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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Using the Meta Object Compiler</h1>



<p> <!-- index moc --><a name="moc"></a>
<p> The Meta Object Compiler, moc among friends, is the program which
handles Qt's <a href="metaobjects.html">C++ extensions.</a>
<p> The moc reads a C++ source file. If it finds one or more class
declarations that contain the <a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a> macro, it produces another
C++ source file which contains the <a href="metaobjects.html#meta-object">meta object</a> code for the classes
that use the Q_OBJECT macro. Among other things, meta object code is
required for the signal/slot mechanism, runtime type information and
the dynamic property system.
<p> The C++ source file generated by the moc must be compiled and linked
with the implementation of the class (or it can be #included into the
class's source file).
<p> If you use <a href="qmake-manual.html">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 moc, see <a href="templates.html">Why doesn't Qt
use templates for signals and slots?</a>.
<p> <h2> Usage
</h2>
<a name="1"></a><p> The moc is typically used with an input file containing class declarations
like this:
<p> <pre>
    class MyClass : public <a href="qobject.html">QObject</a>
    {
        Q_OBJECT
    public:
        MyClass( <a href="qobject.html">QObject</a> * parent=0, const char * name=0 );
        ~MyClass();

    signals:
        void mySignal();

    public slots:
        void mySlot();

    };
</pre>
 
<p> In addition to the signals and slots shown above, the moc also
implements object properties as in the next example. The Q_PROPERTY
macro declares an object property, while Q_ENUMS declares a list of
enumeration types within the class to be usable inside the 
<a href="properties.html">property system</a>.  In this particular
case we declare a property of the enumeration type <tt>Priority</tt> that is
also called "priority" and has a get function <tt>priority()</tt> and a set
function <tt>setPriority()</tt>.
<p> <pre>
    class MyClass : public <a href="qobject.html">QObject</a>
    {
        Q_OBJECT
        Q_PROPERTY( Priority priority READ priority WRITE setPriority )
        Q_ENUMS( Priority )
    public:
        MyClass( <a href="qobject.html">QObject</a> * parent=0, const char * name=0 );
        ~MyClass();

        enum Priority { High, Low, VeryHigh, VeryLow };
        void setPriority( Priority );
        Priority priority() const;
    };
</pre>
 
<p> Properties can be modified in subclasses with the Q_OVERRIDE
macro. The Q_SETS macro declares enums that are to be used as
sets, i.e. OR'ed together. Another macro, Q_CLASSINFO, can be used to
attach additional name/value-pairs to the class' meta object:
<p> <pre>
    class MyClass : public <a href="qobject.html">QObject</a>
    {
        Q_OBJECT
        Q_CLASSINFO( "Author", "Oscar Peterson")
        Q_CLASSINFO( "Status", "Active")
    public:
        MyClass( <a href="qobject.html">QObject</a> * parent=0, const char * name=0 );
        ~MyClass();
    };
</pre>
 
<p> The three concepts, signals and slots, properties and class
meta-data, can be combined.
<p> The output produced by the moc must be compiled and linked, just like
the other C++ code in your program; otherwise the build will fail in
the final link phase. By convention, this is done in one of the
following two ways:
<p> <dl>
<p> <dt><b>Method A: The class declaration is found in a header
(<em>.h</em>) file</b>
<p> <dd>If the class declaration above is found in the file
<em>myclass.h</em>, the moc output should be put in a file called
<em>moc_myclass.cpp</em>. This file should then be compiled as
usual, resulting in an object file <em>moc_myclass.o</em> (on Unix)
or <em>moc_myclass.obj</em> (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> <dt><b>Method B: The class declaration is found in an implementation
(<em>.cpp</em>) file</b>
<p> <dd>If the class declaration above is found in the file
<em>myclass.cpp</em>, the moc output should be put in a file called
<em>myclass.moc</em>. This file should be #included in the
implementation file, i.e. <em>myclass.cpp</em> should contain the
line
<pre>
    #include "myclass.moc"
</pre>
 
at the end. This will cause the moc-generated code to be compiled and
linked together with the normal class definition in <em>myclass.cpp</em>, so
it is not necessary to compile and link it separately, as in Method A.
<p> </dl>
<p> Method A is the normal method. Method B can be used in cases where you
want the implementation file to be self-contained, or in cases where
the Q_OBJECT class is implementation-internal and thus should not be
visible in the header file.
<p> <h2> Automating moc Usage with Makefiles
</h2>
<a name="2"></a><p> For anything but the simplest test programs, it is recommended that
you automate running the moc. By adding some rules to your program's
Makefile, <em>make</em> can take care of running moc when necessary and
handling the moc output.
<p> We recommend using Trolltech's free makefile generation tool, <a href="qmake-manual.html">qmake</a>, for building your Makefiles. This tool
recognizes both Method A and B style source files, and generates a
Makefile that does all the necessary moc handling.
<p> If you want to create your Makefiles yourself, here are some tips on
how to include moc handling.
<p> For Q_OBJECT class declarations in header files, here is a useful
makefile rule if you only use GNU make:
<p> <pre>
    moc_%.cpp: %.h
            moc $&lt; -o $@
</pre>
 
