QSettings Class

When creating a QSettings object, you must pass the name of your company or organization as well as the name of your application. For example, if your product is called Star Runner and your company is called MySoft, you would construct the QSettings object as follows:

    QSettings settings("MySoft", "Star Runner");

QSettings objects can be created either on the stack or on the heap (i.e. using new). Constructing and destroying a QSettings object is very fast.

If you use QSettings from many places in your application, you might want to specify the organization name and the application name using QCoreApplication::setOrganizationName() and QCoreApplication::setApplicationName(), and then use the default QSettings constructor:

    QCoreApplication::setOrganizationName("MySoft");
    QCoreApplication::setOrganizationDomain("mysoft.com");
    QCoreApplication::setApplicationName("Star Runner");
    ...
    QSettings settings;

(Here, we also specify the organization's Internet domain. When the Internet domain is set, it is used on macOS and iOS instead of the organization name, since macOS and iOS applications conventionally use Internet domains to identify themselves. If no domain is set, a fake domain is derived from the organization name. See the Platform-Specific Notes below for details.)

QSettings stores settings. Each setting consists of a QString that specifies the setting's name (the key) and a QVariant that stores the data associated with the key. To write a setting, use setValue(). For example:

    settings.setValue("editor/wrapMargin", 68);

If there already exists a setting with the same key, the existing value is overwritten by the new value. For efficiency, the changes may not be saved to permanent storage immediately. (You can always call sync() to commit your changes.)

You can get a setting's value back using value():

    int margin = settings.value("editor/wrapMargin").toInt();

If there is no setting with the specified name, QSettings returns a null QVariant (which can be converted to the integer 0). You can specify another default value by passing a second argument to value():

    int margin = settings.value("editor/wrapMargin", 80).toInt();

To test whether a given key exists, call contains(). To remove the setting associated with a key, call remove(). To obtain the list of all keys, call allKeys(). To remove all keys, call clear().

Because QVariant is part of the Qt Core module, it cannot provide conversion functions to data types such as QColor, QImage, and QPixmap, which are part of Qt GUI. In other words, there is no toColor(), toImage(), or toPixmap() functions in QVariant.

Instead, you can use the QVariant::value() template function. For example:

QSettings settings("MySoft", "Star Runner");
QColor color = settings.value("DataPump/bgcolor").value<QColor>();

The inverse conversion (e.g., from QColor to QVariant) is automatic for all data types supported by QVariant, including GUI-related types:

QSettings settings("MySoft", "Star Runner");
QColor color = palette().background().color();
settings.setValue("DataPump/bgcolor", color);

Custom types registered using qRegisterMetaType() that have operators for streaming to and from a QDataStream can be stored using QSettings.

Setting keys can contain any Unicode characters. The Windows registry and INI files use case-insensitive keys, whereas the CFPreferences API on macOS and iOS uses case-sensitive keys. To avoid portability problems, follow these simple rules:

1. Always refer to the same key using the same case. For example, if you refer to a key as "text fonts" in one place in your code, don't refer to it as "Text Fonts" somewhere else.

2. Avoid key names that are identical except for the case. For example, if you have a key called "MainWindow", don't try to save another key as "mainwindow".

3. Do not use slashes ('/' and '\') in section or key names; the backslash character is used to separate sub keys (see below). On windows '\' are converted by QSettings to '/', which makes them identical.

You can form hierarchical keys using the '/' character as a separator, similar to Unix file paths. For example:

    settings.setValue("mainwindow/size", win->size());
    settings.setValue("mainwindow/fullScreen", win->isFullScreen());
    settings.setValue("outputpanel/visible", panel->isVisible());

If you want to save or restore many settings with the same prefix, you can specify the prefix using beginGroup() and call endGroup() at the end. Here's the same example again, but this time using the group mechanism:

    settings.beginGroup("mainwindow");
    settings.setValue("size", win->size());
    settings.setValue("fullScreen", win->isFullScreen());
    settings.endGroup();

    settings.beginGroup("outputpanel");
    settings.setValue("visible", panel->isVisible());
    settings.endGroup();

If a group is set using beginGroup(), the behavior of most functions changes consequently. Groups can be set recursively.

In addition to groups, QSettings also supports an "array" concept. See beginReadArray() and beginWriteArray() for details.

Let's assume that you have created a QSettings object with the organization name MySoft and the application name Star Runner. When you look up a value, up to four locations are searched in that order:

1. a user-specific location for the Star Runner application

2. a user-specific location for all applications by MySoft

3. a system-wide location for the Star Runner application

4. a system-wide location for all applications by MySoft

(See Platform-Specific Notes below for information on what these locations are on the different platforms supported by Qt.)

