Was ist der unterschied von qapplication und qcoreapplication

The Application example shows how to implement a standard GUI application with menus, toolbars, and a status bar. The example itself is a simple text editor program built around QPlainTextEdit.

Inhaltsverzeichnis Show

MainWindow Class Definition
MainWindow Class Implementation
The main() Function
The Resource File

Nearly all of the code for the Application example is in the

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

3 class, which inherits QMainWindow. QMainWindow provides the framework for windows that have menus, toolbars, dock windows, and a status bar. The application provides File, Edit, and Help entries in the menu bar, with the following popup menus:

The status bar at the bottom of the main window shows a description of the menu item or toolbar button under the cursor.

To keep the example simple, recently opened files aren't shown in the File menu, even though this feature is desired in 90% of applications. Furthermore, this example can only load one file at a time. The SDI and MDI examples show how to lift these restrictions and how to implement recently opened files handling.

MainWindow Class Definition

Here's the class definition:

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow();

    void loadFile(const QString &fileName);

protected:
    void closeEvent(QCloseEvent *event) override;

private slots:
    void newFile();
    void open();
    bool save();
    bool saveAs();
    void about();
    void documentWasModified();
#ifndef QT_NO_SESSIONMANAGER
    void commitData(QSessionManager &);
#endif

private:
    void createActions();
    void createStatusBar();
    void readSettings();
    void writeSettings();
    bool maybeSave();
    bool saveFile(const QString &fileName);
    void setCurrentFile(const QString &fileName);
    QString strippedName(const QString &fullFileName);

    QPlainTextEdit *textEdit;
    QString curFile;
};

The public API is restricted to the constructor. In the

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

4 section, we reimplement QWidget::closeEvent() to detect when the user attempts to close the window, and warn the user about unsaved changes. In the

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

5 section, we declare slots that correspond to menu entries, as well as a mysterious

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

6 slot. Finally, in the

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

7 section of the class, we have various members that will be explained in due time.

MainWindow Class Implementation

#include 

#include "mainwindow.h"

We start by including

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

8, a header file that contains the definition of all classes in the Qt Core, Qt GUI and Qt Widgets modules. This saves us from the trouble of having to include every class individually. We also include

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

You might wonder why we don't include

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

8 in

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

9 and be done with it. The reason is that including such a large header from another header file can rapidly degrade performances. Here, it wouldn't do any harm, but it's still generally a good idea to include only the header files that are strictly necessary from another header file.

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

In the constructor, we start by creating a QPlainTextEdit widget as a child of the main window (the

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

2 object). Then we call QMainWindow::setCentralWidget() to tell that this is going to be the widget that occupies the central area of the main window, between the toolbars and the status bar.

Then we call

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

3 and

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

4, two private functions that set up the user interface. After that, we call

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

5 to restore the user's preferences.

We establish a signal-slot connection between the QPlainTextEdit's document object and our

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

6 slot. Whenever the user modifies the text in the QPlainTextEdit, we want to update the title bar to show that the file was modified.

At the end, we set the window title using the private

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

7 function. We'll come back to this later.

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

When the user attempts to close the window, we call the private function

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

8 to give the user the possibility to save pending changes. The function returns true if the user wants the application to close; otherwise, it returns false. In the first case, we save the user's preferences to disk and accept the close event; in the second case, we ignore the close event, meaning that the application will stay up and running as if nothing happened.

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

The

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

9 slot is invoked when the user selects File|New from the menu. We call

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

8 to save any pending changes and if the user accepts to go on, we clear the QPlainTextEdit and call the private function

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

7 to update the window title and clear the windowModified flag.

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

The

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

2 slot is invoked when the user clicks File|Open. We pop up a QFileDialog asking the user to choose a file. If the user chooses a file (i.e.,

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

3 is not an empty string), we call the private function

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

4 to actually load the file.

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

The

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

5 slot is invoked when the user clicks File|Save. If the user hasn't provided a name for the file yet, we call

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

6; otherwise, we call the private function

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

7 to actually save the file.

bool MainWindow::saveAs()
{
    QFileDialog dialog(this);
    dialog.setWindowModality(Qt::WindowModal);
    dialog.setAcceptMode(QFileDialog::AcceptSave);
    if (dialog.exec() != QDialog::Accepted)
        return false;
    return saveFile(dialog.selectedFiles().first());
}

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

6, we start by popping up a QFileDialog asking the user to provide a name. If the user clicks Cancel, the returned file name is empty, and we do nothing.

void MainWindow::about()
{
   QMessageBox::about(this, tr("About Application"),
            tr("The Application example demonstrates how to "
               "write modern GUI applications using Qt, with a menu bar, "
               "toolbars, and a status bar."));
}

The application's About box is done using one statement, using the QMessageBox::about() static function and relying on its support for an HTML subset.

The tr() call around the literal string marks the string for translation. It is a good habit to call tr() on all user-visible strings, in case you later decide to translate your application to other languages. The Internationalization with Qt overview covers tr() in more detail.

void MainWindow::documentWasModified()
{
    setWindowModified(textEdit->document()->isModified());
}

The

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

6 slot is invoked each time the text in the QPlainTextEdit changes because of user edits. We call QWidget::setWindowModified() to make the title bar show that the file was modified. How this is done varies on each platform.

