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QSettings Class Reference

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

#include <qsettings.h>

List of all member functions.

Public Members


Detailed Description

The QSettings class provides persistent platform-independent application settings.

On Unix systems, QSettings uses text files to store settings. On Windows systems, QSettings uses the system registry. On Mac OS X, QSettings uses the Carbon preferences API.

Each setting comprises an identifying key and the data associated with the key. A key is a unicode string which consists of two or more subkeys. A subkey is a slash, '/', followed by one or more unicode characters (excluding slashes, newlines, carriage returns and equals, '=', signs). The associated data, called the entry or value, may be a boolean, an integer, a double, a string or a list of strings. Entry strings may contain any unicode characters.

If you want to save and restore the entire desktop's settings, i.e. which applications are running, use QSettings to save the settings for each individual application and QSessionManager to save the desktop's session.

Example settings:

    /MyCompany/MyApplication/background color
    /MyCompany/MyApplication/foreground color
    /MyCompany/MyApplication/geometry/x
    /MyCompany/MyApplication/geometry/y
    /MyCompany/MyApplication/geometry/width
    /MyCompany/MyApplication/geometry/height
    /MyCompany/MyApplication/recent files/1
    /MyCompany/MyApplication/recent files/2
    /MyCompany/MyApplication/recent files/3
    
Each line above is a complete key, made up of subkeys.

A typical usage pattern for reading settings at application startup:

    QSettings settings;
    settings.setPath( "MyCompany.com", "MyApplication" );

    QString bgColor = settings.readEntry( "/colors/background", "white" );
    int width = settings.readNumEntry( "/geometry/width", 640 );
    // ...
    

A typical usage pattern for saving settings at application exit or 'save preferences':

    QSettings settings;
    settings.setPath( "MyCompany.com", "MyApplication" );

    settings.writeEntry( "/colors/background", bgColor );
    settings.writeEntry( "/geometry/width", width );
    // ...
    

A key prefix can be prepended to all keys using beginGroup(). The application of the prefix is stopped using endGroup(). For example:

    QSettings settings;

    settings.beginGroup( "/MainWindow" );
        settings.beginGroup( "/Geometry" );
            int x = settings.readEntry( "/x" );
            // ...
        settings.endGroup();
        settings.beginGroup( "/Toolbars" );
            // ...
        settings.endGroup();
    settings.endGroup();
    

You can get a list of entry-holding keys by calling entryList(), and a list of key-holding keys using subkeyList().

    QStringList keys = entryList( "/MyApplication" );
    // keys contains 'background color' and 'foreground color'.

    QStringList keys = entryList( "/MyApplication/recent files" );
    // keys contains '1', '2' and '3'.

    QStringList subkeys = subkeyList( "/MyApplication" );
    // subkeys contains 'geometry' and 'recent files'

    QStringList subkeys = subkeyList( "/MyApplication/recent files" );
    // subkeys is empty.
    

Since settings for Windows are stored in the registry there are some size limitations as follows:

These limitations are not enforced on Unix or Mac OS X.

Notes for Mac OS X Applications

The location where settings are stored is not formally defined by the CFPreferences API (for the sake of Mac OS 9 support).

At the time of writing settings are stored (either on a global or user basis, preferring locally) into a plist file in $ROOT/System/Library/Preferences (in XML format). QSettings will create an appropriate plist file (com.<first group name>.plist) out of the full path to a key.

For further information on CFPreferences see Apple's Specifications

Notes for Unix Applications

There is no universally accepted place for storing application settings under Unix. In the examples the settings file will be searched for in the following directories:

  1. SYSCONF - the default value is INSTALL/etc/settings
  2. /opt/MyCompany/share/etc
  3. /opt/MyCompany/share/MyApplication/etc
  4. $HOME/.qt
When reading settings the files are searched in the order shown above, with later settings overriding earlier settings. Files for which the user doesn't have read permission are ignored. When saving settings QSettings works in the order shown above, writing to the first settings file for which the user has write permission. (INSTALL is the directory where Qt was installed. This can be modified by using the configure script's -prefix argument )

If you want to put the settings in a particular place in the filesystem you could do this:

    settings.insertSearchPath( QSettings::Unix, "/opt/MyCompany/share" );
    

But in practice you may prefer not to use a search path for Unix. For example the following code:

    settings.writeEntry( "/MyApplication/geometry/width", width );
    
will end up writing the "geometry/width" setting to the file $HOME/.qt/myapplicationrc (assuming that the application is being run by an ordinary user, i.e. not by root).

For cross-platform applications you should ensure that the Windows size limitations are not exceeded.

