Goals

  • We teach basic Qt knowledge by implementing a simple Notepad application using C++ and the QtWidgets module.
    • The application is a small text editor which allows you to…
      • create a text file,
      • save it,
      • print it,
      • or reopen and edit it again.
      • set the font to be used.

Notepad Example

notepad_example

Running the example

  • To run the example from Qt Creator, open the Welcome mode and select the example from Examples.

Creating the Notepad Project

  • Setting up a new project in Qt Creator is aided by a wizard that guides you step-by-step through the project creation process.
    • The wizard prompts you to enter the settings needed for that particular type of project and creates the project for you.

qtcreator_new_project qtcreator_new_project_cmake

  • To create the Notepad project, select File > New Project > Application > Qt Widgets Application > Choose…, and follow the instructions of the wizard.
  • In the Class Information dialog, type Notepad as the class name and select QMainWindow as the base class.

qtcreator_class_information

  • The Qt Widgets Application wizard creates a project that contains a main source file and a set of files that specify a user interface (Notepad widget):

    file name description
    CMakeLists.txt the project file
    main.cpp the main source file for the application
    notepad.cpp the source file of the notepad class of the Notepad widget
    notepad.h the header file of the notepad class for the Notepad widget
    notepad.ui the UI form for the Notepad widget

  • The files come with the necessary boiler plate code for you to be able to build and run the project.

Main Source File

  • The wizard generates the following code in th main.cpp file:
#include "notepad.h"    // 1.
#include <QApplication>

int main(int argc, char *argv[])    // 2.
{
    QApplication a(argc, argv);   // 3.
    Notepad w;    // 4.
    w.show();   // 5.
    return a.exec();    // 6.
}
  1. Include the header files for the Notepad widget and QApplication.
  2. Defines the main function that is the entry point for all C and C++ based applications.
  3. Creates a QApplication object.
    • This object manages application-wide resources and is necessary to run any Qt program that uses Qt Widgets.
    • It constructs an application object with argc command line arguments run in argv.
    • (For GUI applications that do not use Qt Widgets, you can use QGuiApplication instead.)
  4. Creates the Notepad object.
    • This is the object for which the wizard created the class and the UI file.
    • The UI contains visual elements that are called widgets in Qt.
      • Examples of widgets are
        • text edits,
        • scroll bars,
        • labels,
        • and radio buttons.
      • A widget can also be a container for other widgets;
        • a dialog
        • or a main application window,
  5. Shows the Notepad widget on the screen in its own window.
    • Widgets can also function as containers.
    • e.g. QMainWindow which often contains several types of widgets.
      • Widgets are not visible by default;
      • the function show() makes the widget visible.
  6. Makes the QApplication enter its event loop.
    • When a Qt application is running, events are generated and sent to the widgets of the application.
    • e.g. mouse presses and key strokes

Designing a UI

  • The wizard generates a UI definition in XML format: notepad.ui.
    • When you open the notepad.ui file in Qt Creator, it automatically opens in the integrated Qt Designer.
  • When you build the application, Qt Creator launches the Qt User Interface Compiler (uic) that reads the .ui file and creates a corresponding C++ header file, ui_notepad.h.

Using Qt Designer

  • The wizard creates an application that uses a QMainWindow.
    • It has its own layout to which you can add
      • a menu bar,
      • dock widgets,
      • toolbars,
      • a status bar.
    • The center area can be occupied by any kind of widget.
      • The wizard places the Notepad widget there.
  • To add widgets in Qt Designer:
    1. In the Qt Creator Edit mode, double-click the notepad.ui file in the Projects view to launch the file in the integrated Qt Designer.
    2. Drag and drop widgets Text Edit (QTextEdit) to the form.
    3. Press Ctrl+A (or Cmd+A) to select the widgets and click Lay out Vertically (or press Ctrl+L) to apply a vertical layout (QVBoxLayout).
    4. Press Ctrl+S (or Cmd+S) to save your changes.
  • The UI now looks as follows in Qt Designer:

qtcreator_qtdesigner_ui

  • You can view the generated XML file in the code editor:
<?xml version="1.0" encoding="UTF-8"?>    <!-- 1. -->
<ui version="4.0">    <!-- 2. -->
 <class>Notepad</class>
 <widget class="QMainWindow" name="Notepad">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Notepad</string>
  </property>
  <widget class="QWidget" name="centralwidget"/>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>20</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>
  1. Contains the XML declaration, which specifies the XML version and character encoding used in the document:
  2. The rest of the file specifies an ui element that defines a Notepad widget:
  • The UI file is used together with the header and source file of the Notepad class.

Notepad Header File

  • The wizard generated a header file for the Notepad class that has the necessary #includes, a constructor, a destructor, and the Ui object.
#ifndef NOTEPAD_H
#define NOTEPAD_H

#include <QMainWindow>    // 1.

