02 - Basics of Qt

Basics of Qt

Andreas Jakl
Senior Technical Consultant
Forum Nokia

20 September, 2010
v3.0.0

Contents
– Signals and Slots
– Widgets, Layouts and Styles
– Meta-Objects and Memory Management

Callbacks?
• Traditional callbacks
– Callback is pointer to a function
– Called when appropriate (event notification, ...)
• Problems
– Not type-safe: does the caller use the correct arguments?
– Less generic: callback strongly coupled to processing function.
Processing function must know which callback to call.

Signals & Slots
• Signal
– Emitted when a particular event occurs (e.g., clicked())
– Qt widgets: predefined signals
– Also create your own signals
• Slot
– Function called in response to a signal
– Qt widgets: predefined slots (e.g., quit())
– Also create your own slots
• Connection signals  slots established by developer,
handled by Qt framework

Connections
Signals
Signals connect( Object1, signal1, Object2, slot1 ) signal1
connect( Object1, signal1, Object2, slot2 )
signal1
signal2 Slots

slot1
Slots slot2
slot1
slot2
slot3

Signals
signal1 Signals

Slots
slot1 Slots
slot1
slot2
slot3

Find Signals and Slots
• Look up signals and slots of Qt
classes in the documentation

Example: Multiple Signal-Slot Connections
Restore window to
normal size

Show an about dialog
and then maximize the
window

Exit the application

Example: Connections QApplication
app

Signals
QPushButton
but1
Signals
Slots
clicked
aboutQt()
Slots quit()
QPushButton
but2
Signals
clicked QWidget
win
Slots
Signals
QPushButton
but3 clicked
Signals
clicked Slots
showNormal()
Slots showMaximized()

#include <QApplication>
#include <QPushButton>
#include <QVBoxLayout>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QWidget* win = new QWidget();
QVBoxLayout* layout = new QVBoxLayout(win);

QPushButton* but1 = new QPushButton("Normal size");
but1->resize(150, 30);
layout->addWidget(but1);

QPushButton* but2 = new QPushButton("About and Maximize");

QPushButton* but3 = new QPushButton("Exit");
aboutQt() and
QObject::connect(but1, SIGNAL(clicked()), win, SLOT(showNormal())); showMaximized()
QObject::connect(but2, SIGNAL(clicked()), &app, SLOT(aboutQt()));
executed one after
QObject::connect(but2, SIGNAL(clicked()), win, SLOT(showMaximized()));
QObject::connect(but3, SIGNAL(clicked()), &app, SLOT(quit())); another
win->show();

return app.exec();
}

Layouts
Most common layouts
– QHBoxLayout: horizontal, from left to right
(right to left for some cultures)
– QVBoxLayout: vertical, top to bottom
– QGridLayout: grid
– QFormLayout: manages input widgets and associated labels
– QStackedLayout: widgets on top of each other, switch index
of currently visible widget
• Layouts can be nested
– Add new layout to existing layout like a widget:
l->addLayout(l2)

Styles Style can be set using
-style <name> command-
line option.

Windows XP/Vista/7 and
plastique cleanlooks
Macintosh styles are only
available on native platforms
(relies on platform’s theme
engines)

It’s possible to create own
windowsvista windowsxp windows styles.

macintosh motif cde

Example 2
• Synchronize two widgets
– Changing the value in one automatically changes the other

Signals Signals
valueChanged(int) valueChanged(int)

Slots Slots
setValue(int) setValue(int)

#include <QApplication>
#include <QHBoxLayout>
#include <QSpinBox>
#include <QSlider>

int main(int argc, char *argv[]) {
win->setWindowTitle("Synchronized Widgets");

QHBoxLayout* layout = new QHBoxLayout(win);

QSpinBox* spin = new QSpinBox();
spin->setMinimum(0);
spin->setMaximum(100);
layout->addWidget(spin);

QSlider* slider = new QSlider(Qt::Horizontal);
slider->setMinimum(0);
slider->setMaximum(100);
layout->addWidget(slider);

QObject::connect( spin, SIGNAL( valueChanged(int) ),
slider, SLOT( setValue(int) ) );
QObject::connect( slider, SIGNAL( valueChanged(int) ),
spin, SLOT( setValue(int) ) );

spin->setValue(50);

win->show();
return app.exec();
}

Signal Parameters
Transmit additional information
QObject::connect( spin, SIGNAL( valueChanged(int) ),
slider, SLOT( setValue(int) ) );

– Specify type of argument(s)
– Signal has to be compatible to the slot
(same parameter type(s))

Signal Parameters
• Types not automatically casted by Qt
– Signals & slots with exactly the specified parameters have to exist
– Otherwise: no compilation error / warning
– But: warning at runtime when executing connect():
QObject::connect: Incompatible sender/receiver arguments
QSpinBox::valueChanged(QString) --> QSlider::setValue(int)

• → Signals & slots are type safe
– No connection if either sender or receiver doesn’t exist
– Or if signatures of signal and slot do not match
• connect() returns boolean value indicating success

Signal & Slot Processing
setValue(50) is called in the source code

QSpinBox emits valueChanged(int) signal with int argument of 50

signal  slot
Argument is passed to QSlider’s setValue(int) slot, sets slider value to 50

QSlider emits valueChanged(int) signal with int argument of 50

signal  slot
Argument is passed to QSpinbox’s setValue(int) slot, but value is already set to 50.
 doesn’t emit any further signal to prevent infinite recursion.

