QSettings Class Reference

The QSettings class provides persistent platform-independent application settings. More...

 #include <QSettings>

Inherits: QObject.

Note: All functions in this class are reentrant, but registerFormat() is also thread-safe.

Public Types

enum Format { NativeFormat, IniFormat, InvalidFormat }
typedef ReadFunc
enum Scope { UserScope, SystemScope }
typedef SettingsMap
enum Status { NoError, AccessError, FormatError }
typedef WriteFunc

Public Functions

QSettings ( const QString & organization, const QString & application = QString(), QObject * parent = 0 )
QSettings ( Scope scope, const QString & organization, const QString & application = QString(), QObject * parent = 0 )
QSettings ( Format format, Scope scope, const QString & organization, const QString & application = QString(), QObject * parent = 0 )
QSettings ( const QString & fileName, Format format, QObject * parent = 0 )
QSettings ( QObject * parent = 0 )
~QSettings ()
QStringList allKeys () const
QString applicationName () const
void beginGroup ( const QString & prefix )
int beginReadArray ( const QString & prefix )
void beginWriteArray ( const QString & prefix, int size = -1 )
QStringList childGroups () const
QStringList childKeys () const
void clear ()
bool contains ( const QString & key ) const
void endArray ()
void endGroup ()
bool fallbacksEnabled () const
QString fileName () const
Format format () const
QString group () const
QTextCodec * iniCodec () const
bool isWritable () const
QString organizationName () const
void remove ( const QString & key )
Scope scope () const
void setArrayIndex ( int i )
void setFallbacksEnabled ( bool b )
void setIniCodec ( QTextCodec * codec )
void setIniCodec ( const char * codecName )
void setValue ( const QString & key, const QVariant & value )
Status status () const
void sync ()
QVariant value ( const QString & key, const QVariant & defaultValue = QVariant() ) const
  • 29 public functions inherited from QObject

Static Public Members

Format defaultFormat ()
Format registerFormat ( const QString & extension, ReadFunc readFunc, WriteFunc writeFunc, Qt::CaseSensitivity caseSensitivity = Qt::CaseSensitive )
void setDefaultFormat ( Format format )
void setPath ( Format format, Scope scope, const QString & path )
  • 7 static public members inherited from QObject

Reimplemented Protected Functions

virtual bool event ( QEvent * event )
  • 8 protected functions inherited from QObject

Additional Inherited Members

  • 1 property inherited from QObject
  • 1 public slot inherited from QObject
  • 1 signal inherited from QObject
  • 8 protected functions inherited from QObject

Detailed Description

The QSettings class provides persistent platform-independent application settings.

Users normally expect an application to remember its settings (window sizes and positions, options, etc.) across sessions. This information is often stored in the system registry on Windows, and in XML preferences files on Mac OS X. On Unix systems, in the absence of a standard, many applications (including the KDE applications) use INI text files.

QSettings is an abstraction around these technologies, enabling you to save and restore application settings in a portable manner. It also supports custom storage formats.

QSettings's API is based on QVariant, allowing you to save most value-based types, such as QString, QRect, and QImage, with the minimum of effort.

If all you need is a non-persistent memory-based structure, consider using QMap<QString, QVariant> instead.

Basic Usage

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 Mac OS X instead of the organization name, since Mac OS X 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().

QVariant and GUI Types

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

Instead, you can use the QVariant::value() or the qVariantValue() 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() and qRegisterMetaTypeStreamOperators() can be stored using QSettings.

Section and Key Syntax

Setting keys can contain any Unicode characters. The Windows registry and INI files use case-insensitive keys, whereas the Carbon Preferences API on Mac OS X 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.

Fallback Mechanism

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.

Locationsobj1obj2obj3obj4
1. User, ApplicationX
2. User, OrganizationoX
3. System, ApplicationoX
4. System, OrganizationoooX

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");

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

Restoring the State of a GUI Application

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("size", size());
     settings.setValue("pos", pos());
     settings.endGroup();
 }

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

     settings.beginGroup("MainWindow");
     resize(settings.value("size", QSize(400, 400)).toSize());
     move(settings.value("pos", QPoint(200, 200)).toPoint());
     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.

Accessing Settings from Multiple Threads or Processes Simultaneously

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. It uses advisory file locking and a smart merging algorithm to ensure data integrity. Note that sync() imports changes made by other processes (in addition to writing the changes from this QSettings).

Platform-Specific Notes

Locations Where Application Settings Are Stored

As mentioned in the Fallback Mechanism section, QSettings stores settings for an application in up to four locations, depending on whether the settings are user-specific or system-wide and whether the settings are application-specific or organization-wide. For simplicity, we're assuming the organization is called MySoft and the application is called Star Runner.

