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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.
If you use qmake to create your makefiles, build rules will be included that call the moc when required, so you will not need to use the moc directly. For more background information on moc, see Why doesn't Qt use templates for signals and slots?
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.
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.
We recommend using Trolltech's makefile generation tool, qmake, for building your makefiles. This tool generates a makefile 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 $< -o $@
If you want to write portably, you can use individual rules with the following form:
moc_foo.cpp: foo.h moc $< -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.
(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 -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.
Here are the command-line options supported by the moc:
Option | Description |
---|---|
-o<file> | Write output to <file> rather than to standard output. |
-f[<file>] | Force the generation of an #include statement in the output. This is the default for header files whose extension starts with H or h. This option is useful if you have header files that do not follow the standard naming conventions. The <file> part is optional. |
-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. |
-nw | Do not generate any warnings. (Not recommended.) |
-p<path> | Makes the moc prepend <path>/ to the file name in the generated #include statement. |
-I<dir> | Add dir to the include path for header files. |
-E | Preprocess only; do not generate meta-object code. |
-D<macro>[=<def>] | Define macro, with optional definition. |
-U<macro> | Undefine macro. |
-h | Display the usage and the list of options. |
-v | Display moc's version number. |
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.
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.
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 mySignal(int); };
Another limitation is that moc does not expand macros, so you for example cannot use a macro to declare a signal/slot or use one to define a base class for a QObject.
Less importantly, 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.
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.
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.
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); };
Since moc doesn't expand #defines, 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 { Q_OBJECT signals: void someSignal(SIGNEDNESS(int)); };
A macro without parameters will work.
Here's an example of the offending construct:
class A { public: class B { Q_OBJECT public slots: // WRONG void b(); }; };
moc will complain if you try to put other constructs in the signals or slots sections of a class than signals and slots.
See also Meta-Object System, Signals and Slots, and Qt's Property System.
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