This document summarizes a webinar on Qt's object model. It discusses how Qt's classes can be divided into identity types like QObject that derive from C++ and extend it with features using a meta-object system, and value types like QVariant that use copy-on-write. It covers how QObject is the heart of Qt's object model and includes features like memory management, object properties, introspection, signals and slots, and event handling. It also discusses QVariant's role as a union for common value types, and how to make custom classes use QVariants and properties.

1. © Integrated Computer Solutions, Inc. All Rights Reserved
QVariant, QObject
June 26, 2018
Qt for Beginners Summer Webinar Series
Part II
Copyright 2018, Integrated Computers Solutions, Inc.
This work may not be reproduced in whole or in part without the express written consent of Integrated
Computer Solutions, Inc.

2. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Qt’s Object Model
• Qt’s 1,500 classes can be divided into two groups
• Identity types
• Value types
• Identity types
• derive from QObject
• extend C++ with many dynamic features using a meta‐object system
• cannot be copied as the copy constructor and assignment operator equal to
delete
• QWidget, QWindow, QApplication, QEventLoop, QThread, QFile, QTcpSocket
• Value types are standard C++ classes
• QColor, QEvent, QDataStream, QMetaType
• ~100 value types use copy‐on‐write pattern – implicitly shared
• QString, QByteArray, QList, QVector, QHash, QCache, QDir, QPixmap, QImage,
QBrush, QPen
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3. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Qt's C++ Object Model - QObject
• QObject is the heart of Qt's object model
• Include these features:
• Memory management
• Object properties
• Introspection
• Signals and Slots
• Event handling
• QObject has no visual representation
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4. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Let’s Start With Some Code!
const QMetaObject *metaObj = object->metaObject();
const QString className = metaObj->className();
const int propertyCount = metaObj->propertyCount();
for (int i=0; i<propertyCount; ++i) {
const QMetaProperty metaProp = metaObj->property(i);
const QString typeName = metaProp.typeName();
const QString propertyName = metaProp.name();
const QVariant value = object-
>property(propertyName);
}
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5. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Meet QVariant
• QVariant
• Union for common Qt "value types" (copyable, assignable)
• Supports implicit sharing (fast copying)
• Supports user types
• A generic data object
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6. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Let’s Start With Some Code!
const QMetaObject *metaObj = object->metaObject();
const QString className = metaObj->className();
const int propertyCount = metaObj->propertyCount();
for (int i=0; i<propertyCount; ++i) {
const QMetaProperty metaProp = metaObj->property(i);
const QString typeName = metaProp.typeName();
const QString propertyName = metaProp.name();
const QVariant value = object-
>property(propertyName);
}
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7. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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QVariant
• For QtCore types
QVariant variant(42);
int value = variant.toInt(); // read back as integer
QString text = variant.toString(); // read back as string
qDebug() << variant.typeName(); // int
• For non-core and custom types:
QVariant variant = QVariant::fromValue(QColor(Qt::red));
QColor color = variant.value<QColor>(); // read back
qDebug() << variant.typeName(); // "QColor"
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8. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Making Your Custom Classes
QVariants
#include <QMetaType>
class Contact
{
public:
void setName(const QString & name);
QString name() const;
...
};
Q_DECLARE_METATYPE(Contact);
● Type must support default construction, copy and assignment.
● Q_DECLARE_METATYPE should be after class definition in header file.
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9. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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qRegisterMetaType
• Q_DECLARE_METATYPE makes the type known to
QVariant and template based functions
• qRegisterMetaType will register the type
const int typeId = qRegisterMetaType<Contact>();
• Type can be used with property system and as signal
parameters in queued connections
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10. © 2018 Integrated Computer Solutions, Inc., The Qt Company
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Properties
• Q_PROPERTY is a macro:
Q_PROPERTY(type name READ getFunction
[WRITE setFunction] [RESET resetFunction]
[NOTIFY notifySignal])
• Property access methods:
QVariant property(const char* name) const;
void setProperty(const char* name,const QVariant& value);
• Note:
• Q_OBJECT macro is required for Q_PROPERTY to work
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Connecting Signals to Slots
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QObject Tree
• QObjects organize themselves in object trees
• Based on parent‐child relationship
• QObject(QObject *parent = Q_NULLPTR)
• Parent adds object to list of children
• const QObjectList &children();
• QList<QWidget> findChildren()
• Parent owns children
• Construction/Destruction
• Tree can be constructed in any order
• Tree can be destroyed in any order
• if object has parent: object first removed from parent
• if object has children: deletes each child first
• No object is deleted twice
• Note: Parent-child relationship is NOT inheritance
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Further Reading
- The Qt Property System
http://doc.qt.io/qt-5/properties.html
- How Signals and Slots Work
https://woboq.com/blog/how-qt-signals-slots-work.html
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In the Next Webinar
• QML — July 19
• QWidgets — August 2
• Model/View — August 16
So Tune in, Same Qt Channel, Same Qt Time!
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© Integrated Computer Solutions
This document is based on a Creative Commons training course, written by a
community of Certified Qt Training Partners. It is provided "as is" with no
warranty whatsoever.
Prior to 2012, parts of this document were written or further developed by
people who are or were employed by Trolltech, KDAB, Nokia, ICS, and possibly
others.
Since 2013, this particular fork is maintained by ICS.
Qt is developed by The Qt Company (a subsidiary of Digia) together
with the Qt Project under open governance.