QSettings Class Reference
^{[QtCore module]}

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

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.

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.

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);

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.

See also setPath().

typedef QSettings::SettingsMap

Typedef for QMap<QString, QVariant>.

See also registerFormat().

enum QSettings::Status

The following status values are possible:

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);

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 QSettings::UserScope and the format is QSettings::NativeFormat.

See also 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 always QSettings::NativeFormat.

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

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.

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 QSettings::NativeFormat.

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(), and QCoreApplication::setApplicationName().

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().

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");
     login.password = settings.value("password");
     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/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();
 // group: ["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.

See also value() and setValue().

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::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().

QString QSettings::group () const

Returns the current group.

See also beginGroup() and endGroup().

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().

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 constuctor. 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::UserSettings, "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"]

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

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::setFallbacksEnabled ( bool b )

Sets whether fallbacks are enabled to b.

By default, fallbacks are enabled.

See also fallbacksEnabled().

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:

The default UserScope paths on Unix and Mac OS X ($HOME/.config) 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.

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.

void QSettings::sync ()

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

Unless you use QSettings as a communication mechanism between different processes, you normally don't need to call this function.

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.

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().