#include 

#include "mainwindow.h"

The

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

3 private function, which is called from the

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

3 constructor, creates QActions and populates the menus and two toolbars. The code is very repetitive, so we show only the actions corresponding to File|New, File|Open, and Help|About Qt.

A QAction is an object that represents one user action, such as saving a file or invoking a dialog. An action can be put in a QMenu or a QToolBar, or both, or in any other widget that reimplements QWidget::actionEvent().

An action has a text that is shown in the menu, an icon, a shortcut key, a tooltip, a status tip (shown in the status bar), a "What's This?" text, and more. It emits a triggered() signal whenever the user invokes the action (e.g., by clicking the associated menu item or toolbar button).

Instances of QAction can be created by passing a parent QObject or by using one of the convenience functions of QMenu, QMenuBar or QToolBar. We create the actions that are in a menu as well as in a toolbar parented on the window to prevent ownership issues. For actions that are only in the menu, we use the convenience function QMenu::addAction(), which allows us to pass text, icon and the target object and its slot member function.

Creating toolbars is very similar to creating menus. The same actions that we put in the menus can be reused in the toolbars. After creating the action, we add it to the toolbar using QToolBar::addAction().

The code above contains one more idiom that must be explained. For some of the actions, we specify an icon as a QIcon to the QAction constructor. We use QIcon::fromTheme() to obtain the correct standard icon from the underlying window system. If that fails due to the platform not supporting it, we pass a file name as fallback. Here, the file name starts with

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

2. Such file names aren't ordinary file names, but rather path in the executable's stored resources. We'll come back to this when we review the

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

3 file that's part of the project.

#include 

#include "mainwindow.h"

The Edit|Cut and Edit|Copy actions must be available only when the QPlainTextEdit contains selected text. We disable them by default and connect the QPlainTextEdit::copyAvailable() signal to the QAction::setEnabled() slot, ensuring that the actions are disabled when the text editor has no selection.

Just before we create the Help menu, we call QMenuBar::addSeparator(). This has no effect for most widget styles (e.g., Windows and macOS styles), but for some styles this makes sure that Help is pushed to the right side of the menu bar.

#include 

#include "mainwindow.h"

QMainWindow::statusBar() returns a pointer to the main window's QStatusBar widget. Like with QMainWindow::menuBar(), the widget is automatically created the first time the function is called.

#include 

#include "mainwindow.h"

The

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

5 function is called from the constructor to load the user's preferences and other application settings. The QSettings class provides a high-level interface for storing settings permanently on disk. On Windows, it uses the (in)famous Windows registry; on macOS, it uses the native XML-based CFPreferences API; on Unix/X11, it uses text files.

The QSettings constructor takes arguments that identify your company and the name of the product. This ensures that the settings for different applications are kept separately.

We use QSettings::value() to extract the value of the geometry setting. The second argument to QSettings::value() is optional and specifies a default value for the setting if there exists none. This value is used the first time the application is run.

We use QWidget::saveGeometry() and Widget::restoreGeometry() to save the position. They use an opaque QByteArray to store screen number, geometry and window state.

#include 

#include "mainwindow.h"

The

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

5 function is called from

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

6. Writing settings is similar to reading them, except simpler. The arguments to the QSettings constructor must be the same as in

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

#include 

#include "mainwindow.h"

The

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

8 function is called to save pending changes. If there are pending changes, it pops up a QMessageBox giving the user to save the document. The options are QMessageBox::Yes, QMessageBox::No, and QMessageBox::Cancel. The Yes button is made the default button (the button that is invoked when the user presses Return) using the QMessageBox::Default flag; the Cancel button is made the escape button (the button that is invoked when the user presses Esc) using the QMessageBox::Escape flag.

The

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

8 function returns

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

0 in all cases, except when the user clicks Cancel or saving the file fails. The caller must check the return value and stop whatever it was doing if the return value is

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

#include 

#include "mainwindow.h"

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

4, we use QFile and QTextStream to read in the data. The QFile object provides access to the bytes stored in a file.

We start by opening the file in read-only mode. The QFile::Text flag indicates that the file is a text file, not a binary file. On Unix and macOS, this makes no difference, but on Windows, it ensures that the "\r\n" end-of-line sequence is converted to "\n" when reading.

If we successfully opened the file, we use a QTextStream object to read in the data. QTextStream automatically converts the 8-bit data into a Unicode QString and supports various encodings. If no encoding is specified, QTextStream assumes the file is written using the system's default 8-bit encoding (for example, Latin-1; see QTextCodec::codecForLocale() for details).

Since the call to QTextStream::readAll() might take some time, we set the cursor to be Qt::WaitCursor for the entire application while it goes on.

At the end, we call the private

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

7 function, which we'll cover in a moment, and we display the string "File loaded" in the status bar for 2 seconds (2000 milliseconds).