See also Input/Output and Networking and Miscellaneous Classes.


Member Type Documentation

QSettings::Format

QSettings::Scope

QSettings::System


Member Function Documentation

QSettings::QSettings ()

Creates a settings object.

QSettings::QSettings ( Format format )

Creates a settings object. If format is 'Ini' the settings will be stored in a text file, using the Unix strategy (see above). If format is 'Native', the settings will be stored in a platform specific way (ie. the Windows registry).

QSettings::~QSettings ()

Destroys the settings object. All modifications made to the settings will automatically be saved.

void QSettings::beginGroup ( const QString & group )

Appends group to the current key prefix.

    QSettings settings;
    settings.beginGroup( "/MainWindow" );
    // read values
    settings.endGroup();
    

void QSettings::endGroup ()

Undo previous calls to beginGroup(). Note that a single beginGroup("a/b/c") is undone by a single call to endGroup().

    QSettings settings;
    settings.beginGroup( "/MainWindow/Geometry" );
    // read values
    settings.endGroup();
    

QStringList QSettings::entryList ( const QString & key ) const

Returns a list of the keys which contain entries under key. Does not return any keys that contain keys.

Example settings:

    /MyCompany/MyApplication/background color
    /MyCompany/MyApplication/foreground color
    /MyCompany/MyApplication/geometry/x
    /MyCompany/MyApplication/geometry/y
    /MyCompany/MyApplication/geometry/width
    /MyCompany/MyApplication/geometry/height
    
    QStringList keys = entryList( "/MyCompany/MyApplication" );
    
keys contains 'background color' and 'foreground color'. It does not contain 'geometry' because this key contains keys not entries.

To access the geometry values could either use subkeyList() to read the keys and then read each entry, or simply read each entry directly by specifying its full key, e.g. "/MyCompany/MyApplication/geometry/y".

See also subkeyList().

QString QSettings::group () const

Returns the current key prefix, or a null string if there is no key prefix set.

See also beginGroup().

void QSettings::insertSearchPath ( System s, const QString & path )

Inserts path into the settings search path. The semantics of path depends on the system s.

When s is Windows and the execution environment is not Windows the function does nothing. Similarly when s is Unix and the execution environment is not Unix the function does nothing.

When s is Windows, and the execution environment is Windows, the search path list will be used as the first subfolder of the "Software" folder in the registry.

When reading settings the folders are searched forwards from the first folder (listed below) to the last, returning the first settings found, and ignoring any folders for which the user doesn't have read permission.

  1. HKEY_CURRENT_USER/Software/MyCompany/MyApplication
  2. HKEY_LOCAL_MACHINE/Software/MyCompany/MyApplication
  3. HKEY_CURRENT_USER/Software/MyApplication
  4. HKEY_LOCAL_MACHINE/Software/MyApplication

  QSettings settings;
  settings.insertSearchPath( QSettings::Windows, "/MyCompany" );
  settings.writeEntry( "/MyApplication/Tip of the day", TRUE );
  
The code above will write the subkey "Tip of the day" into the first of the registry folders listed below that is found and for which the user has write permission.
  1. HKEY_LOCAL_MACHINE/Software/MyCompany/MyApplication
  2. HKEY_CURRENT_USER/Software/MyCompany/MyApplication
  3. HKEY_LOCAL_MACHINE/Software/MyApplication
  4. HKEY_CURRENT_USER/Software/MyApplication
If a setting is found in the HKEY_CURRENT_USER space, this setting is overwritten independently of write permissions in the HKEY_LOCAL_MACHINE space.

When s is Unix, and the execution environment is Unix, the search path list will be used when trying to determine a suitable filename for reading and writing settings files. By default, there are two entries in the search path:

  1. SYSCONF - where SYSCONF is a directory specified when configuring Qt; by default it is INSTALL/etc/settings.
  2. $HOME/.qt/ - where $HOME is the user's home directory.

All insertions into the search path will go before $HOME/.qt/. For example:

  QSettings settings;
  settings.insertSearchPath( QSettings::Unix, "/opt/MyCompany/share/etc" );
  settings.insertSearchPath( QSettings::Unix, "/opt/MyCompany/share/MyApplication/etc" );
  // ...
  
Will result in a search path of:
  1. SYSCONF
  2. /opt/MyCompany/share/etc
  3. /opt/MyCompany/share/MyApplication/etc
  4. $HOME/.qt
When reading settings the files are searched in the order shown above, with later settings overriding earlier settings. Files for which the user doesn't have read permission are ignored. When saving settings QSettings works in the order shown above, writing to the first settings file for which the user has write permission.

