Home > Articles > Programming > C/C++

  • Print
  • + Share This
This chapter is from the book

9.3 QApplication and the Event Loop

Interactive Qt applications with GUI have a different control flow from console applications and filter applications2 because they are event-based, and often multithreaded. Objects are frequently sending messages to each other, making a linear hand-trace through the code rather difficult.

The Qt class QEvent encapsulates the notion of an event. QEvent is the base class for several specific event classes such as QActionEvent, QFileOpenEvent, QHoverEvent, QInputEvent, QMouseEvent, and so forth. QEvent objects can be created by the window system in response to actions of the user (e.g., QMouseEvent) at specified time intervals (QTimerEvent) or explicitly by an application program. The type() member function returns an enum that has nearly a hundred specific values that can identify the particular kind of event.

A typical Qt program creates objects, connects them, and then tells the application to exec(). At that point, the objects can send information to each other in a variety of ways. QWidgets send QEvents to other QObjects in response to user actions such as mouse clicks and keyboard events. A widget can also respond to events from the window manager such as repaints, resizes, or close events. Furthermore, QObjects can transmit information to one another by means of signals and slots.

Each QWidget can be specialized to handle keyboard and mouse events in its own way. Some widgets will emit a signal in response to receiving an event.

An event loop is a program structure that permits events to be prioritized, enqueued, and dispatched to objects. Writing an event-based application means implementing a passive interface of functions that only get called in response to certain events. The event loop generally continues running until a terminating event occurs (e.g., the user clicks on the QUIT button).

Example 9.5 shows a simple application that initiates the event loop by calling exec().

Example 9.5. src/eventloop/main.cpp

[ . . . . ]

int main(int argc, char * argv[]) {
    QApplication myapp(argc, argv);          <-- 1

    QWidget rootWidget;

    rootWidget.show();                       <-- 2
    return myapp.exec();                     <-- 3

  • (1)Every GUI, multithreaded, or event-driven Qt Application must have a QApplication object defined at the top of main().
  • (2)Show our widget on the screen.
  • (3)Enter the event loop.

When we run this app, we first see a widget on the screen as shown in the following figure.

We can type in the QTextEdit on the screen, or click on the Shout button. When Shout is clicked, a widget is superimposed on our original widget as shown in the next figure.

This message dialog knows how to self-destruct, because it has its own buttons and actions.

9.3.1 Layouts: A First Look

Whenever more than a single widget needs to be displayed, they must be arranged in some form of a layout (see Section 11.5). Layouts are derived from the abstract base class, QLayout, which is derived from QObject. Layouts are geometry managers that fit into the composition hierarchy of a graphical interface. Typically, we start with a widget that will contain all of the parts of our graphical construction. We select one or more suitable layouts to be children of our main widget (or of one another) and then we add widgets to the layouts.

In Example 9.6, we are laying out widgets in a vertical fashion with QVBoxLayout.

Example 9.6. src/eventloop/main.cpp

[ . . . . ]

QWidget* setGui(QWidget *box) {
    QLayout* layout = new QVBoxLayout;
    box->setLayout(layout);                   <-- 1

    QTextEdit *te = new QTextEdit;            <-- 2
    layout->addWidget(te);                    <-- 3
    te->setHtml("Some <b>text</b> in the <tt>QTextEdit</tt>"
         "edit window <i>please</i>?");

    QPushButton *quitButton=new QPushButton("Quit");

    QPushButton *shoutButton = new QPushButton("Shout");

    Messager *msgr = new Messager("This dialog will self-
    destruct.", box);

    QObject::connect(quitButton, SIGNAL(clicked()),
                 qApp, SLOT(quit()));          <-- 4

    qApp->connect(shoutButton, SIGNAL(clicked()), msgr, SLOT(shout()));
    return box;

  • (1)box is the parent of layout.
  • (2)This is the window for qDebug messages.
  • (3)te is the child of layout.
  • (4)qApp is a global variable that points to the current QApplication object.