Signals & Slots
• Type safe
– Signal signature must match signature of receiving slot
– (Slot might also have shorter signature and ignore rest of the arguments)
• Loosely coupled
– Emitter doesn’t know or care which slots receive signal
– Information encapsulation
• Integrated, independent components
– Slots are normal C++ member functions
– Don’t know if signals are connected to them

Manual Signal & Slots
counter.h counter.cpp
#ifndef COUNTER_H #include "counter.h"
#define COUNTER_H
void Counter::setValue(int value)
#include <QObject> {
if (value != m_value)
class Counter : public QObject {
{ m_value = value;
Q_OBJECT emit valueChanged(value);
}
public: }
Counter() { m_value = 0; }
int value() const { return m_value; }

public slots:
void setValue(int value);

signals: Own Class that represents
void valueChanged(int newValue); behaviour of Example 2
private:
int m_value;
};

#endif // COUNTER_H

main.cpp
#include <QtGui/QApplication>
#include <QMessageBox>
#include "counter.h"

int main(int argc, char *argv[])
{

Counter a, b;
QObject::connect(&a, SIGNAL(valueChanged(int)),
&b, SLOT(setValue(int)));

a.setValue(12); // a.value() == 12, b.value() == 12

QMessageBox msgBox;
msgBox.setText("a = " + QString::number(a.value()) + ", b = " + QString::number(b.value()));
msgBox.exec();

b.setValue(48); // a.value() == 12, b.value() == 48

msgBox.setText("a = " + QString::number(a.value()) + ", b = " + QString::number(b.value()));
msgBox.exec();

app.quit();
return 1;
}

Qt Meta-Object System
• C++ extended with meta-object mechanism:
Introspection
– Obtain meta-information about QObject
subclasses at runtime
– Used for: getting list of signals & slots,
properties and text translation

QObject
• Meta information not supported by standard C++
– QObject class
• Base class for objects that use meta-object system #include <QObject>
class Counter : public QObject
– Q_OBJECT macro {
• Enables meta-object features Q_OBJECT
• Without ; [...]
};
– “moc” tool
• Parses Q_OBJECT macro
• Extends source code with additional functions (extra files)
• Removes the signals, slots and emit keywords so compiler sees standard C++
• Advantage: works with any C++ compiler

Meta-Object Features
• Features provided by meta-object code:
– Signals and slots
– metaObject() – returns associated meta-object for the class
– QMetaObject::className() – returns class name as a string,
without requiring native run-time type information (RTTI) support
through the C++ compiler
– inherits() – returns whether this instance inherits the specified QObject class
– tr() – translate strings
– setProperty() and property() – dynamically set and get properties by name

Casting
• Meta-object information can be used for casting:
if (widget->inherits("QAbstractButton")) {
QAbstractButton *button = static_cast<QAbstractButton*>(widget);
button->toggle();
}

• Similar, but less error-prone:
if (QAbstractButton *button = qobject_cast<QAbstractButton*>(widget))
button->toggle();

More on... Signals
• Emitted signal
– Slot usually executed immediately
(like normal function call)
– Code after emit keyword executed after
all slots returned
– Queued connections: slots executed later
#include <QObject>
• 1 signal  n slots class Counter : public QObject {
– Slots executed one after another (arbitrary order) Q_OBJECT
[...]
• Implementation signals:
– Automatically generated by moc void valueChanged(int newValue);
[...]
– Define in .h, do not implement in .cpp file };
– Can never have return values ( use void)
– Good style: arguments should not use special types (reusability)

More on... Slots #include <QObject>
class Counter : public QObject {
Q_OBJECT
• C++ function [...]
public slots:
– Can be called directly/normally void setValue(int value);
– If connected to and invoked by signal: works [...]
};
regardless of access level. Arbitrary class can
invoke private slot of an unrelated class instance.
– Slots can be virtual, but not static
• Overhead compared to direct call
– Does not matter in practice
– Around 10x slower than direct, non-virtual call
– Sounds a lot, but isn’t compared to for example new/delete operations
– i586-500: emit per second
2,000,000 signals  1 slot.
1,200,000 signals  2 slots.

Qt Class Hierarchy
QLayoutItem QObject QPaintDevice QString

QLayout QWidget QImage

...
... ... Many objects
QBoxLayout QDialog
(and all widgets)
derived from QObject
(directly or indirectly)
... ... ... ...

Memory Management
• Parent-child hierarchy implemented in QObject
– Initialization with pointer to parent QObject 
parent adds new object to list of its children
– Delete parent: automatically deletes all its children (recursively)
– Delete child: automatically removed from parent’s child list
–  Some memory management handled by Qt, only delete objects created
with new without parent
• Widgets
– Parent has additional meaning: child widgets shown within parent’s area

Example: Parent-Child QWidget

QVBoxLayout* layout = new QVBoxLayout(win); QVBoxLayout QPushButton
QPushButton* but = new QPushButton("Label");
layout->addWidget(but);
win->show();

– Layout is a child of parent widget
– Push button added to layout, but widget takes ownership
QPushButton* but = new QPushButton("Label", win);
win->show();

• Directly adding push button to the parent widget:
– Also displays push button, but not managed by layout manager

Example: Parent-Child II
• Helpful: print parent-child relationship
QPushButton* but = new QPushButton("Label");
layout->addWidget(but);
win->dumpObjectTree();

Console
QWidget::
QVBoxLayout::
QPushButton::

Creating Objects
• Objects inheriting from QObject are allocated on the heap using new
– If a parent object is assigned, it takes ownership of the newly created object – and
eventually calls delete
QLabel *label = new QLabel("Some Text", parent);
• Objects not inheriting QObject are allocated on the stack, not the heap
QStringList list;
QColor color;

• Exceptions
– QFile and QApplication (inheriting QObject) are usually allocated on the stack
– Modal dialogs are often allocated on the stack, too

02 - Basics of Qt

More Related Content

What's hot (20)

Viewers also liked (9)

Similar to 02 - Basics of Qt (20)

More from Andreas Jakl (20)

Recently uploaded (20)

02 - Basics of Qt