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The QSettings class provides persistent platform-independent application settings. More...
#include <QSettings>
Inherits QObject.
Note: All the functions in this class are reentrant, except registerFormat().
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.
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().
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.
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 two simple rules:
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:
(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.
| Locations | obj1 | obj2 | obj3 | obj4 |
|---|---|---|---|---|
| 1. User, Application | X | |||
| 2. User, Organization | o | X | ||
| 3. System, Application | o | X | ||
| 4. System, Organization | o | o | o | X |
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");
Sometimes you do want to access settings stored in a specific file or registry path. In that case, you can use a constructor that takes a file name (or registry path) and a file format. For example:
QSettings settings("starrunner.ini", QSettings::IniFormat);
The file format can either be QSettings::IniFormat or QSettings::NativeFormat. On Mac OS X, the native format is an XML-based format called plist. On Windows, the native format is the Windows registry, and the first argument is a path in the registry rather than a file name, for example:
QSettings settings("HKEY_CURRENT_USER\\Software\\Microsoft",
QSettings::NativeFormat);
On Unix systems, QSettings::IniFormat and QSettings::NativeFormat have the same meaning.
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 we will 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.
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. Changes performed by another process aren't visible in the current process until sync() is called.
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 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:
On Mac OS X versions 10.2 and 10.3, these files are used by default:
On Windows, NativeFormat settings are stored in the following registry paths:
If the file format is IniFormat, the following files are used on Unix and Mac OS X:
On Windows, the following files are used:
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.
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 by setting the XDG_CONFIG_HOME environment variable; see setPath() for details.
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:
#ifdef Q_WS_MAC
QSettings settings("grenoullelogique.fr", "Squash");
#else
QSettings settings("Grenoulle Logique", "Squash");
#endif
See also QVariant, QSessionManager, and Settings Editor Example.
This enum type specifies the storage format used by QSettings.
| Constant | Value | Description |
|---|---|---|
| QSettings::NativeFormat | 0 | Store 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::IniFormat | 1 | Store the settings in INI files. |
| QSettings::InvalidFormat | 16 | Special 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:
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.
windir = C:\Windows
QSettings always treats backslash as a special character and provides no API for reading or writing such entries.
See also registerFormat() and setPath().
Typedef for a pointer to a function with the following signature:
bool myReadFunc(QIODevice &device, QSettings::SettingsMap &map);
See also WriteFunc and registerFormat().
This enum specifies whether settings are user-specific or shared by all users of the same system.
| Constant | Value | Description |
|---|---|---|
| QSettings::UserScope | 0 | Store settings in a location specific to the current user (e.g., in the user's home directory). |
| QSettings::SystemScope | 1 | Store settings in a global location, so that all users on the same machine access the same set of settings. |
See also setPath().
Typedef for QMap<QString, QVariant>.
See also registerFormat().
The following status values are possible:
| Constant | Value | Description |
|---|---|---|
| QSettings::NoError | 0 | No error occurred. |
| QSettings::AccessError | 1 | An access error occurred (e.g. trying to write to a read-only file). |
| QSettings::FormatError | 2 | A format error occurred (e.g. loading a malformed INI file). |
See also status().
Typedef for a pointer to a function with the following signature:
bool myWriteFunc(QIODevice &device, const QSettings::SettingsMap &map);
See also ReadFunc and registerFormat().
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.
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.
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.
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().
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().
Destroys the QSettings object.
Any unsaved changes will eventually be written to permanent storage.
See also sync().
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().
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:
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().
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().
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
To read back an array, use beginReadArray().
See also beginReadArray(), endArray(), and setArrayIndex().
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().
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().
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().
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().
Closes the array that was started using beginReadArray() or beginWriteArray().
See also beginReadArray() and beginWriteArray().
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().
Returns true if fallbacks are enabled; returns false otherwise.
By default, fallbacks are enabled.
See also setFallbacksEnabled().
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().
Returns the current group.
See also beginGroup() and endGroup().
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().
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", readXmlFunc, writeXmlFunc);
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().
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().
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.
Sets whether fallbacks are enabled to b.
By default, fallbacks are enabled.
See also fallbacksEnabled().
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:
| Platform | Format | Scope | Path |
|---|---|---|---|
| Windows | IniFormat | UserScope | %APPDATA% |
| SystemScope | %COMMON_APPDATA% | ||
| Unix | NativeFormat, IniFormat | UserScope | $HOME/.config |
| SystemScope | /etc/xdg | ||
| Mac OS X | IniFormat | UserScope | $HOME/.config |
| SystemScope | /etc/xdg |
The default UserScope paths on Unix and Mac OS X ($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().
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().
Returns a status code indicating the first error that was met by QSettings, or QSettings::NoError if no error occurred.
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.
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().
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