<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>
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<p>For <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> class declarations in implementation (<tt>.cpp</tt>) files, we suggest a makefile rule like this:</p>
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<pre> foo.o: foo.moc
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moc $(DEFINES) $(INCPATH) -i $< -o $@</pre>
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moc $(DEFINES) $(INCPATH) -i $< -o $@</pre>
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<p>This guarantees that make will run the moc before it compiles <tt>foo.cpp</tt>. You can then put</p>
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<pre> #include "foo.moc"</pre>
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<pre> #include "foo.moc"</pre>
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<p>at the end of <tt>foo.cpp</tt>, where all the classes declared in that file are fully known.</p>
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<a name="command-line-options"></a>
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<h2>Command-Line Options</h2>
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<p>Here are the command-line options supported by the moc:</p>
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<p><table align="center" cellpadding="2" cellspacing="1" border="0">
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<tr valign="top" bgcolor="#a2c511"><th>Option</th><th>Description</th></tr>
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<tr valign="top" bgcolor="#f0f0f0"><td><tt>-o<file></tt></td><td>Write output to <tt><file></tt> rather than to standard output.</td></tr>
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<tr valign="top" bgcolor="#e0e0e0"><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>
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<tr valign="top" bgcolor="#f0f0f0"><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>
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<tr valign="top" bgcolor="#e0e0e0"><td><tt>-nw</tt></td><td>Do not generate any warnings. (Not recommended.)</td></tr>
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<tr valign="top" bgcolor="#f0f0f0"><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>
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<tr valign="top" bgcolor="#e0e0e0"><td><tt>-I<dir></tt></td><td>Add dir to the include path for header files.</td></tr>
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<tr valign="top" bgcolor="#f0f0f0"><td><tt>-E</tt></td><td>Preprocess only; do not generate meta-object code.</td></tr>
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<tr valign="top" bgcolor="#e0e0e0"><td><tt>-D<macro>[=<def>]</tt></td><td>Define macro, with optional definition.</td></tr>
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<tr valign="top" bgcolor="#f0f0f0"><td><tt>-U<macro></tt></td><td>Undefine macro.</td></tr>
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<tr valign="top" bgcolor="#e0e0e0"><td><tt>-h</tt></td><td>Display the usage and the list of options.</td></tr>
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<tr valign="top" bgcolor="#f0f0f0"><td><tt>-v</tt></td><td>Display <tt>moc</tt>'s version number.</td></tr>
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<thead><tr valign="top" class="qt-style"><th>Option</th><th>Description</th></tr></thead>
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<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>
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<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>
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<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>
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<tr valign="top" class="even"><td><tt>-nw</tt></td><td>Do not generate any warnings. (Not recommended.)</td></tr>
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<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>
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<tr valign="top" class="even"><td><tt>-I<dir></tt></td><td>Add dir to the include path for header files.</td></tr>
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<tr valign="top" class="odd"><td><tt>-E</tt></td><td>Preprocess only; do not generate meta-object code.</td></tr>
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<tr valign="top" class="even"><td><tt>-D<macro>[=<def>]</tt></td><td>Define macro, with optional definition.</td></tr>
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<tr valign="top" class="odd"><td><tt>-U<macro></tt></td><td>Undefine macro.</td></tr>
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<tr valign="top" class="even"><td><tt>-h</tt></td><td>Display the usage and the list of options.</td></tr>
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<tr valign="top" class="odd"><td><tt>-v</tt></td><td>Display <tt>moc</tt>'s version number.</td></tr>
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<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>
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<pre> #ifndef Q_MOC_RUN
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<pre> #ifndef Q_MOC_RUN
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<p>is skipped by the <tt>moc</tt>.</p>
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<a name="diagnostics"></a>
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<h2>Diagnostics</h2>
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<a name="limitations"></a>
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<h2>Limitations</h2>
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<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>
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<pre> class SomeTemplate<int> : public QFrame
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<pre> class SomeTemplate<int> : public QFrame
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<p>Another limitation is that moc does not expand macros, so you for example cannot use a macro to declare a signal/slot or use one to define a base class for a <a href="qobject.html">QObject</a>.</p>
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<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>