On Unix systems, if the file format is NativeFormat, the following files are used by default:

  1. $HOME/.config/MySoft/Star Runner.conf (Qt for Embedded Linux: $HOME/Settings/MySoft/Star Runner.conf)
  2. $HOME/.config/MySoft.conf (Qt for Embedded Linux: $HOME/Settings/MySoft.conf)
  3. /etc/xdg/MySoft/Star Runner.conf
  4. /etc/xdg/MySoft.conf

On Mac OS X versions 10.2 and 10.3, these files are used by default:

  1. $HOME/Library/Preferences/com.MySoft.Star Runner.plist
  2. $HOME/Library/Preferences/com.MySoft.plist
  3. /Library/Preferences/com.MySoft.Star Runner.plist
  4. /Library/Preferences/com.MySoft.plist

On Windows, NativeFormat settings are stored in the following registry paths:

  1. HKEY_CURRENT_USER\Software\MySoft\Star Runner
  2. HKEY_CURRENT_USER\Software\MySoft
  3. HKEY_LOCAL_MACHINE\Software\MySoft\Star Runner
  4. HKEY_LOCAL_MACHINE\Software\MySoft

Note: On Windows, for 32-bit programs running in WOW64 mode, settings are stored in the following registry path: HKEY_LOCAL_MACHINE\Software\WOW6432node.

On BlackBerry only a single file is used (see Platform Limitations). If the file format is NativeFormat, this is "Settings/MySoft/Star Runner.conf" in the application's home directory.

If the file format is IniFormat, the following files are used on Unix and Mac OS X:

  1. $HOME/.config/MySoft/Star Runner.ini (Qt for Embedded Linux: $HOME/Settings/MySoft/Star Runner.ini)
  2. $HOME/.config/MySoft.ini (Qt for Embedded Linux: $HOME/Settings/MySoft.ini)
  3. /etc/xdg/MySoft/Star Runner.ini
  4. /etc/xdg/MySoft.ini

On Windows, the following files are used:

  1. %APPDATA%\MySoft\Star Runner.ini
  2. %APPDATA%\MySoft.ini
  3. %COMMON_APPDATA%\MySoft\Star Runner.ini
  4. %COMMON_APPDATA%\MySoft.ini

The %APPDATA% path is usually C:\Documents and Settings\User Name\Application Data; the %COMMON_APPDATA% path is usually C:\Documents and Settings\All Users\Application Data.

On BlackBerry only a single file is used (see Platform Limitations). If the file format is IniFormat, this is "Settings/MySoft/Star Runner.ini" in the application's home directory.

On Symbian, the following files are used for both IniFormat and NativeFormat (in this example, we assume that the application is installed on the e-drive and its Secure ID is 0xECB00931):

  1. c:\data\.config\MySoft\Star Runner.conf
  2. c:\data\.config\MySoft.conf
  3. e:\private\ecb00931\MySoft\Star Runner.conf
  4. e:\private\ecb00931\MySoft.conf

The SystemScope settings location is determined from the installation drive and Secure ID (UID3) of the application. If the application is built-in on the ROM, the drive used for SystemScope is c:.

Note: Symbian SystemScope settings are by default private to the application and not shared between applications, unlike other environments.

The paths for the .ini and .conf files can be changed using setPath(). On Unix and Mac OS X, the user can override them by setting the XDG_CONFIG_HOME environment variable; see setPath() for details.

Accessing INI and .plist Files Directly

Sometimes you do want to access settings stored in a specific file or registry path. On all platforms, if you want to read an INI file directly, you can use the QSettings constructor that takes a file name as first argument and pass QSettings::IniFormat as second argument. For example:

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

You can then use the QSettings object to read and write settings in the file.

On Mac OS X, you can access XML-based .plist files by passing QSettings::NativeFormat as second argument. For example:

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

Accessing the Windows Registry Directly

On Windows, QSettings lets you access settings that have been written with QSettings (or settings in a supported format, e.g., string data) in the system registry. This is done by constructing a QSettings object with a path in the registry and QSettings::NativeFormat.

For example:

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

All the registry entries that appear under the specified path can be read or written through the QSettings object as usual (using forward slashes instead of backslashes). For example:

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

Note that the backslash character is, as mentioned, used by QSettings to separate subkeys. As a result, you cannot read or write windows registry entries that contain slashes or backslashes; you should use a native windows API if you need to do so.

Accessing Common Registry Settings on Windows

On Windows, it is possible for a key to have both a value and subkeys. Its default value is accessed by using "Default" or "." in place of a subkey:

 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"

On other platforms than Windows, "Default" and "." would be treated as regular subkeys.

Securing application settings in Symbian

UserScope settings in Symbian are writable by any application by default. To protect the application settings from access and tampering by other applications, the settings need to be placed in the private secure area of the application. This can be done by specifying the settings storage path directly to the private area. The following snippet changes the UserScope to c:/private/ecb00931/MySoft.conf (provided the application is installed on the c-drive and its Secure ID is 0xECB00931:

 QSettings settings(QApplication::applicationDirPath() + "/MySoft.conf");

Framework libraries (like Qt itself) may store configuration and cache settings using UserScope, which is accessible and writable by other applications. If the application is very security sensitive or uses high platform security capabilities, it may be prudent to also force framework settings to be stored in the private directory of the application. This can be done by changing the default path of UserScope before QApplication is created:

 #include <QSettings>
 #include <QDesktopServices>
 int main(int argc, char *argv[])
 {
 #ifdef Q_OS_SYMBIAN
     // Use QDesktopServices:storageLocation as QApplication is not yet created
     QSettings::setPath(
         QSettings::NativeFormat, QSettings::UserScope,
         QDesktopServices::storageLocation(QDesktopServices::DataLocation) + "/settings");
 #endif
     QApplication app(argc, argv);

     ...
 }

Note that this may affect framework libraries' functionality if they expect the settings to be shared between applications.