The widgets are arranged vertically in this layout, from top to bottom, in the order that they were added to the layout.

9.3.2 Connecting to Slots

In Example 9.7, we saw the following connections established:

QObject::connect(quitButton, SIGNAL(clicked()), qApp, SLOT(quit()));
qApp->connect(shoutButton, SIGNAL(clicked()), msgr, SLOT(shout()));

connect() is actually a static member of QObject and can be called with any QObject or, as we showed, by means of its class scope resolution operator. qApp is a global pointer that points to the currently running QApplication.

The second connect goes to a slot that we declare in Example 9.7.

Example 9.7. src/eventloop/messager.h

#ifndef MESSAGER_H
#define MESSAGER_H

#include <QObject>
#include <QString>
#include <QErrorMessage>
class Messager : public QObject {
    Messager (QString msg, QWidget* parent=0);

 public slots:
    void shout();

    QWidget* m_Parent;
    QErrorMessage* message;


Declaring a member function to be a slot enables it to be connected to a signal so that it can be called passively in response to some event. For its definition, shown in Example 9.8, we have kept things quite simple: the shout() function simply pops up a message box on the screen.

Example 9.8. src/eventloop/messager.cpp

#include "messager.h"

Messager::Messager(QString msg, QWidget* parent)
        : m_Parent(parent) {
    message = new QErrorMessage(parent);

void Messager::shout() {

9.3.3 Signals and Slots

When the main thread of a C++ program calls qApp->exec(), it enters into an event loop, where messages are handled and dispatched. While qApp is executing its event loop, it is possible for QObjects to send messages to one another.

A signal is a message that is presented in a class definition like a void function declaration. It has a parameter list but no function body. A signal is part of the interface of a class. It looks like a function but it cannot be called—it must be emitted by an object of that class. A signal is implicitly protected, and so are all the identifiers that follow it in the class definition until another access specifier appears.

A slot is a void member function. It can be called as a normal member function.

A signal of one object can be connected to the slots of one or more3 other objects, provided the objects exist and the parameter lists are assignment compatible4 from the signal to the slot. The syntax of the connect statement is:

bool QObject::connect(senderqobjptr,

Any QObject that has a signal can emit that signal. This will result in an indirect call to all connected slots.

QWidgets already emit signals in response to events, so you only need to make the proper connections to receive those signals. Arguments passed in the emit statement are accessible as parameters in the slot function, similar to a function call, except that the call is indirect. The argument list is a way to transmit information from one object to another.

Example 9.9 defines a class that uses signals and slots to transmit a single int parameter.

Example 9.9. src/widgets/sliderlcd/sliderlcd.h

[ . . . . ]
class QSlider;
class QLCDNumber;
class LogWindow;
class QErrorMessage;

class SliderLCD : public QMainWindow {
    SliderLCD(int minval = -273, int maxval = 360);
    void initSliderLCD();

 public slots:
    void checkValue(int newValue);
    void showMessage();

    void toomuch();
    int m_Minval, m_Maxval;
    LogWindow* m_LogWin;
    QErrorMessage *m_ErrorMessage;
    QLCDNumber* m_LCD;
    QSlider* m_Slider;
[ . . . . ]

In Example 9.10, we can see how the widgets are initially created and connected.

Example 9.10. src/widgets/sliderlcd/sliderlcd.cpp

[ . . . . ]