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<a name="multiple-inheritance-requires-qobject-to-be-first"></a>
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<h3>Multiple Inheritance Requires QObject to Be First</h3>
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<p>If you are using multiple inheritance, <tt>moc</tt> assumes that the first inherited class is a subclass of <a href="qobject.html">QObject</a>. Also, be sure that only the first inherited class is a <a href="qobject.html">QObject</a>.</p>
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<pre> // correct
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class SomeClass : public QObject, public OtherClass
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<pre><span class="comment"> // correct</span>
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class SomeClass : public QObject, public OtherClass
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<p>Virtual inheritance with <a href="qobject.html">QObject</a> is <i>not</i> supported.</p>
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<a name="function-pointers-cannot-be-signal-or-slot-parameters"></a>
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<h3>Function Pointers Cannot Be Signal or Slot Parameters</h3>
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<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>
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<pre> class SomeClass : public QObject
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<pre> class SomeClass : public QObject
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void apply(void (*apply)(List *, void *), char *); // WRONG
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void apply(void (*apply)(List *, void *), char *); <span class="comment">// WRONG</span>
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<p>You can work around this restriction like this:</p>
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<pre> typedef void (*ApplyFunction)(List *, void *);
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class SomeClass : public QObject
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void apply(ApplyFunction, char *);
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<pre> typedef void (*ApplyFunction)(List *, void *);
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class SomeClass : public QObject
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void apply(ApplyFunction, char *);
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<p>It may sometimes be even better to replace the function pointer with inheritance and virtual functions.</p>
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<a name="enums-and-typedefs-must-be-fully-qualified-for-signal-and-slot-parameters"></a>
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<h3>Enums and Typedefs Must Be Fully Qualified for Signal and Slot Parameters</h3>
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<p>When checking the signatures of its arguments, <a href="qobject.html#connect">QObject::connect</a>() compares the data types literally. Thus, <a href="qt.html#AlignmentFlag-enum">Alignment</a> and <a href="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>
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<pre> class MyClass : public QObject
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void stateChanged(MyClass::Error error);
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<pre> class MyClass : public QObject
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void stateChanged(MyClass::Error error);
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<a name="type-macros-cannot-be-used-for-signal-and-slot-parameters"></a>
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<h3>Type Macros Cannot Be Used for Signal and Slot Parameters</h3>
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<p>Since <tt>moc</tt> doesn't expand <tt>#define</tt>s, type macros that take an argument will not work in signals and slots. Here is an illegal example:</p>
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<pre> #ifdef ultrix
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#define SIGNEDNESS(a) unsigned a
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#define SIGNEDNESS(a) a
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class Whatever : public QObject
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void someSignal(SIGNEDNESS(int));
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#define SIGNEDNESS(a) unsigned a
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#define SIGNEDNESS(a) a
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class Whatever : public QObject
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void someSignal(SIGNEDNESS(int));
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<p>A macro without parameters will work.</p>
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<a name="nested-classes-cannot-have-signals-or-slots"></a>
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<h3>Nested Classes Cannot Have Signals or Slots</h3>
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<p>Here's an example of the offending construct:</p>
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public slots: // WRONG
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public slots: <span class="comment">// WRONG</span>
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<a name="signal-slot-return-types-cannot-be-references"></a>
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<h3>Signal/Slot return types cannot be references</h3>
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<p>Signals and slots can have return types, but signals or slots returning references will be treated as returning void.</p>
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<a name="only-signals-and-slots-may-appear-in-the-and-sections-of-a-class"></a>
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<h3>Only Signals and Slots May Appear in the <tt>signals</tt> and <tt>slots</tt> Sections of a Class</h3>
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<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>
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