Using the Meta-Object Compiler (moc)

Usage

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

 class MyClass : public QObject
 {
     Q_OBJECT

 public:
     MyClass(QObject *parent = 0);
     ~MyClass();

 signals:
     void mySignal();

 public slots:
     void mySlot();
 };

In addition to the signals and slots shown above, 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 the following example, 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:
     enum Priority { High, Low, VeryHigh, VeryLow };

     MyClass(QObject *parent = 0);
     ~MyClass();

     void setPriority(Priority priority);
     Priority priority() const;
 };

The Q_FLAGS() macro declares enums that are to be used as flags, i.e. OR'd together. Another macro, Q_CLASSINFO(), allows you to attach additional name/value pairs to the class's meta-object:

 class MyClass : public QObject
 {
     Q_OBJECT
     Q_CLASSINFO("Author", "Oscar Peterson")
     Q_CLASSINFO("Status", "Active")

 public:
     MyClass(QObject *parent = 0);
     ~MyClass();
 };

The output produced by 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. If you use qmake, this is done automatically. Whenever qmake is run, it parses the project's header files and generates make rules to invoke moc for those files that contain a Q_OBJECT macro.

If the class declaration 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, e.g., 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.

 moc_%.cpp: %.h
         moc $(DEFINES) $(INCPATH) $< -o $@

 moc_foo.cpp: foo.h
         moc $(DEFINES) $(INCPATH) $< -o $@

 foo.o: foo.moc

 foo.moc: foo.cpp
         moc $(DEFINES) $(INCPATH) -i $< -o $@

 #include "foo.moc"

 #ifndef Q_MOC_RUN
     ...
 #endif

 class SomeTemplate<int> : public QFrame
 {
     Q_OBJECT
     ...

 signals:
     void mySignal(int);
 };

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

 class SomeClass : public QObject
 {
     Q_OBJECT

 public slots:
     void apply(void (*apply)(List *, void *), char *); // WRONG
 };

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

 class SomeClass : public QObject
 {
     Q_OBJECT

 public slots:
     void apply(ApplyFunction, char *);
 };

 class MyClass : public QObject
 {
     Q_OBJECT

     enum Error {
         ConnectionRefused,
         RemoteHostClosed,
         UnknownError
     };

 signals:
     void stateChanged(MyClass::Error error);
 };

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

 class Whatever : public QObject
 {
     Q_OBJECT

 signals:
     void someSignal(SIGNEDNESS(int));
 };

 class A
 {
 public:
     class B
     {
         Q_OBJECT

     public slots:   // WRONG
         void b();
     };
 };