QT_BEGIN_NAMESPACE
namespace Ui { class Notepad; }   / /2.
QT_END_NAMESPACE

class Notepad : public QMainWindow    // 3.
{
    Q_OBJECT

public:
    Notepad(QWidget *parent = nullptr);   // 4.
    ~Notepad();   // 5.

private:
    Ui::Notepad *ui;    // 6.
};
#endif // NOTEPAD_H
  1. Includes QMainWindow that provides a main application window.
  2. Declares the Notepad class in the Ui namespace, which is the standard namespace for the UI classes generated from .ui files by the uic tool.
  3. The class declaration contains the Q_OBJECT macro. It must come first in the class definition, and declares our class as a QObject.
    • Naturally, it must also inherit from QObject.
    • A QObject adds several abilities to a normal C++ class.
    • Notably, the class name and slot names can be queried at runtime.
    • It is also possible to query a slot’s parameter types and invoke it.
  4. Declares a constructor that has a default argument called parent.
    • The value nullptr indicates that the widget has no parent (it is a top-level widget).
  5. Declares a virtual destructor to free the resources that were acquired by the object during its life-cycle.
    • According to the C++ naming convention, destructors have the same name as the class they are associated with, prefixed with a tilde(~).
    • In QObject , destructors are virtual to ensure that the destructors of derived classes are invoked properly when an object is deleted through a pointer-to-base-class.
  6. Declares a member variable which is a pointer to the Notepad UI class.
    • A member variable is associated with a specific class, and accessible for all its methods.

Notepad Source File

  • The source file that the wizard generated for the Notepad class looks as follows:
#include "notepad.h"    // 1.
#include "./ui_notepad.h"

Notepad::Notepad(QWidget *parent)   // 2.
    : QMainWindow(parent)   // 3.
    , ui(new Ui::Notepad)   // 4.
{
    ui->setupUi(this);    // 5.
}

Notepad::~Notepad()   // 6.
{
    delete ui;
}
  1. Includes the Notepad class header file that was generated by the wizard and the UI header file that was generated by the uic tool.
  2. Defines the Notepad constructor.
  3. Calls the QMainWindow constructor, which is the base class for the Notepad class.
  4. Creates the UI class instance and assigns it to the ui member.
  5. Sets up the UI.
  6. In the destructor, we delete the ui.

Project File

  • The wizard generates the following project file, CMakeLists.txt, for us.
cmake_minimum_required(VERSION 3.5)

project(NotepadEx VERSION 0.1 LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(QT NAMES Qt6 Qt5 COMPONENTS Widgets REQUIRED)
find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)

set(PROJECT_SOURCES
        main.cpp
        notepad.cpp
        notepad.h
        notepad.ui
)

if(${QT_VERSION_MAJOR} GREATER_EQUAL 6)
    qt_add_executable(NotepadEx
        MANUAL_FINALIZATION
        ${PROJECT_SOURCES}
    )
# Define target properties for Android with Qt 6 as:
#    set_property(TARGET NotepadEx APPEND PROPERTY QT_ANDROID_PACKAGE_SOURCE_DIR
#                 ${CMAKE_CURRENT_SOURCE_DIR}/android)
# For more information, see https://doc.qt.io/qt-6/qt-add-executable.html#target-creation
else()
    if(ANDROID)
        add_library(NotepadEx SHARED
            ${PROJECT_SOURCES}
        )
# Define properties for Android with Qt 5 after find_package() calls as:
#    set(ANDROID_PACKAGE_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/android")
    else()
        add_executable(NotepadEx
            ${PROJECT_SOURCES}
        )
    endif()
endif()

target_link_libraries(NotepadEx PRIVATE Qt${QT_VERSION_MAJOR}::Widgets)

set_target_properties(NotepadEx PROPERTIES
    MACOSX_BUNDLE_GUI_IDENTIFIER my.example.com
    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
    MACOSX_BUNDLE TRUE
    WIN32_EXECUTABLE TRUE
)

if(QT_VERSION_MAJOR EQUAL 6)
    qt_finalize_executable(NotepadEx)
endif()
  • The project file specifies the source, header, and UI files included in the project.

Adding User Interaction

qtcreator_ui_type_here

  • To add functionality to the editor, we start by adding menu items and buttons on a toolbar.
  • Click on “Type Here”, and add the options New, Open, Save, Save as, Print and Exit.
    • This creates 6 lines in the Action Editor below.
    • To connect the actions to slots, right-click an action and select Go to slot > triggered(), and complete the code for that given slot by changing the name of the function.

qtcreator_ui_action_slot

  • If we also want to add the actions to a toolbar, we can assign an icon to each QAction, and then drag the QAction to the toolbar.

qtcreator_ui_add_icon_toolbar

  • You assign an icon by entering an icon name in the icon property of the action concerned.
    • When the QAction has been dragged to the toolbar, clicking the icon will launch the associated slot.
    • If the icon shows up fine in the toolbar in Qt Designer, but does not show when the project is running, then we should make a QRC file.
    • Add the notepad.qrc file to CMakeLists.txt.

