moc
SYNOPSIS
moc [-o file] [-i] [-f] [-k] [-ldbg] [-nw] [-p path] [-q
path] [-v] file
DESCRIPTION
This page documents the Meta Object Compiler for the Qt
GUI application framework. The moc reads one or more C++
class declarations from a C++ header or source file and
generates one C++ source file containing meta object
information for the classes. 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).
If you use qmake 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.
In brief, the meta object system is a structure used by Qt
(see http://doc.trolltech.com) for component programming
and run time type information. It adds properties and
inheritance information to (some) classes and provides a
new type of communication between those instances of those
classes, signal-slot connections.
OPTIONS
-o file
Write output to file rather than to stdout.
-f Force the generation of an #include statement in
the output. This is the default for files whose
name matches the regular expression .[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.
-i Do not generate an #include statement in the
output. This may be used to run moc on a C++ file
containing one or more class declarations. You
should then #include the meta object code in the
.cpp file (see USAGE below). If both -f and -i are
present, the last one wins.
-nw Do not generate any warnings. Not recommended.
-ldbg Write a flood of lex debug information to stdout.
-p path
Makes moc prepend path/ to the file name in the
generated #include statement (if one is generated).
-q path
moc is almost always invoked by make(1), not by hand.
moc is typically used with an input file containing class
declarations like this:
class YourClass : public QObject {
Q_OBJECT
Q_PROPERTY( ... )
Q_CLASSINFO( ... )
public:
YourClass( QObject * parent=0, const char * name=0 );
~YourClass();
signals:
public slots:
};
Here is a useful makefile rule if you only use GNU make:
m%.cpp: %.h
moc $< -o $@
If you want to write portably, you can use individual
rules of the following form:
mNAME.cpp: NAME.h
moc $< -o $@
You must also remember to add mNAME.cpp to your SOURCES
(substitute your favorite name) variable and mNAME.o to
your OBJECTS variable.
(While we prefer to name our C++ source files .cpp, the
moc doesn't know that, so you can use .C, .cc, .CC, .cxx
or even .c++ if you prefer.)
If you have class declarations in C++ files, we recommend
that you use a makefile rule like this:
NAME.o: mNAME.cpp
mNAME.cpp: NAME.cpp
moc -i $< -o $@
This guarantees that make(1) will run the moc before it
compiles NAME.cpp. You can then put
#include "nNAME.cpp"
The moc does not expand #include or #define, it simply
skips any preprocessor directives it encounters. This is
regrettable, but is normally not a problem in practice.
The moc does not handle all of C++. The main problem is
that class templates cannot have signals or slots. This
is an important bug. Here is an example:
class SomeTemplate<int> : public QFrame {
Q_OBJECT
....
signals:
void bugInMocDetected( int );
};
Less importantly, the following constructs are illegal.
All of them have have alternatives which we think are
usually better, so removing these limitations is not a
high priority for us.
Multiple inheritance requires QObject to be first.
If you are using multiple inheritance, moc assumes that
the first inherited class is a subclass of QObject. Also,
be sure that only the first inherited class is a QObject.
class SomeClass : public QObject, public OtherClass {
...
};
This bug is almost impossible to fix; since the moc does
not expand #include or #define, it cannot find out which
one of the base classes is a QObject.
Function pointers cannot be arguments to signals or slots.
In most cases where you would consider that, we think
inheritance is a better alternative. Here is an example
of illegal syntax:
class SomeClass : public QObject {
Q_OBJECT
...
public slots:
// illegal
void apply( void (*apply)(List *, void *), void * );
};
You can work around this restriction like this:
typedef void (*ApplyFunctionType)( List *, void * );
class SomeClass : public QObject {
Q_OBJECT
public sections instead. Here is an example of the
illegal syntax:
class SomeClass : public QObject {
Q_OBJECT
...
signals:
friend class ClassTemplate<char>; // illegal
};
Signals and slots cannot be upgraded
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:
class Whatever : public QButtonGroup {
...
public slots:
QButtonGroup::buttonPressed; // illegal
...
};
The QButtonGroup::buttonPressed() slot is protected.
C++ quiz: What happens if you try to upgrade a protected
member function which is overloaded?
- All the functions are upgraded.
- That is not legal C++.
Type macros cannot be used for signal and slot arguments
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:
#ifdef ultrix
#define SIGNEDNESS(a) unsigned a
#else
#define SIGNEDNESS(a) a
#endif
class Whatever : public QObject {
...
signals:
void someSignal( SIGNEDNESS(int) ); // illegal
};
A #define without arguments works.
Nested classes cannot be in the signals or slots sections nor
have signals or slots
Here's an example:
};
Constructors cannot be used in signals or slots sections
It is a mystery to us why anyone would put a constructor
on 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:
class SomeClass : public QObject {
Q_OBJECT
public slots:
SomeClass( QObject *parent, const char *name )
: QObject( parent, name ) {} // illegal
...
};
Properties need to be declared before the public section that
contains the respective get and set functions
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:
class SomeClass : public QObject {
Q_OBJECT
public:
...
// illegal
Q_PROPERTY( Priority priority READ priority WRITE setPriority )
Q_ENUMS( Priority )
enum Priority { High, Low, VeryHigh, VeryLow };
void setPriority( Priority );
Priority priority() const;
...
};
Work around this limitation by declaring all properties at
the beginning of the class declaration, right after
Q_OBJECT:
class SomeClass : public QObject {
Q_OBJECT
Q_PROPERTY( Priority priority READ priority WRITE setPriority )
Q_ENUMS( Priority )
public:
...
enum Priority { High, Low, VeryHigh, VeryLow };
void setPriority( Priority );
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