#include 

#include "mainwindow.h"

Saving a file is very similar to loading one. Here, the QFile::Text flag ensures that on Windows, "\n" is converted into "\r\n" to conform to the Windows convension.

#include 

#include "mainwindow.h"

The

void MainWindow::closeEvent(QCloseEvent *event)
{
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}

7 function is called to reset the state of a few variables when a file is loaded or saved, or when the user starts editing a new file (in which case

void MainWindow::newFile()
{
    if (maybeSave()) {
        textEdit->clear();
        setCurrentFile(QString());
    }
}

3 is empty). We update the

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

6 variable, clear the QTextDocument::modified flag and the associated

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

7 flag, and update the window title to contain the new file name (or

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

8).

The

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

9 function call around

bool MainWindow::save()
{
    if (curFile.isEmpty()) {
        return saveAs();
    } else {
        return saveFile(curFile);
    }
}

6 in the QWidget::setWindowTitle() call shortens the file name to exclude the path. Here's the function:

#include 

#include "mainwindow.h"

The main() Function

The

bool MainWindow::saveAs()
{
    QFileDialog dialog(this);
    dialog.setWindowModality(Qt::WindowModal);
    dialog.setAcceptMode(QFileDialog::AcceptSave);
    if (dialog.exec() != QDialog::Accepted)
        return false;
    return saveFile(dialog.selectedFiles().first());
}

1 function for this application is typical of applications that contain one main window:

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

The main function uses QCommandLineParser to check whether some file argument was passed to the application and loads it via MainWindow::loadFile().

The Resource File

As you will probably recall, for some of the actions, we specified icons with file names starting with

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

2 and mentioned that such file names aren't ordinary file names, but path in the executable's stored resources. These resources are compiled

The resources associated with an application are specified in a

bool MainWindow::saveAs()
{
    QFileDialog dialog(this);
    dialog.setWindowModality(Qt::WindowModal);
    dialog.setAcceptMode(QFileDialog::AcceptSave);
    if (dialog.exec() != QDialog::Accepted)
        return false;
    return saveFile(dialog.selectedFiles().first());
}

3 file, an XML-based file format that lists files on the disk. Here's the

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

3 file that's used by the Application example:

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

The

bool MainWindow::saveAs()
{
    QFileDialog dialog(this);
    dialog.setWindowModality(Qt::WindowModal);
    dialog.setAcceptMode(QFileDialog::AcceptSave);
    if (dialog.exec() != QDialog::Accepted)
        return false;
    return saveFile(dialog.selectedFiles().first());
}

5 files listed in the

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

3 file are files that are part of the Application example's source tree. Paths are relative to the directory where the

void MainWindow::open()
{
    if (maybeSave()) {
        QString fileName = QFileDialog::getOpenFileName(this);
        if (!fileName.isEmpty())
            loadFile(fileName);
    }
}

3 file is located (the

bool MainWindow::saveAs()
{
    QFileDialog dialog(this);
    dialog.setWindowModality(Qt::WindowModal);
    dialog.setAcceptMode(QFileDialog::AcceptSave);
    if (dialog.exec() != QDialog::Accepted)
        return false;
    return saveFile(dialog.selectedFiles().first());
}

8 directory).

The resource file must be mentioned in the

bool MainWindow::saveAs()
{
    QFileDialog dialog(this);
    dialog.setWindowModality(Qt::WindowModal);
    dialog.setAcceptMode(QFileDialog::AcceptSave);
    if (dialog.exec() != QDialog::Accepted)
        return false;
    return saveFile(dialog.selectedFiles().first());
}

9 file so that

void MainWindow::about()
{
   QMessageBox::about(this, tr("About Application"),
            tr("The Application example demonstrates how to "
               "write modern GUI applications using Qt, with a menu bar, "
               "toolbars, and a status bar."));
}

0 knows about it:

MainWindow::MainWindow()
    : textEdit(new QPlainTextEdit)
{
    setCentralWidget(textEdit);

    createActions();
    createStatusBar();

    readSettings();

    connect(textEdit->document(), &QTextDocument::contentsChanged,
            this, &MainWindow::documentWasModified);

#ifndef QT_NO_SESSIONMANAGER
    QGuiApplication::setFallbackSessionManagementEnabled(false);
    connect(qApp, &QGuiApplication::commitDataRequest,
            this, &MainWindow::commitData);
#endif

    setCurrentFile(QString());
    setUnifiedTitleAndToolBarOnMac(true);
}

void MainWindow::about()
{
   QMessageBox::about(this, tr("About Application"),
            tr("The Application example demonstrates how to "
               "write modern GUI applications using Qt, with a menu bar, "
               "toolbars, and a status bar."));
}

0 will produce make rules to generate a file called

void MainWindow::about()
{
   QMessageBox::about(this, tr("About Application"),
            tr("The Application example demonstrates how to "
               "write modern GUI applications using Qt, with a menu bar, "
               "toolbars, and a status bar."));
}

2 that is linked into the application. This file contains all the data for the images and other resources as static C++ arrays of compressed binary data. See The Qt Resource System for more information about resources.