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Using the Meta Object Compiler


The Meta Object Compiler, moc among friends, is the program which handles the C++ extensions in Qt.

The moc reads a C++ source file. If it finds one or more class declarations that contain the "Q_OBJECT" macro, it produces another C++ source file which contains the meta object code for this class. Among other things, meta object code is required for the signal/slot mechanism, runtime type information and the dynamic property system.

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' source file).

Using the moc is introduced in chapter 7 of the Qt Tutorial. Chapter 7 includes a simple Makefile that uses the moc and of course source code that uses signals and slots.

Usage

The moc is typically used with an input file containing class declarations like this skeleton:

    class MyClass : public QObject
    {
        Q_OBJECT
    public:
        MyClass( QObject * parent=0, const char * name=0 );
        ~MyClass();

    signals:
        void mySignal();

    public slots:
        void mySlot();

    };

In addition to the signals and slots shown above, the moc also implements object properties as in the next skeleton. 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 property system. In this particular case we declare a property of the enumeration type Priority that is also called "priority" and has a get function priority() and a set function setPriority().

    class MyClass : public QObject
    {
        Q_OBJECT
        Q_PROPERTY( Priority priority READ priority WRITE setPriority )
        Q_ENUMS( Priority )
    public:
        MyClass( QObject * parent=0, const char * name=0 );
        ~MyClass();

        enum Priority { High, Low, VeryHigh, VeryLow };
        void setPriority( Priority );
        Priority priority() const;
    };

Properties can be modified in subclasses with the "Q_OVERRIDE" macro. The "Q_SETS" macro declares enums to actually be used as sets. Another macro "Q_CLASSINFO" can be used to attach additional name/value-pairs to the classes' meta object:

    class MyClass : public QObject
    {
        Q_OBJECT
        Q_CLASSINFO( "Author", "Oscar Peterson")
        Q_CLASSINFO( "Status", "Very nice class")
    public:
        MyClass( QObject * parent=0, const char * name=0 );
        ~MyClass();
    };

The three concepts, signals and slots, properties and class informations, can be combined.

The output produced by the moc must be compiled and linked, just as the other C++ code of your program; otherwise the building of your program will fail in the final link phase. By convention, this is done in one of the following two ways:

Method A: The class declaration is found in a header (.h) file

If the class declaration above is found in the file myclass.h, the moc output should be put in a file called moc_myclass.cpp. This file should then be compiled as usual, resulting in an object file moc_myclass.o (on Unix) or moc_myclass.obj (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.

Method B: The class declaration is found in an implementation (.cpp) file

If the class declaration above is found in the file myclass.cpp, the moc output should be put in a file called myclass.moc. This file should be #included in the implementation file, i.e. myclass.cpp should contain the line
#include "myclass.moc"
after the other code. This will cause the moc-generated code to be compiled and linked together with the normal class definition in myclass.cpp, so it is not necessary to compile and link it separately, as in Method A.

Method A is the normal method. Method B can be used in cases where one for some reason wants 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.

Automating moc Usage with Makefiles

For anything but the simplest test programs, it is recommended to automate the running of the moc. By adding some rules to the Makefile of your program, make can take care of running moc when necessary and handling the moc output.

We recommend using Trolltech's free makefile generation tool tmake for building your Makefiles. This tool recognizes both Method A and B style source files, and generates a Makefile that does all necessary moc handling. tmake is available from http://www.trolltech.com/freebies/tmake.html.

If, on the other hand, you want to build your Makefiles yourself, here are some tips on how to include moc handling.

For Q_OBJECT class declarations in header files, here is a useful makefile rule if you only use GNU make:

    moc_%.cpp: %.h
            moc $< -o $@

If you want to write portably, you can use individual rules of the following form:

    moc_NAME.cpp: NAME.h
            moc $< -o $@

You must also remember to add moc_NAME.cpp to your SOURCES (substitute your favorite name) variable and moc_NAME.o or moc_NAME.objto 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.)

For Q_OBJECT class declarations in implementation (.cpp) files, we suggest a makefile rule like this:

    NAME.o: NAME.moc

    NAME.moc: NAME.cpp
            moc -i $< -o $@

This guarantees that make will run the moc before it compiles NAME.cpp. You can then put

    #include "NAME.moc"

at the end of NAME.cpp, where all the classes declared in that file are fully known.