Changing the location of global Qt settings on Mac OS X

On Mac OS X, the global Qt settings (stored in com.trolltech.plist) are stored in the application settings file in two situations:

  1. If the application runs in a Mac OS X sandbox (on Mac OS X 10.7 or later) or
  2. If the Info.plist file of the application contains the key "ForAppStore" with the value "yes"

In these situations, the application settings file is named using the bundle identifier of the application, which must consequently be set in the application's Info.plist file.

This feature is provided to ease the acceptance of Qt applications into the Mac App Store, as the default behaviour of storing global Qt settings in the com.trolltech.plist file does not conform with Mac App Store file system usage requirements. For more information about submitting Qt applications to the Mac App Store, see Preparing a Qt application for Mac App Store submission.

Platform Limitations

While QSettings attempts to smooth over the differences between the different supported platforms, there are still a few differences that you should be aware of when porting your application:

  • The Windows system registry has the following limitations: A subkey may not exceed 255 characters, an entry's value may not exceed 16,383 characters, and all the values of a key may not exceed 65,535 characters. One way to work around these limitations is to store the settings using the IniFormat instead of the NativeFormat.
  • On Mac OS X, allKeys() will return some extra keys for global settings that apply to all applications. These keys can be read using value() but cannot be changed, only shadowed. Calling setFallbacksEnabled(false) will hide these global settings.
  • On Mac OS X, the CFPreferences API used by QSettings expects Internet domain names rather than organization names. To provide a uniform API, QSettings derives a fake domain name from the organization name (unless the organization name already is a domain name, e.g. OpenOffice.org). The algorithm appends ".com" to the company name and replaces spaces and other illegal characters with hyphens. If you want to specify a different domain name, call QCoreApplication::setOrganizationDomain(), QCoreApplication::setOrganizationName(), and QCoreApplication::setApplicationName() in your main() function and then use the default QSettings constructor. Another solution is to use preprocessor directives, for example:
     #ifdef Q_WS_MAC
         QSettings settings("grenoullelogique.fr", "Squash");
     #else
         QSettings settings("Grenoulle Logique", "Squash");
     #endif
  • On Unix and Mac OS X systems, the advisory file locking is disabled if NFS (or AutoFS or CacheFS) is detected to work around a bug in the NFS fcntl() implementation, which hangs forever if statd or lockd aren't running. Also, the locking isn't performed when accessing .plist files.
  • On the BlackBerry platform, applications run in a sandbox. They are not allowed to read or write outside of this sandbox. This involves the following limitations:
    • As there is only a single scope the scope is simply ignored, i.e. there is no difference between SystemScope and UserScope.
    • The Fallback Mechanism is not applied, i.e. only a single location is considered.
    • It is advised against setting and using custom file paths.

See also QVariant, QSessionManager, Settings Editor Example, and Application Example.

Member Type Documentation

enum QSettings::Format

This enum type specifies the storage format used by QSettings.

ConstantValueDescription
QSettings::NativeFormat0Store the settings using the most appropriate storage format for the platform. On Windows, this means the system registry; on Mac OS X, this means the CFPreferences API; on Unix, this means textual configuration files in INI format.
QSettings::IniFormat1Store the settings in INI files.
QSettings::InvalidFormat16Special value returned by registerFormat().

On Unix, NativeFormat and IniFormat mean the same thing, except that the file extension is different (.conf for NativeFormat, .ini for IniFormat).

The INI file format is a Windows file format that Qt supports on all platforms. In the absence of an INI standard, we try to follow what Microsoft does, with the following exceptions:

  • If you store types that QVariant can't convert to QString (e.g., QPoint, QRect, and QSize), Qt uses an @-based syntax to encode the type. For example:
     pos = @Point(100 100)

    To minimize compatibility issues, any @ that doesn't appear at the first position in the value or that isn't followed by a Qt type (Point, Rect, Size, etc.) is treated as a normal character.

  • Although backslash is a special character in INI files, most Windows applications don't escape backslashes (\) in file paths:
     windir = C:\Windows

    QSettings always treats backslash as a special character and provides no API for reading or writing such entries.

  • The INI file format has severe restrictions on the syntax of a key. Qt works around this by using % as an escape character in keys. In addition, if you save a top-level setting (a key with no slashes in it, e.g., "someKey"), it will appear in the INI file's "General" section. To avoid overwriting other keys, if you save something using the a key such as "General/someKey", the key will be located in the "%General" section, not in the "General" section.
  • Following the philosophy that we should be liberal in what we accept and conservative in what we generate, QSettings will accept Latin-1 encoded INI files, but generate pure ASCII files, where non-ASCII values are encoded using standard INI escape sequences. To make the INI files more readable (but potentially less compatible), call setIniCodec().