<p> If you want to write portably, you can use individual rules with the
following form:
<p> <pre>
    moc_NAME.cpp: NAME.h
            moc $&lt; -o $@
</pre>
 
<p> You must also remember to add <em>moc_NAME.cpp</em> to your SOURCES
(substitute your favorite name) variable and <em>moc_NAME.o</em> or
<em>moc_NAME.obj</em> to your OBJECTS variable.
<p> (While we prefer to name our C++ source files .cpp, the moc doesn't
care, so you can use .C, .cc, .CC, .cxx or even .c++ if you
prefer.)
<p> For Q_OBJECT class declarations in implementation (.cpp) files, we
suggest a makefile rule like this:
<p> <pre>
    NAME.o: NAME.moc

    NAME.moc: NAME.cpp
            moc -i $&lt; -o $@
</pre>
 
<p> This guarantees that make will run the moc before it compiles
<em>NAME.cpp</em>. You can then put
<p> <pre>
    #include "NAME.moc"
</pre>
 
<p> at the end of <em>NAME.cpp</em>, where all the classes declared in
that file are fully known.
<p> <h2> Invoking moc
</h2>
<a name="3"></a><p> Here are the command-line options supported by the moc:
<p> <center><table cellpadding="4" cellspacing="2" border="0">
<tr bgcolor="#a2c511"> <th valign="top">Option <th valign="top">Meaning
<tr bgcolor="#f0f0f0">
<td valign="top">-o <em>file</em> 
<td valign="top">Write output to <em>file</em> rather than to stdout.
<tr bgcolor="#d0d0d0">
<td valign="top">-f 
<td valign="top">Force the generation of an #include statement in the
output.  This is the default for files whose name matches the <a href="qregexp.html#regular-expression">regular expression</a> &#92;.[hH][^.]* (i.e. the extension starts with H or h).  This
option is only useful if you have header files that do not follow the
standard naming conventions.
<tr bgcolor="#f0f0f0">
<td valign="top">-i 
<td valign="top">Do not generate an #include 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 #include the meta object
code in the .cpp
file.  If both -i and -f are present, the last one wins.
<tr bgcolor="#d0d0d0">
<td valign="top">-nw 
<td valign="top">Do not generate any warnings.  Not recommended.
<tr bgcolor="#f0f0f0">
<td valign="top">-ldbg 
<td valign="top">Write a flood of lex debug information to stdout.
<tr bgcolor="#d0d0d0">
<td valign="top">-p <em>path</em> 
<td valign="top">Makes the moc prepend <em>path</em>/ to
the file name in the generated #include statement (if one is
generated).
<tr bgcolor="#f0f0f0">
<td valign="top">-q <em>path</em> 
<td valign="top">Makes the moc prepend <em>path</em>/ to
the file name of qt #include files in the generated code.
</table></center>
<p> You can explicitly tell the moc not to parse parts of a header
file. It recognizes any C++ comment (//) that contains the substrings
MOC_SKIP_BEGIN or MOC_SKIP_END. They work as you would expect and you
can have several levels of them. The net result as seen by the moc is
as if you had removed all lines between a MOC_SKIP_BEGIN and a
MOC_SKIP_END.
<p> <h2> Diagnostics
</h2>
<a name="4"></a><p> The moc will warn you about a number of dangerous or illegal
constructs in the Q_OBJECT class declarations.
<p> If you get linkage errors in the final building phase of your
program, saying that YourClass::className() is undefined or that
YourClass lacks a vtbl, something has been done wrong. Most often,
you have forgotten to compile or #include the moc-generated C++ code, or
(in the former case) include that object file in the link command.
<p> <h2> Limitations
</h2>
<a name="5"></a><p> The moc does not expand #include or #define, it simply skips any
preprocessor directives it encounters. This is regrettable, but is
not usually a problem in practice.
<p> The moc 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 SomeTemplate&lt;int&gt; : public <a href="qframe.html">QFrame</a> {
        Q_OBJECT
        ...
    signals:
        void bugInMocDetected( int );
    };
</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> <h3> Multiple inheritance requires <a href="qobject.html">QObject</a> to be first
</h3>
<a name="5-1"></a><p> If you are using multiple inheritance, moc assumes that the <em>first</em>
inherited class is a subclass of QObject.  Also, be sure that <em>only</em>
the first inherited class is a QObject.
<p> <pre>
    class SomeClass : public <a href="qobject.html">QObject</a>, public OtherClass {
        ...
    };
</pre>
 