If a key cannot be found in the first location, the search goes on in the second location, and so on. This enables you to store system-wide or organization-wide settings and to override them on a per-user or per-application basis. To turn off this mechanism, call setFallbacksEnabled(false).

Although keys from all four locations are available for reading, only the first file (the user-specific location for the application at hand) is accessible for writing. To write to any of the other files, omit the application name and/or specify QSettings::SystemScope (as opposed to QSettings::UserScope, the default).

Let's see with an example:

    QSettings obj1("MySoft", "Star Runner");
    QSettings obj2("MySoft");
    QSettings obj3(QSettings::SystemScope, "MySoft", "Star Runner");
    QSettings obj4(QSettings::SystemScope, "MySoft");

The table below summarizes which QSettings objects access which location. "X" means that the location is the main location associated to the QSettings object and is used both for reading and for writing; "o" means that the location is used as a fallback when reading.

The beauty of this mechanism is that it works on all platforms supported by Qt and that it still gives you a lot of flexibility, without requiring you to specify any file names or registry paths.

If you want to use INI files on all platforms instead of the native API, you can pass QSettings::IniFormat as the first argument to the QSettings constructor, followed by the scope, the organization name, and the application name:

    QSettings settings(QSettings::IniFormat, QSettings::UserScope,
                       "MySoft", "Star Runner");

Note that INI files lose the distinction between numeric data and the strings used to encode them, so values written as numbers shall be read back as QString. The numeric value can be recovered using QString::toInt(), QString::toDouble() and related functions.

The Settings Editor example lets you experiment with different settings location and with fallbacks turned on or off.

QSettings is often used to store the state of a GUI application. The following example illustrates how to use QSettings to save and restore the geometry of an application's main window.

void MainWindow::writeSettings()
{
    QSettings settings("Moose Soft", "Clipper");

    settings.beginGroup("MainWindow");
    settings.setValue("geometry", saveGeometry());
    settings.endGroup();
}

void MainWindow::readSettings()
{
    QSettings settings("Moose Soft", "Clipper");

    settings.beginGroup("MainWindow");
    const auto geometry = settings.value("geometry", QByteArray()).toByteArray();
    if (geometry.isEmpty())
        setGeometry(200, 200, 400, 400);
    else
        restoreGeometry(geometry)
    settings.endGroup();
}

See Window Geometry for a discussion on why it is better to call QWidget::resize() and QWidget::move() rather than QWidget::setGeometry() to restore a window's geometry.

The readSettings() and writeSettings() functions must be called from the main window's constructor and close event handler as follows:

MainWindow::MainWindow()
{
    ...
    readSettings();
}

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

See the Application example for a self-contained example that uses QSettings.

QSettings is reentrant. This means that you can use distinct QSettings object in different threads simultaneously. This guarantee stands even when the QSettings objects refer to the same files on disk (or to the same entries in the system registry). If a setting is modified through one QSettings object, the change will immediately be visible in any other QSettings objects that operate on the same location and that live in the same process.

QSettings can safely be used from different processes (which can be different instances of your application running at the same time or different applications altogether) to read and write to the same system locations, provided certain conditions are met. For QSettings::IniFormat, it uses advisory file locking and a smart merging algorithm to ensure data integrity. The condition for that to work is that the writeable configuration file must be a regular file and must reside in a directory that the current user can create new, temporary files in. If that is not the case, then one must use setAtomicSyncRequired() to turn the safety off.

Note that sync() imports changes made by other processes (in addition to writing the changes from this QSettings).

Locations Where Application Settings Are Stored▲

Accessing INI and .plist Files Directly▲

QSettings settings("/home/petra/misc/myapp.ini",
                   QSettings::IniFormat);

QSettings settings("/Users/petra/misc/myapp.plist",
                   QSettings::NativeFormat);

Accessing the Windows Registry Directly▲

QSettings settings("HKEY_CURRENT_USER\\Software\\Microsoft\\Office",
                   QSettings::NativeFormat);

settings.setValue("11.0/Outlook/Security/DontTrustInstalledFiles", 0);

Accessing Common Registry Settings on Windows▲

settings.setValue("HKEY_CURRENT_USER\\MySoft\\Star Runner\\Galaxy", "Milkyway");
settings.setValue("HKEY_CURRENT_USER\\MySoft\\Star Runner\\Galaxy\\Sun", "OurStar");
settings.value("HKEY_CURRENT_USER\\MySoft\\Star Runner\\Galaxy\\Default"); // returns "Milkyway"

Platform Limitations▲