Using the Meta-Object Compiler (moc)

The Meta-Object Compiler, moc, is the program that handles Qt's C++ extensions.

The moc tool reads a C++ header file. If it finds one or more class declarations that contain the Q_OBJECT macro, it produces a C++ source file containing the meta-object code for those classes. Among other things, meta-object code is required for the signals and slots mechanism, the run-time type information, and the dynamic property system.

The C++ source file generated by moc must be compiled and linked with the implementation of the class.

Both qmake and CMake generate makefiles with build rules that will invoke moc accordingly, so you will not need to use the moc directly. qmake will add these build rules by default, whereas with CMake, you can use the AUTOMOC property to handle moc automatically. For more background information on moc, see Why Does Qt Use Moc for Signals and Slots?

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_ENUM() 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)

 public:
     enum Priority { High, Low, VeryHigh, VeryLow };
     Q_ENUM(Priority)

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

     void setPriority(Priority priority) { m_priority = priority; }
     Priority priority() const { return m_priority; }

 private:
     Priority m_priority;
 };

The Q_FLAG() 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. Similarly, when setting AUTOMOC to ON, CMake will scan the header and source files at build time and invoke moc accordingly.

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.

Writing Make Rules for Invoking moc

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, make can take care of running moc when necessary and handling the moc output.

You can use CMake or qmake to generate makefiles that does all the necessary moc handling.

If you want to create 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 $(DEFINES) $(INCPATH) $< -o $@

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

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

You must also remember to add moc_foo.cpp to your SOURCES (substitute your favorite name) variable and moc_foo.o or moc_foo.obj to your OBJECTS variable.

Both examples assume that $(DEFINES) and $(INCPATH) expand to the define and include path options that are passed to the C++ compiler. These are required by moc to preprocess the source files.

While we prefer to name our C++ source files .cpp, you can use any other extension, such as .C, .cc, .CC, .cxx, and .c++, if you prefer.

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

 foo.o: foo.moc

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

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

 #include "foo.moc"

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

Command-Line Options

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

You can explicitly tell the moc not to parse parts of a header file. moc defines the preprocessor symbol Q_MOC_RUN. Any code surrounded by

 #ifndef Q_MOC_RUN
     ...
 #endif

is skipped by the moc.

Diagnostics

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 vtable, 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. If you use qmake, try rerunning it to update your makefile. This should do the trick.

Build Systems

Including header moc files

qmake and CMake behave differently with regards to including header moc files.

To illustrate this with an example, suppose that you have two headers with corresponding source files: a.h, a.cpp, b.h, and b.cpp. Each header has a Q_OBJECT macro:

 // a.h
 class A : public QObject
 {
     Q_OBJECT

     public:
         // ...
 };

 // a.cpp
 #include "a.h"

 // ...

 #include "moc_a.cpp"

 // b.h
 class B : public QObject
 {
     Q_OBJECT

     public:
         // ...
 };

 // b.cpp
 #include "b.h"

 // ...

 #include "moc_b.cpp"

With qmake, if you don't include the moc-generated file (moc_a.cpp/moc_b.cpp), a.cpp, b.cpp, moc_a.cpp, and moc_b.cpp will be compiled separately. This can result in slower builds. If you include the moc generated files, only a.cpp and b.cpp will need to be compiled, as the moc generated code is included in those files.

With CMake, if you don't include the files, an single additional file is generated by moc (let's call it cmake.cpp for the sake of the example). cmake.cpp would include both moc_a.cpp and moc_b.cpp. Including the moc-generated file is still allowed with CMake, but it's not necessary.

For more information on CMake's moc support regarding this topic, see Including header moc files in sources.

Limitations

moc does not handle all of C++. The main problem is that class templates cannot have the Q_OBJECT macro. Here is an example:

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

 signals:
     void mySignal(int);
 };

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.

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.

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

Virtual inheritance with QObject is not supported.

Function Pointers Cannot Be Signal or Slot Parameters

In most cases where you would consider using function pointers as signal or slot parameters, 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 (*apply)(List *, void *), char *); // WRONG
 };

You can work around this restriction like this:

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

 class SomeClass : public QObject
 {
     Q_OBJECT

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

It may sometimes be even better to replace the function pointer with inheritance and virtual functions.

Enums and Typedefs Must Be Fully Qualified for Signal and Slot Parameters

When checking the signatures of its arguments, QObject::connect() compares the data types literally. Thus, Alignment and Qt::Alignment are treated as two distinct types. To work around this limitation, make sure to fully qualify the data types when declaring signals and slots, and when establishing connections. For example:

 class MyClass : public QObject
 {
     Q_OBJECT

     enum Error {
         ConnectionRefused,
         RemoteHostClosed,
         UnknownError
     };

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

Nested Classes Cannot Have Signals or Slots

Here's an example of the offending construct:

 class A
 {
 public:
     class B
     {
         Q_OBJECT

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

Signal/Slot return types cannot be references

Signals and slots can have return types, but signals or slots returning references will be treated as returning void.

Only Signals and Slots May Appear in the signals and slots Sections of a Class

moc will complain if you try to put other constructs in the signals or slots sections of a class than signals and slots.