<p> (This limitation is almost impossible to remove; since the moc does not expand
#include or #define, it cannot find out which one of the base classes
is a QObject.)
<p> <h3> Function pointers cannot be arguments to signals or slots
</h3>
<a name="5-2"></a><p> In most cases where you would consider using function pointers as
signal/slot arguments, we think inheritance is a better alternative.
Here is an example of illegal syntax:
<p> <pre>
    class SomeClass : public <a href="qobject.html">QObject</a> {
        Q_OBJECT
        ...
    public slots:
        // illegal
        void apply( void (*apply)(List *, void *), char * );
    };
</pre>
 
<p> You can work around this restriction like this:
<pre>
    typedef void (*ApplyFunctionType)( List *, void * );

    class SomeClass : public <a href="qobject.html">QObject</a> {
        Q_OBJECT
        ...
    public slots:
        void apply( ApplyFunctionType, char * );
    };
</pre>
 
<p> It may sometimes be even better to replace the function pointer with
inheritance and virtual functions, signals or slots.
<p> <h3> Friend declarations cannot be placed in signals or slots sections
</h3>
<a name="5-3"></a><p> Sometimes it will work, but in general, friend declarations cannot be
placed in signals or slots sections.  Put them in the private,
protected or public sections instead.  Here is an example of the
illegal syntax:
<p> <pre>
    class SomeClass : public <a href="qobject.html">QObject</a> {
        Q_OBJECT
        ...
    signals:
        friend class ClassTemplate&lt;char&gt;; // WRONG
    };
</pre>
 
<p> <h3> Signals and slots cannot be upgraded
</h3>
<a name="5-4"></a><p> The C++ feature of upgrading an inherited member function to
public status is not extended to cover signals and slots.  Here is an
illegal example:
<p> <pre>
    class Whatever : public <a href="qbuttongroup.html">QButtonGroup</a> {
        ...
    public slots:
        <a href="qbuttongroup.html">QButtonGroup</a>::buttonPressed; // WRONG
        ...
    };
</pre>
 
<p> The QButtonGroup::buttonPressed() slot is protected.
<p> C++ quiz: What happens if you try to upgrade a protected member
function which is overloaded?
<ol type=1>
<li> All the functions are overloaded.
<li> That is not legal C++.
</ol>
<p> 
<p> <h3> Type macros cannot be used for signal and slot parameters
</h3>
<a name="5-5"></a><p> Since the moc does not expand #define, type macros that take an argument
will not work in signals and slots. Here is an illegal example:
<p> <pre>
    #ifdef ultrix
    #define SIGNEDNESS(a) unsigned a
    #else
    #define SIGNEDNESS(a) a
    #endif

    class Whatever : public <a href="qobject.html">QObject</a> {
        ...
    signals:
        void someSignal( SIGNEDNESS(int) );
        ...
    };
</pre>
 
<p> A #define without parameters will work as expected.
<p> <h3> Nested classes cannot be in the signals or slots sections nor have
signals or slots
</h3>
<a name="5-6"></a><p> Here's an example:
<p> <pre>
    class A {
        Q_OBJECT
    public:
        class B {
        public slots:   // WRONG
            void b();
            ...
        };
    signals:
        class B {       // WRONG
            void b();
            ...
        }:
    };
</pre>
 
<p> <h3> Constructors cannot be used in signals or slots sections
</h3>
<a name="5-7"></a><p> It is a mystery to us why anyone would put a constructor in
either the signals or slots sections.  You can't anyway (except
that it happens to work in some cases).  Put them in private,
protected or public sections, where they belong.  Here is an example
of the illegal syntax:
<p> <pre>
    class SomeClass : public <a href="qobject.html">QObject</a> {
        Q_OBJECT
    public slots:
        SomeClass( <a href="qobject.html">QObject</a> *parent, const char *name )
            : <a href="qobject.html">QObject</a>( parent, name ) { } // WRONG
        ...
    };
</pre>
 
<p> <h3> Properties need to be declared before the public section that
contains the respective get and set functions
</h3>
<a name="5-8"></a><p> Declaring the first property within or after the public section that
contains the type definition and the respective get and set functions
does not work as expected. The moc will complain that it can neither
find the functions nor resolve the type. Here is an example of the
illegal syntax:
<p> <pre>
    class SomeClass : public <a href="qobject.html">QObject</a> {
        Q_OBJECT
    public:
        ...
        Q_PROPERTY( Priority priority READ priority WRITE setPriority ) // WRONG
        Q_ENUMS( Priority ) // WRONG
        enum Priority { High, Low, VeryHigh, VeryLow };
        void setPriority( Priority );
        Priority priority() const;
        ...
    };
</pre>
 
<p> Work around this limitation by declaring all properties at the
beginning of the class declaration, right after Q_OBJECT:
<p> <pre>
    class SomeClass : public <a href="qobject.html">QObject</a> {
        Q_OBJECT
        Q_PROPERTY( Priority priority READ priority WRITE setPriority )
        Q_ENUMS( Priority )
    public:
        ...
        enum Priority { High, Low, VeryHigh, VeryLow };
        void setPriority( Priority );
        Priority priority() const;
        ...
    };
</pre>
 
<p> 
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