SliderLCD::SliderLCD(int min, int max) : m_Minval(min),
m_Maxval(max) {

void SliderLCD::initSliderLCD() {
    m_LogWin = new LogWindow();                                    <-- 1
    QDockWidget *logDock = new QDockWidget("Debug Log");
    logDock->setFeatures(0);                                       <-- 2

    m_LCD = new QLCDNumber();
    QDockWidget *lcdDock = new QDockWidget("LCD");
    lcdDock->setFeatures(QDockWidget::DockWidgetClosable);         <-- 3
    addDockWidget(Qt::LeftDockWidgetArea, lcdDock);

    m_Slider = new QSlider( Qt::Horizontal);
    QDockWidget* sliderDock = new QDockWidget("How cold is it
    /* Can be moved between doc areas */
    addDockWidget(Qt::BottomDockWidgetArea, sliderDock);

    m_Slider->setRange(m_Minval, m_Maxval);
    m_Slider->setSingleStep(1);                                    <-- 4
    m_Slider->setPageStep(20);                                     <-- 5
    m_Slider->setFocus();                                          <-- 6

    connect(m_Slider, SIGNAL(valueChanged(int)),  /*SliderLCD is a
    QObject so
        connect does not need scope resolution. */
            this, SLOT(checkValue(int)));
    connect(m_Slider, SIGNAL(valueChanged(int)),
            m_LCD, SLOT(display(int)));

    connect(this, SIGNAL(toomuch()),
            this, SLOT(showMessage()));                           <-- 7
    m_ErrorMessage = NULL;

  • (1)a class defined in the utils library
  • (2)cannot be closed, moved, or floated
  • (3)can be closed
  • (4)Step each time left or right arrow key is pressed.
  • (5)Step each time PageUp/PageDown key is pressed.
  • (6)Give the slider focus.
  • (7)Normally there is no point in connecting a signal to a slot on the same object, but we do it for demonstration purposes.

Only the argument types belong in the connect statement; for example, the following is not legal:

connect( button, SIGNAL(valueChanged(int)), lcd, SLOT(setValue(3)))

Example 9.11 defines the two slots, one of which conditionally emits another signal.

Example 9.11. src/widgets/sliderlcd/sliderlcd.cpp

[ . . . . ]

void SliderLCD::checkValue(int newValue) {
    if (newValue> 120) {
        emit toomuch();                                 <-- 1

/* This slot is called indirectly via emit because
   of the connect */
void SliderLCD::showMessage() {
    if (m_ErrorMessage == NULL) {
        m_ErrorMessage = new QErrorMessage(this);
    if (!m_ErrorMessage->isVisible()) {
        QString message("Too hot outside! Stay in. ");
        m_ErrorMessage->showMessage(message);          <-- 2

  • (1)Emit a signal to anyone interested.
  • (2)This is a direct call to a slot. It's a member function.

Example 9.12 contains client code to test this class.

Example 9.12. src/widgets/sliderlcd/sliderlcd-demo.cpp

#include "sliderlcd.h"
#include <QApplication>
#include <QDebug>

int main(int argc, char ** argv) {
    QApplication app(argc, argv);
    SliderLCD slcd;
    qDebug() << QString("This is a debug message.");
    return app.exec();

Whenever the slider produces a new value, that value is transmitted as an argument from the valueChanged(int) signal to the display(int) slot of the lcd.

Exercises: Signals and Slots

  1. Modify the sliderlcd program as follows:
    • Make the lcd display show the temperatures as hexadecimal integers.
    • Make the lcd display characters have a different ("flat") style.
    • Give the slider a vertical orientation.
    • Give the slider and the lcd display more interesting colors.
    • Add a push button that the user can click to switch the lcd display from decimal mode to hexadecimal mode.
    • Make the push button into a toggle that allows the user to switch back and forth between decimal and hexadecimal modes.
  2. Write an application, similar to the one in Section 9.3, but that has four buttons. The first one, labeled Advice, should be connected to a slot that randomly selects a piece of text (such as a fortune cookie) and displays it in the QTextEdit window. The second one, labeled Weather, randomly selects a sentence about the weather and displays it in the QTextEdit window. The third one, labeled Next Meeting, pops up a message dialog with a randomly generated (fictitious) meeting time and descriptive message in it. The fourth one, labeled Quit, terminates the program. Use signals and slots to connect the button clicks with the appropriate functions.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020