Using the Meta Object Compiler

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

    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 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 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 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:

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

The three concepts, signals and slots, properties and class meta-data, can be combined.

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:

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"

at the end. 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 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.

Automating moc Usage with Makefiles

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

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

    NAME.o: NAME.moc

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

    #include "NAME.moc"

    class SomeTemplate<int> : public QFrame {
        Q_OBJECT
        ...
    signals:
        void bugInMocDetected( int );
    };

    class SomeClass : public QObject, public OtherClass {
        ...
    };

    class SomeClass : public QObject {
        Q_OBJECT
        ...
    public slots:
        // illegal
        void apply( void (*apply)(List *, void *), char * );
    };

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

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

    class SomeClass : public QObject {
        Q_OBJECT
        ...
    signals:
        friend class ClassTemplate<char>; // WRONG
    };

    class Whatever : public QButtonGroup {
        ...
    public slots:
        QButtonGroup::buttonPressed; // WRONG
        ...
    };

    #ifdef ultrix
    #define SIGNEDNESS(a) unsigned a
    #else
    #define SIGNEDNESS(a) a
    #endif

    class Whatever : public QObject {
        ...
    signals:
        void someSignal( SIGNEDNESS(int) );
        ...
    };

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

    class SomeClass : public QObject {
        Q_OBJECT
    public slots:
        SomeClass( QObject *parent, const char *name )
            : QObject( parent, name ) { } // WRONG
        ...
    };

    class SomeClass : public QObject {
        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;
        ...
    };

    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;
        ...
    };