See also registerFormat() and setPath().

typedef QSettings::ReadFunc

Typedef for a pointer to a function with the following signature:

 bool myReadFunc(QIODevice &device, QSettings::SettingsMap &map);

ReadFunc is used in registerFormat() as a pointer to a function that reads a set of key/value pairs. ReadFunc should read all the options in one pass, and return all the settings in the SettingsMap container, which is initially empty.

See also WriteFunc and registerFormat().

enum QSettings::Scope

This enum specifies whether settings are user-specific or shared by all users of the same system.

ConstantValueDescription
QSettings::UserScope0Store settings in a location specific to the current user (e.g., in the user's home directory).
QSettings::SystemScope1Store settings in a global location, so that all users on the same machine access the same set of settings.

See also setPath().

typedef QSettings::SettingsMap

Typedef for QMap<QString, QVariant>.

See also registerFormat().

enum QSettings::Status

The following status values are possible:

ConstantValueDescription
QSettings::NoError0No error occurred.
QSettings::AccessError1An access error occurred (e.g. trying to write to a read-only file).
QSettings::FormatError2A format error occurred (e.g. loading a malformed INI file).

See also status().

typedef QSettings::WriteFunc

Typedef for a pointer to a function with the following signature:

 bool myWriteFunc(QIODevice &device, const QSettings::SettingsMap &map);

WriteFunc is used in registerFormat() as a pointer to a function that writes a set of key/value pairs. WriteFunc is only called once, so you need to output the settings in one go.

See also ReadFunc and registerFormat().

Member Function Documentation

QSettings::QSettings ( const QString & organization, const QString & application = QString(), QObject * parent = 0 )

Constructs a QSettings object for accessing settings of the application called application from the organization called organization, and with parent parent.

Example:

 QSettings settings("Moose Tech", "Facturo-Pro");

The scope is set to QSettings::UserScope, and the format is set to QSettings::NativeFormat (i.e. calling setDefaultFormat() before calling this constructor has no effect).

See also setDefaultFormat() and Fallback Mechanism.

QSettings::QSettings ( Scope scope, const QString & organization, const QString & application = QString(), QObject * parent = 0 )

Constructs a QSettings object for accessing settings of the application called application from the organization called organization, and with parent parent.

If scope is QSettings::UserScope, the QSettings object searches user-specific settings first, before it searches system-wide settings as a fallback. If scope is QSettings::SystemScope, the QSettings object ignores user-specific settings and provides access to system-wide settings.

The storage format is set to QSettings::NativeFormat (i.e. calling setDefaultFormat() before calling this constructor has no effect).

If no application name is given, the QSettings object will only access the organization-wide locations.

See also setDefaultFormat().

QSettings::QSettings ( Format format, Scope scope, const QString & organization, const QString & application = QString(), QObject * parent = 0 )

Constructs a QSettings object for accessing settings of the application called application from the organization called organization, and with parent parent.

If scope is QSettings::UserScope, the QSettings object searches user-specific settings first, before it searches system-wide settings as a fallback. If scope is QSettings::SystemScope, the QSettings object ignores user-specific settings and provides access to system-wide settings.

If format is QSettings::NativeFormat, the native API is used for storing settings. If format is QSettings::IniFormat, the INI format is used.

If no application name is given, the QSettings object will only access the organization-wide locations.

QSettings::QSettings ( const QString & fileName, Format format, QObject * parent = 0 )

Constructs a QSettings object for accessing the settings stored in the file called fileName, with parent parent. If the file doesn't already exist, it is created.

If format is QSettings::NativeFormat, the meaning of fileName depends on the platform. On Unix, fileName is the name of an INI file. On Mac OS X, fileName is the name of a .plist file. On Windows, fileName is a path in the system registry.

If format is QSettings::IniFormat, fileName is the name of an INI file.

Warning: This function is provided for convenience. It works well for accessing INI or .plist files generated by Qt, but might fail on some syntaxes found in such files originated by other programs. In particular, be aware of the following limitations:

  • QSettings provides no way of reading INI "path" entries, i.e., entries with unescaped slash characters. (This is because these entries are ambiguous and cannot be resolved automatically.)
  • In INI files, QSettings uses the @ character as a metacharacter in some contexts, to encode Qt-specific data types (e.g., @Rect), and might therefore misinterpret it when it occurs in pure INI files.

See also fileName().

QSettings::QSettings ( QObject * parent = 0 )

Constructs a QSettings object for accessing settings of the application and organization set previously with a call to QCoreApplication::setOrganizationName(), QCoreApplication::setOrganizationDomain(), and QCoreApplication::setApplicationName().

The scope is QSettings::UserScope and the format is defaultFormat() (QSettings::NativeFormat by default). Use setDefaultFormat() before calling this constructor to change the default format used by this constructor.