qtcreator_ui_add_icon_resource

  • Complete the method newDocument():
void Notepad::on_actionNew_triggered()
{
    currentFile.clear();
    ui->textEdit->setText(QString());
}
  • The currentFile variable is a global variable containing the file presently being edited, and clear() clears the text buffer.
  • The currentFile variable is defined in the private part of notepad.h:
private:
  Ui::Notepad *ui;
  QString currentFile;

Opening a file

  • In notepad.ui, right click on actionOpen and select Go to Slot
  • Complete method open().
void Notepad::on_actionOpen_triggered()
{
    QString fileName = QFileDialog::getOpenFileName(this, "Open the file");
    if (fileName.isEmpty()) return;

    QFile file(fileName);
    currentFile = fileName;
    if (!file.open(QIODevice::ReadOnly | QFile::Text)) {
        QMessageBox::warning(this, "Warning", "Cannot open file: " + file.errorString());
        return;
    }
    setWindowTitle(fileName);

    QTextStream in(&file);
    QString text = in.readAll();
    ui->textEdit->setText(text);
    file.close();
}
  • QFileDialog::getOpenFileName opens a dialog enabling you to select a file. QFile object myfile has the selected file_name as parameter.
    • We store the selected file also into the global variable currentFile for later purposes.
    • We open the file with file.open as a readonly text file.
    • If it cannot be opened, a warning is issued, and the program stops.
  • We define a QTextStream instream for parameter myfile.
    • The contents of file myfile is copied into QString text.
    • setText(text) fills the buffer of out editor with text.

Saving a file

  • We create the method for saving a file in the same way as for Opening a file, by right clinking on actionSave, and selecting Go to Slot.
void Notepad::on_actionSave_triggered()
{
    QString fileName;

    // If we don't have a filename from before, get one.
    if (currentFile.isEmpty()) {
        fileName = QFileDialog::getSaveFileName(this, "Save");
        if (fileName.isEmpty()) return;
        currentFile = fileName;
    }
    else {
        fileName = currentFile;
    }

    QFile file(fileName);
    if (!file.open(QIODevice::WriteOnly | QFile::Text)) {
        QMessageBox::warning(this, "Warning", "Cannot save file: " + file.errorString());
        return;
    }
    setWindowTitle(fileName);
    QTextStream out(&file);
    QString text = ui->textEdit->toPlainText();
    out << text;
    file.close();
}
  • QFile object myfile is linked to a member variable current_file, the variable that contains the file we were working with.
    • If we cannot open myfile, an error message is issued and the method stops.
    • We create a QTextStream outstream.
    • The contents of the editor buffer is converted to plain text, and then written to outstream.

Saving a file under another name

void Notepad::on_actionSave_as_triggered()
{
    QString fileName = QFileDialog::getSaveFileName(this, "Save as");
    if (fileName.isEmpty()) return;

    QFile file(fileName);
    if (!file.open(QFile::WriteOnly | QFile::Text)) {
        QMessageBox::warning(this, "Warning", "Cannot save file: " + file.errorString());
        return;
    }
    currentFile = fileName;
    setWindowTitle(fileName);
    QTextStream out(&file);
    QString text = ui->textEdit->toPlainText();
    out << text;
    file.close();
}
  • This is the same procedure as for Saving a file, the only difference being that here you need to enter a new file name for the file to be created.

Printing a File

If you want to use print functionalities, you need to add PrintSupport to the project file:

find_package(Qt${QT_VERSION_MAJOR}
  REQUIRED COMPONENTS Core Gui Widgets
  OPTIONAL_COMPONENTS PrintSupport
)

In notepad.cpp, we declare a QPrinter object called printDev:

void Notepad::on_actionPrint_triggered()
{
#if defined(QT_PRINTSUPPORT_LIB) && QT_CONFIG(printer)
    QPrinter printDev;
#if QTCONFIG(printdialog)
    QPrintDialog dialog(&printDev, this);
    if (dialog.exec() == QDialog::Rejected)
        return;
#endif	// QT_CONFIG(printdialog)
    ui->textEdit->print(&printDev);
#endif	// QT_CONFIG(printer)
}
  • We launch a printer dialog box and store the selected printer in object printDev.
    • If we clicked on Cancel and did not select a printer, the methods returns.
    • The actual printer command is given with ui->textEdit->print with our QPrinter object as parameter.

Select a Font

void Notepad::on_actionFont_triggered()
{
    bool fontSelected;
    QFont font = QFontDialog::getFont(&fontSelected, this);
    if (fontSelected)
        ui->textEdit->setFont(font);
}
  • We declare a boolean indicating if we did select a font with QFontDialog.
    • If so, we set the font with ui->textEdit->setFont(myfont).

Copy, Cut, Paste, Undo and Redo

  • If you select some text, and want to copy it to the clipboard, you call the appropriate method of ui->textEdit. The same counts for cut, paste, undo and redo.

  • This table shows the method name to use.

Task Method called
Copy ui->textEdit->copy()
Cut ui->textEdit->cut()
Paste ui->textEdit->paste()
Undo ui->textEdit->undo()
Redo ui->textEdit->redo()