Invoking moc

Here are the command-line options supported by the moc:

-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][^.]* (ie. 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 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.
-nw
Do not generate any warnings. Discouraged.
-ldbg
Write a flood of lex debug information on stdout.
-p path
Makes the moc prepend path/ to the file name in the generated #include statement (if one is generated).
-q path
Makes the moc prepend path/ to the file name of qt #include files in the generated code.

You can explicitly tell the moc to not 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

Diagnostics

The moc will warn you about a number of dangerous or illegal constructs in the Q_OBJECT class declarations.

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 forgot to compile or #include the moc-generated C++ code, or (in the former case) include that object file in the link command.

Limitations

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. 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 workarounds which we think are better alternatives, 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 limitation is almost impossible to remove; since the moc does not expand is a QObject.)

Virtual functions cannot be slots when using multiple inheritance

This problem occurs if you are using multiple inheritance. If you reimplement a virtual function as a slot and that function was originally declared in a class that does not inherit QObject, your program will crash when a signal triggers the slot. (This may not happen on all platforms.)

The following example shows one wrong and two correct slot definitions.

    class BaseClass {
    [...]
        virtual void setValue( int );
    };

    class SubClass : public QObject, public BaseClass {
    [...]
    public slots:
        void setValue( int ); //virtual from BaseClass, error.
        void slotSetValue( int i ) { setValue(i); } //new function, ok.
        void setName( const char* ); // virtual from QObject, ok.
    };

(For those interested in C++ internals: The cause of this problem is that a slot is internally represented as a function pointer, and invoked on a QObject pointer. )

Function pointers can not 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:
        void apply(void (*applyFunction)(QList*, void*), char*); // illegal
    };

You can work around this restriction like this:

    typedef void (*ApplyFunctionType)(QList*, void*);

    class someClass : public QObject {
        Q_OBJECT
    [...]
    public slots:
        void apply( ApplyFunctionType, char *);
    };

(It may sometimes be even better to replace the function pointer with inheritance and virtual functions, signals or slots.)

Friend declarations can not be placed in signals or slots sections

Sometimes it will work, but in general, friend declarations can not be placed in signals or slots sections. Put them in the good old private, protected or 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:
        void 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?

  1. All the functions are overloaded.
  2. That is not legal C++.

Type macros can not 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(a) );
    [...]
    };

A #define without arguments will work as expected.

Nested classes cannot be in the signals or slots sections nor have signals or slots

Here's an example:

    class A {
        Q_OBJECT
    public:
        class B {
        public slots:   // illegal
            void b();
        [....]
        };
    signals:
        class B {       // illegal
            void b();

        [....]
        }:
    };

Constructors can not be used in signals or slots sections

It is a mystery to me why anyone would put a constructor on either the signals or slots sections. You can not, 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
    [...]
    };

Signals and slots may not have default arguments

Since signal->slot binding occurs at run-time, it is conceptually difficult to use default parameters, which are a compile-time phenomenon. This will fail:

    class SomeClass : public QObject {
        Q_OBJECT
    public slots:
        void someSlot(int x=100); // illegal
    };

Signals and slots may not have template arguments

Declaring signals and slots with template-type parameters will not work as expected, even though the moc will not complain. Connecting the signal to the slot in the following example, the slot will not get executed when the signal is emitted:

   [...]
   public slots:
       void MyWidget::setLocation (pair<int,int> location);

   [...]
   public signals:
       void MyObject::moved (pair<int,int> location);

However, you can work around this limitation by explicitly typedef'ing the parameter types, like this:

   typedef pair<int,int> IntPair;       
   [...]
   public slots:
       void MyWidget::setLocation (IntPair location);

   [...]
   public signals:
       void MyObject::moved (IntPair location);

This will work as expected.

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:
    [...]
        Q_PROPERTY( Priority priority READ priority WRITE setPriority ) // illegal
        Q_ENUMS( Priority ) // illegal
        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 );
        Priority priority() const;
    [...]
    };


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