The code

 QSettings settings("Moose Soft", "Facturo-Pro");

is equivalent to

 QCoreApplication::setOrganizationName("Moose Soft");
 QCoreApplication::setApplicationName("Facturo-Pro");
 QSettings settings;

If QCoreApplication::setOrganizationName() and QCoreApplication::setApplicationName() has not been previously called, the QSettings object will not be able to read or write any settings, and status() will return AccessError.

On Mac OS X, if both a name and an Internet domain are specified for the organization, the domain is preferred over the name. On other platforms, the name is preferred over the domain.

See also QCoreApplication::setOrganizationName(), QCoreApplication::setOrganizationDomain(), QCoreApplication::setApplicationName(), and setDefaultFormat().

QSettings::~QSettings ()

Destroys the QSettings object.

Any unsaved changes will eventually be written to permanent storage.

See also sync().

QStringList QSettings::allKeys () const

Returns a list of all keys, including subkeys, that can be read using the QSettings object.

Example:

 QSettings settings;
 settings.setValue("fridge/color", Qt::white);
 settings.setValue("fridge/size", QSize(32, 96));
 settings.setValue("sofa", true);
 settings.setValue("tv", false);

 QStringList keys = settings.allKeys();
 // keys: ["fridge/color", "fridge/size", "sofa", "tv"]

If a group is set using beginGroup(), only the keys in the group are returned, without the group prefix:

 settings.beginGroup("fridge");
 keys = settings.allKeys();
 // keys: ["color", "size"]

See also childGroups() and childKeys().

QString QSettings::applicationName () const

Returns the application name used for storing the settings.

This function was introduced in Qt 4.4.

See also QCoreApplication::applicationName(), format(), scope(), and organizationName().

void QSettings::beginGroup ( const QString & prefix )

Appends prefix to the current group.

The current group is automatically prepended to all keys specified to QSettings. In addition, query functions such as childGroups(), childKeys(), and allKeys() are based on the group. By default, no group is set.

Groups are useful to avoid typing in the same setting paths over and over. For example:

 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();

This will set the value of three settings:

  • mainwindow/size
  • mainwindow/fullScreen
  • outputpanel/visible

Call endGroup() to reset the current group to what it was before the corresponding beginGroup() call. Groups can be nested.

See also endGroup() and group().

int QSettings::beginReadArray ( const QString & prefix )

Adds prefix to the current group and starts reading from an array. Returns the size of the array.

Example:

 struct Login {
     QString userName;
     QString password;
 };
 QList<Login> logins;
 ...

 QSettings settings;
 int size = settings.beginReadArray("logins");
 for (int i = 0; i < size; ++i) {
     settings.setArrayIndex(i);
     Login login;
     login.userName = settings.value("userName").toString();
     login.password = settings.value("password").toString();
     logins.append(login);
 }
 settings.endArray();

Use beginWriteArray() to write the array in the first place.

See also beginWriteArray(), endArray(), and setArrayIndex().

void QSettings::beginWriteArray ( const QString & prefix, int size = -1 )

Adds prefix to the current group and starts writing an array of size size. If size is -1 (the default), it is automatically determined based on the indexes of the entries written.

If you have many occurrences of a certain set of keys, you can use arrays to make your life easier. For example, let's suppose that you want to save a variable-length list of user names and passwords. You could then write:

 struct Login {
     QString userName;
     QString password;
 };
 QList<Login> logins;
 ...

 QSettings settings;
 settings.beginWriteArray("logins");
 for (int i = 0; i < logins.size(); ++i) {
     settings.setArrayIndex(i);
     settings.setValue("userName", list.at(i).userName);
     settings.setValue("password", list.at(i).password);
 }
 settings.endArray();

The generated keys will have the form

  • logins/size
  • logins/1/userName
  • logins/1/password
  • logins/2/userName
  • logins/2/password
  • logins/3/userName
  • logins/3/password
  • ...

To read back an array, use beginReadArray().

See also beginReadArray(), endArray(), and setArrayIndex().

QStringList QSettings::childGroups () const

Returns a list of all key top-level groups that contain keys that can be read using the QSettings object.

Example:

 QSettings settings;
 settings.setValue("fridge/color", Qt::white);
 settings.setValue("fridge/size", QSize(32, 96));
 settings.setValue("sofa", true);
 settings.setValue("tv", false);

 QStringList groups = settings.childGroups();
 // groups: ["fridge"]

If a group is set using beginGroup(), the first-level keys in that group are returned, without the group prefix.

 settings.beginGroup("fridge");
 groups = settings.childGroups();
 // groups: []

You can navigate through the entire setting hierarchy using childKeys() and childGroups() recursively.

See also childKeys() and allKeys().

QStringList QSettings::childKeys () const

Returns a list of all top-level keys that can be read using the QSettings object.

Example:

 QSettings settings;
 settings.setValue("fridge/color", Qt::white);
 settings.setValue("fridge/size", QSize(32, 96));
 settings.setValue("sofa", true);
 settings.setValue("tv", false);

 QStringList keys = settings.childKeys();
 // keys: ["sofa", "tv"]

If a group is set using beginGroup(), the top-level keys in that group are returned, without the group prefix:

 settings.beginGroup("fridge");
 keys = settings.childKeys();
 // keys: ["color", "size"]

You can navigate through the entire setting hierarchy using childKeys() and childGroups() recursively.

