DataObject: An Extension of QObject

DataObject An Extension of QObject

We have developed an extension to QObject named DataObject that we can use as a base class for other data types that require any of the following features:

  • A virtual interface for obtaining properties, MetaProperties, and metaClass information
  • Convenience functions for copying and comparing property values of DataObjects
  • A toString() function that returns a presentation of all the properties of the object in XML format

This improved interface makes DataObject a Façade for QObject.

DataObject, shown in Figure 15.3, is used in several forthcoming examples to demonstrate various design patterns.

Figure 15.3. DataObject

DataObject is Qt-property aware and takes advantage of this interface for reading and writing properties of arbitrary QObjects, as shown in Example 15.7.

Example 15.7. src/libs/dataobjects/dataobject.cpp

[ . . . . ]

bool DataObject::readFrom(const QObject& source) {
 bool retval = true;
 const QMetaObject* meta = source.metaObject();
 int count = meta->propertyCount();
 for (int i=0; Iproperty(i);
 const char* pname =;
 if (metap.isWritable()) {
 retval = setProperty(pname,
 && retval;
 return retval;

[ . . . . ]

bool DataObject::writeTo(QObject& dest) const {
 bool result = true;
 foreach (QString propname, propertyNames()) {
 if (metaProperty(propname).isWritable()) {
 QVariant val = property(propname);
 result = dest.setProperty(propname.toAscii(), val)
 && result;
 return result;

Exercises: Meta Objects, Properties, and Reflective Programming


Many people have several pets, each of which require periodic maintenance (visits to the veterinarian, immunizations, vitamins, grooming, etc.).

  • Derive a class named Pet from DataObject. Supply this class with appropriate attributes that can uniquely describe the pet (type, breed, name, ID, birthday, etc.). For each attribute, you will need a getter and a setter so that you can set up a Q_PROPERTY.
  • Derive another class from DataObject that you can call Maintenance (if you can't think of a better name). This class should have attributes that uniquely describe a particular maintenance event (type of event, date, cost, etc.). Maintenance objects will be stored as children of Pet objects.
  • Derive a PetList class from DataObject.Pet objects will be children of PetList.
  • Serialization is facilitated by the DataObject::toString() function. Find out how to use this for your PetWriter class and how to reverse the action in your PetReader class.
  • Design a nice GUI for these classes so that the user can enter, store, and display data.

Do the exercise in Section 25.1.

Property Containers PropsMap

Part I: Introduction to C++ and Qt 4

C++ Introduction


Introduction to Qt



Inheritance and Polymorphism

Part II: Higher-Level Programming


Introduction to Design Patterns


Generics and Containers

Qt GUI Widgets


Validation and Regular Expressions

Parsing XML

Meta Objects, Properties, and Reflective Programming

More Design Patterns

Models and Views

Qt SQL Classes

Part III: C++ Language Reference

Types and Expressions

Scope and Storage Class

Statements and Control Structures

Memory Access

Chapter Summary

Inheritance in Detail

Miscellaneous Topics

Part IV: Programming Assignments

MP3 Jukebox Assignments

Part V: Appendices

MP3 Jukebox Assignments


MP3 Jukebox Assignments

An Introduction to Design Patterns in C++ with Qt 4
An Introduction to Design Patterns in C++ with Qt 4
ISBN: 0131879057
EAN: 2147483647
Year: 2004
Pages: 268 © 2008-2020.
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