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Inside Qt meta Object

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Created: mahbub_s60 (28 Dec 2009)
Last edited: hamishwillee (11 Oct 2012)


In this article we will discuss how and why we use meta-object in Qt.

The word meta indicates that the word prefixed is about itself. So a meta-object is an object describing the object. QT Meta-Object Compiler, moc, is the program that handles Qt's C++ extensions.The moc reads C++ source files, if it finds one or more class declarations that contain the Q_OBJECT macro, it produces another C++ source file which contains the meta object code for those classes and those are automatically included by build system.

QMetaObject Class

A meta-object contains information about a class that inherits QObject, e.g. class name, super class name, properties, signals and slots. Every QObject subclass that contains the Q_OBJECT macro will have a meta-object. Following code declares MyQTClass which has a property declared by Q_PROPERTY macro. This will allow us to update the property in several ways.

 class MyQTClass : public QObject
Q_PROPERTY(int Myage READ Age WRITE setAge)
void SomeFunction();
int Age();
void setAge(int aAge);
QString myName;
int Myage;
bool abool;
// test code
MyQTClass *myinstance = new MyQTClass;
QObject *object = myinstance;
myinstance->setAge(100); // using setter function
myinstance->setProperty("Myage", 100); //using property
int retage =myinstance->Age(); // retage should be 100
//Setting property
QVariant v(123);
bool b = object->setProperty("Myage", v); // b is true since we have a Myage property
b = object->setProperty("aVariable", v); // b is not true
QVariant test = object->property("Myage");
b = test.isValid(); // b is true
QVariant wrong = object->property("aVariabdfhdfgle");
b = wrong.isValid(); // b is not true
const QMetaObject *metaobject = object->metaObject();
QString classname(metaobject->className()); // classname shoudl be "MyQTClass"
int count = metaobject->propertyCount(); // number of Property
for (int i=0; i<count; ++i)
QMetaProperty metaproperty = metaobject->property(i);
const char *name =; // property name should come from MyQTClass + QObject
QString propertyName(name);

In the above code,at the very top of the class declaration we find the Q_OBJECT macro. It is important that this macro appears first in the body of the class declaration because it marks the class as a class that needs a meta-object. In the case of Qt, meta-objects are instances of the class. It means that the above code goes into a file called MyQTClass.h. The implementation goes into MyQTClass.cpp, and the moc generates another C++ file from the header file called moc_MyQTClass.cpp. The contents from the generated file can change between Qt versions.

Example Applications

The full source code presented in this article is available here

This page was last modified on 11 October 2012, at 01:17.
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