See also childGroups() and allKeys().

void QSettings::clear ()

Removes all entries in the primary location associated to this QSettings object.

Entries in fallback locations are not removed.

If you only want to remove the entries in the current group(), use remove("") instead.

See also remove() and setFallbacksEnabled().

bool QSettings::contains ( const QString & key ) const

Returns true if there exists a setting called key; returns false otherwise.

If a group is set using beginGroup(), key is taken to be relative to that group.

Note that the Windows registry and INI files use case-insensitive keys, whereas the Carbon Preferences API on Mac OS X uses case-sensitive keys. To avoid portability problems, see the Section and Key Syntax rules.

See also value() and setValue().

Format QSettings::defaultFormat () [static]

Returns default file format used for storing settings for the QSettings(QObject *) constructor. If no default format is set, QSettings::NativeFormat is used.

This function was introduced in Qt 4.4.

See also setDefaultFormat() and format().

void QSettings::endArray ()

Closes the array that was started using beginReadArray() or beginWriteArray().

See also beginReadArray() and beginWriteArray().

void QSettings::endGroup ()

Resets the group to what it was before the corresponding beginGroup() call.

Example:

 settings.beginGroup("alpha");
 // settings.group() == "alpha"

 settings.beginGroup("beta");
 // settings.group() == "alpha/beta"

 settings.endGroup();
 // settings.group() == "alpha"

 settings.endGroup();
 // settings.group() == ""

See also beginGroup() and group().

bool QSettings::event ( QEvent * event ) [virtual protected]

Reimplemented from QObject::event().

bool QSettings::fallbacksEnabled () const

Returns true if fallbacks are enabled; returns false otherwise.

By default, fallbacks are enabled.

See also setFallbacksEnabled().

QString QSettings::fileName () const

Returns the path where settings written using this QSettings object are stored.

On Windows, if the format is QSettings::NativeFormat, the return value is a system registry path, not a file path.

See also isWritable() and format().

Format QSettings::format () const

Returns the format used for storing the settings.

This function was introduced in Qt 4.4.

See also defaultFormat(), fileName(), scope(), organizationName(), and applicationName().

QString QSettings::group () const

Returns the current group.

See also beginGroup() and endGroup().

QTextCodec * QSettings::iniCodec () const

Returns the codec that is used for accessing INI files. By default, no codec is used, so a null pointer is returned.

This function was introduced in Qt 4.5.

See also setIniCodec().

bool QSettings::isWritable () const

Returns true if settings can be written using this QSettings object; returns false otherwise.

One reason why isWritable() might return false is if QSettings operates on a read-only file.

Warning: This function is not perfectly reliable, because the file permissions can change at any time.

See also fileName(), status(), and sync().

QString QSettings::organizationName () const

Returns the organization name used for storing the settings.

This function was introduced in Qt 4.4.

See also QCoreApplication::organizationName(), format(), scope(), and applicationName().

Format QSettings::registerFormat ( const QString & extension, ReadFunc readFunc, WriteFunc writeFunc, Qt::CaseSensitivity caseSensitivity = Qt::CaseSensitive ) [static]

Registers a custom storage format. On success, returns a special Format value that can then be passed to the QSettings constructor. On failure, returns InvalidFormat.

The extension is the file extension associated to the format (without the '.').

The readFunc and writeFunc parameters are pointers to functions that read and write a set of key/value pairs. The QIODevice parameter to the read and write functions is always opened in binary mode (i.e., without the QIODevice::Text flag).

The caseSensitivity parameter specifies whether keys are case sensitive or not. This makes a difference when looking up values using QSettings. The default is case sensitive.

By default, if you use one of the constructors that work in terms of an organization name and an application name, the file system locations used are the same as for IniFormat. Use setPath() to specify other locations.

Example:

 bool readXmlFile(QIODevice &device, QSettings::SettingsMap &map);
 bool writeXmlFile(QIODevice &device, const QSettings::SettingsMap &map);

 int main(int argc, char *argv[])
 {
     const QSettings::Format XmlFormat =
             QSettings::registerFormat("xml", readXmlFile, writeXmlFile);

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

     ...
 }

Note: This function is thread-safe.

This function was introduced in Qt 4.1.

See also setPath().

void QSettings::remove ( const QString & key )

Removes the setting key and any sub-settings of key.

Example:

 QSettings settings;
 settings.setValue("ape");
 settings.setValue("monkey", 1);
 settings.setValue("monkey/sea", 2);
 settings.setValue("monkey/doe", 4);

 settings.remove("monkey");
 QStringList keys = settings.allKeys();
 // keys: ["ape"]

Be aware that if one of the fallback locations contains a setting with the same key, that setting will be visible after calling remove().

