8.4. Q_OBJECT and moc: A checklist

8.4. `Q_OBJECT` and `moc`: A checklist
	Chapter 8. QObject, QApplication, Signals, and Slots

8.4. `Q_OBJECT` and `moc`: A checklist

[ fromfile: qobject.xml id: moc ]

QObject supports features not normally available in C++ objects.
- Signals and Slots (Section 8.5)
- MetaObjects, MetaProperties, MetaMethods (Chapter 12)
- qobject_cast (Section 12.2)

Some of these features are only possible through the use of generated code.
The Meta Object Compiler, moc, generates additional functions for each QObject-derived class that uses the Q_OBJECT macro.

This means that some errors from the compiler/linker may be confuscated^[40] when moc cannot find or process a class in the project.
Guidelines which help avoid common errors:

Each class definition should go in its own .h file.
Its implementation should go in a corresponding .cpp file.
The header file should be "wrapped" (e.g., with #ifndef) to avoid multiple inclusion.
Each .cpp file should be listed in the SOURCES variable of the project file; otherwise it will not be compiled.
Each header file should be listed in the HEADERS variable of the .pro file. Without this, moc will not preprocess the file.
The Q_OBJECT macro must appear inside the class definition of each QObject derived header file so that moc knows to generate code for it.

Multiple Inheritance and QObject

	Multiple Inheritance and QObject
Because each Q_OBJECT macro generates code, it needs to be preprocessed by `moc`. `moc` works under the assumption that you are only deriving from `QObject` once and, further, that `QObject` is the first base class in the list of base classes. If you accidentally inherit from `QObject` more than once, or if it is not the first base class in the inheritance list, you may produce strange errors in the `moc`-generated code.

Because each Q_OBJECT macro generates code, it needs to be preprocessed by moc.
moc works under the assumption that you are only deriving from QObject once and, further, that QObject is the first base class in the list of base classes.
If you accidentally inherit from QObject more than once, or if it is not the first base class in the inheritance list, you may produce strange errors in the moc-generated code.

Undefined reference to vtable

	Undefined reference to vtable
If you define a `QObject`-derived class, build an application, realize that you need to add the `Q_OBJECT` macro to the class definition, and add it after the project was built with an old `Makefile`, you must then rerun `qmake` to update the `Makefile`. `make` is not smart enough to add the `moc` step on such files to your `Makefile` otherwise. A "clean rebuild" does not fix this problem usually. This is an issue that often causes headaches for inexperienced Qt developers. For more information about this error message, see Section C.3.1

If you define a QObject-derived class, build an application, realize that you need to add the Q_OBJECT macro to the class definition, and add it after the project was built with an old Makefile, you must then rerun qmake to update the Makefile.

make is not smart enough to add the moc step on such files to your Makefile otherwise. A "clean rebuild" does not fix this problem usually. This is an issue that often causes headaches for inexperienced Qt developers. For more information about this error message, see Section C.3.1

^[40]confusing + obfuscated


8.3. QApplication and the Event Loop		8.5. Signals and Slots