If key is an empty string, all keys in the current group() are removed. For example:

 QSettings settings;
 settings.setValue("ape");
 settings.setValue("monkey", 1);
 settings.setValue("monkey/sea", 2);
 settings.setValue("monkey/doe", 4);

 settings.beginGroup("monkey");
 settings.remove("");
 settings.endGroup();

 QStringList keys = settings.allKeys();
 // keys: ["ape"]

Note that the Windows registry and INI files use case-insensitive keys, whereas the Carbon Preferences API on Mac OS X uses case-sensitive keys. To avoid portability problems, see the Section and Key Syntax rules.

See also setValue(), value(), and contains().

Scope QSettings::scope () const

Returns the scope used for storing the settings.

This function was introduced in Qt 4.4.

See also format(), organizationName(), and applicationName().

void QSettings::setArrayIndex ( int i )

Sets the current array index to i. Calls to functions such as setValue(), value(), remove(), and contains() will operate on the array entry at that index.

You must call beginReadArray() or beginWriteArray() before you can call this function.

void QSettings::setDefaultFormat ( Format format ) [static]

Sets the default file format to the given format, which is used for storing settings for the QSettings(QObject *) constructor.

If no default format is set, QSettings::NativeFormat is used. See the documentation for the QSettings constructor you are using to see if that constructor will ignore this function.

This function was introduced in Qt 4.4.

See also defaultFormat() and format().

void QSettings::setFallbacksEnabled ( bool b )

Sets whether fallbacks are enabled to b.

By default, fallbacks are enabled.

See also fallbacksEnabled().

void QSettings::setIniCodec ( QTextCodec * codec )

Sets the codec for accessing INI files (including .conf files on Unix) to codec. The codec is used for decoding any data that is read from the INI file, and for encoding any data that is written to the file. By default, no codec is used, and non-ASCII characters are encoded using standard INI escape sequences.

Warning: The codec must be set immediately after creating the QSettings object, before accessing any data.

This function was introduced in Qt 4.5.

See also iniCodec().

void QSettings::setIniCodec ( const char * codecName )

This is an overloaded function.

Sets the codec for accessing INI files (including .conf files on Unix) to the QTextCodec for the encoding specified by codecName. Common values for codecName include "ISO 8859-1", "UTF-8", and "UTF-16". If the encoding isn't recognized, nothing happens.

This function was introduced in Qt 4.5.

See also QTextCodec::codecForName().

void QSettings::setPath ( Format format, Scope scope, const QString & path ) [static]

Sets the path used for storing settings for the given format and scope, to path. The format can be a custom format.

The table below summarizes the default values:

PlatformFormatScopePath
WindowsIniFormatUserScope%APPDATA%
SystemScope%COMMON_APPDATA%
UnixNativeFormat, IniFormatUserScope$HOME/.config
SystemScope/etc/xdg
Qt for Embedded LinuxNativeFormat, IniFormatUserScope$HOME/Settings
SystemScope/etc/xdg
Mac OS XIniFormatUserScope$HOME/.config
SystemScope/etc/xdg
SymbianNativeFormat, IniFormatUserScopec:/data/.config
SystemScope<drive>/private/<uid>

The default UserScope paths on Unix and Mac OS X ($HOME/.config or $HOME/Settings) can be overridden by the user by setting the XDG_CONFIG_HOME environment variable. The default SystemScope paths on Unix and Mac OS X (/etc/xdg) can be overridden when building the Qt library using the configure script's --sysconfdir flag (see QLibraryInfo for details).

Setting the NativeFormat paths on Windows and Mac OS X has no effect.

Warning: This function doesn't affect existing QSettings objects.

This function was introduced in Qt 4.1.

See also registerFormat().

void QSettings::setValue ( const QString & key, const QVariant & value )

Sets the value of setting key to value. If the key already exists, the previous value is overwritten.

Note that the Windows registry and INI files use case-insensitive keys, whereas the Carbon Preferences API on Mac OS X uses case-sensitive keys. To avoid portability problems, see the Section and Key Syntax rules.

Example:

 QSettings settings;
 settings.setValue("interval", 30);
 settings.value("interval").toInt();     // returns 30

 settings.setValue("interval", 6.55);
 settings.value("interval").toDouble();  // returns 6.55

See also value(), remove(), and contains().

Status QSettings::status () const

Returns a status code indicating the first error that was met by QSettings, or QSettings::NoError if no error occurred.

Be aware that QSettings delays performing some operations. For this reason, you might want to call sync() to ensure that the data stored in QSettings is written to disk before calling status().

See also sync().

void QSettings::sync ()

Writes any unsaved changes to permanent storage, and reloads any settings that have been changed in the meantime by another application.

This function is called automatically from QSettings's destructor and by the event loop at regular intervals, so you normally don't need to call it yourself.

See also status().

QVariant QSettings::value ( const QString & key, const QVariant & defaultValue = QVariant() ) const

Returns the value for setting key. If the setting doesn't exist, returns defaultValue.

If no default value is specified, a default QVariant is returned.

Note that the Windows registry and INI files use case-insensitive keys, whereas the Carbon Preferences API on Mac OS X uses case-sensitive keys. To avoid portability problems, see the Section and Key Syntax rules.

Example:

 QSettings settings;
 settings.setValue("animal/snake", 58);
 settings.value("animal/snake", 1024).toInt();   // returns 58
 settings.value("animal/zebra", 1024).toInt();   // returns 1024
 settings.value("animal/zebra").toInt();         // returns 0

See also setValue(), contains(), and remove().

Notes provided by the Qt Community
Informative
  • 5

Votes: 4

Coverage: Qt library 4.7, 4.8, 5.0

Picture of Andre Andre

Robot Herder
30 notes

Nokia Certified Qt Developer

Correction to sample for beginWriteArray

There is a small error in the beginWriteArray documentation. The correct version is:

  1. struct Login  {
  2.     QString userName;
  3.     QString password;
  4. };
  5. QList<Login> logins;
  6. ...
  7.  
  8. QSettings settings;
  9. settings.beginWriteArray ("logins");
  10. for  ( i = 0; i < logins.size (); ++i)  {
  11.     settings.setArrayIndex (i);
  12.     settings.setValue ("userName", logins.at (i).userName);
  13.     settings.setValue ("password", logins.at (i).password);
  14. }
  15. settings.endArray ();

The difference is that in the writing loop, the logins variable is used instead of an undeclared variable called list on lines 12 and 13.

Thanks to alfah [developer.qt.nokia.com] for noticing [developer.qt.nokia.com] .

[Revisions]

Informative
  • 5

Votes: 6

Coverage: Qt library 4.7, 4.8, 5.0

Picture of Volker Volker

Ant Farmer
35 notes

Nokia Certified Qt Developer

File is created on destruction of QSettings object

Class QSettings creates a new .ini file if it not yet present. The file is neither created on creation of the QSettings object nor on writing the first value, but on destruction of the QSettings object.

Use sync() to force the QSettings object to flush the changes to disk.

[Revisions]

Informative
  • 0

Votes: 0

Coverage: Qt 4.8

Picture of koahnig koahnig

Area 51 Engineer
7 notes

Removing an array from QSettings

In this [qt-project.org] post the question of removing an array came up.
An array stored in QSettings may be removed using remove(”“) as described here [qt-project.org]

The following example demonstrates this:

  1. #include <QSettings>
  2. #include <QList>
  3. #include <QDebug>
  4.  
  5. struct Login {
  6.     QString userName;
  7.     QString password;
  8. };
  9.  
  10.  
  11. int main(int argc, char *argv[])
  12. {
  13.     QList<Login> logins;
  14.     for ( int i = 0; i < 10; ++i )
  15.     {
  16.         Login lgin;
  17.         QString name ( "Name%1");
  18.         lgin.userName = name.arg(i);
  19.         name = "Pass%1";
  20.         lgin.password = name.arg(i);
  21.         logins.push_back ( lgin );
  22.     }
  23.  
  24.     QSettings settings ("c:/Source/creator/TestQSettings/test.ini", QSettings::IniFormat );
  25.  
  26.     settings.beginWriteArray("logins");
  27.     for (int i = 0; i < logins.size(); ++i) {
  28.         settings.setArrayIndex(i);
  29.         settings.setValue("userName", logins.at(i).userName);
  30.         settings.setValue("password", logins.at(i).password);
  31.     }
  32.     settings.endArray();
  33.  
  34.     settings.beginGroup ( "AnotherGroup");
  35.     settings.setValue ( "test", 10 );
  36.     settings.endGroup ();
  37.  
  38.     settings.beginGroup ("logins");
  39.     settings.remove ("");
  40.     settings.endGroup ();
  41.  
  42.     return 0;
  43. }

Note: This has been tested using Qt4.8.4. However, it is most likely also valid for Qt5

[Revisions]

Informative
  • 0

Votes: 0

Coverage: Qt 4.8

Picture of tilsitt tilsitt

Ant Farmer
5 notes

using custom types with QSettings

Here is a complete example on how to use custom types with QSettings:

  1. // considering a foo structure
  2.  
  3. struct FooStruct
  4. {
  5.     int foo_member;
  6. };
  7.  
  8. // define stream operators
  9.  
  10. QDataStream& operator<<(QDataStream& stream, const FooStruct& foo)
  11. {
  12.     stream<<foo.foo_member;
  13.  
  14.     return stream;
  15. }
  16.  
  17. QDataStream& operator>>(QDataStream& stream, FooStruct& foo)
  18. {
  19.     stream>>foo.foo_member;
  20.  
  21.     return stream;
  22. }
  23.  
  24. // declare FooStruct as a meta type
  25.  
  26. Q_DECLARE_METATYPE(FooStruct)
  27.  
  28. // register stream operators
  29.  
  30. qRegisterMetaTypeStreamOperators<FooStruct>("FooStruct");
  31.  
  32. // FooStruct can now be used with QSettings methods
  33.  
  34. FooStruct foo=settings.value("fooKey").value<FooStruct>();
  35.  
  36. settings.setValue("fooKey", QVariant::fromValue<FooStruct>(foo));

[Revisions]