Settings under Unix are stored in files whose names are based on the first subkey of the key (not including the search path). The algorithm for creating names is essentially: lowercase the first subkey, replace spaces with underscores and add 'rc', e.g. /MyCompany/MyApplication/background color will be stored in myapplicationrc (assuming that /MyCompany is part of the search path).

See also removeSearchPath().

Example: chart/chartform.cpp.

bool QSettings::readBoolEntry ( const QString & key, bool def = FALSE, bool * ok = 0 ) const

Reads the entry specified by key, and returns a bool, or the default value, def, if the entry couldn't be read. If ok is non-null, *ok is set to TRUE if the key was read, FALSE otherwise.

See also readEntry(), readNumEntry(), readDoubleEntry(), writeEntry(), and removeEntry().

double QSettings::readDoubleEntry ( const QString & key, double def = 0, bool * ok = 0 ) const

Reads the entry specified by key, and returns a double, or the default value, def, if the entry couldn't be read. If ok is non-null, *ok is set to TRUE if the key was read, FALSE otherwise.

See also readEntry(), readNumEntry(), readBoolEntry(), writeEntry(), and removeEntry().

QString QSettings::readEntry ( const QString & key, const QString & def = QString::null, bool * ok = 0 ) const

Reads the entry specified by key, and returns a QString, or the default value, def, if the entry couldn't be read. If ok is non-null, *ok is set to TRUE if the key was read, FALSE otherwise.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), writeEntry(), and removeEntry().

QStringList QSettings::readListEntry ( const QString & key, bool * ok = 0 ) const

Reads the entry specified by key as a string. If ok is not 0, *ok is set to TRUE if the key was read, otherwise *ok is set to FALSE.

Note that if you want to iterate over the list, you should iterate over a copy, e.g.

    QStringList list = mySettings.readListEntry( "recentfiles" );
    QStringList::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }
    

See also readEntry(), readDoubleEntry(), readBoolEntry(), writeEntry(), removeEntry(), and QStringList::split().

QStringList QSettings::readListEntry ( const QString & key, const QChar & separator, bool * ok = 0 ) const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.

Reads the entry specified by key as a string. The separator is used to create a QStringList by calling QStringList::split(separator, entry). If ok is not 0: *ok is set to TRUE if the key was read, otherwise *ok is set to FALSE.

Warning: As the documentation states, QStringList::split() will omit empty strings from the list. Because of this, it is impossible to retrieve identical list data with this function. We recommend using the readListEntry() and writeEntry() overloads that do not take a separator argument.

Note that if you want to iterate over the list, you should iterate over a copy, e.g.

    QStringList list = mySettings.readListEntry( "size", " " );
    QStringList::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }
    

See also readEntry(), readDoubleEntry(), readBoolEntry(), writeEntry(), removeEntry(), and QStringList::split().

int QSettings::readNumEntry ( const QString & key, int def = 0, bool * ok = 0 ) const

Reads the entry specified by key, and returns an integer, or the default value, def, if the entry couldn't be read. If ok is non-null, *ok is set to TRUE if the key was read, FALSE otherwise.

See also readEntry(), readDoubleEntry(), readBoolEntry(), writeEntry(), and removeEntry().

bool QSettings::removeEntry ( const QString & key )

Removes the entry specified by key.

Returns TRUE if the entry existed and was removed; otherwise returns FALSE.

See also readEntry() and writeEntry().

void QSettings::removeSearchPath ( System s, const QString & path )

Removes all occurrences of path (using exact matching) from the settings search path for system s. Note that the default search paths cannot be removed.

See also insertSearchPath().

void QSettings::resetGroup ()

Set the current key prefix to the empty string.

void QSettings::setPath ( const QString & domain, const QString & product, Scope scope = Global )

Insert platform-dependent paths from platform-independent information.

The domain should be an Internet domain name controlled by the producer of the software, eg. Trolltech products use "trolltech.com".

The product should be the official name of the product.

The scope should be QSettings::User for user-specific settings, or QSettings::Global for system-wide settings (generally these will be read-only to many users).

Not all information is relevant on all systems (e.g. scoping is currently used only if QSettings accesses the Windows registry).

QStringList QSettings::subkeyList ( const QString & key ) const

Returns a list of the keys which contain keys under key. Does not return any keys that contain entries.

Example settings:

    /MyCompany/MyApplication/background color
    /MyCompany/MyApplication/foreground color
    /MyCompany/MyApplication/geometry/x
    /MyCompany/MyApplication/geometry/y
    /MyCompany/MyApplication/geometry/width
    /MyCompany/MyApplication/geometry/height
    /MyCompany/MyApplication/recent files/1
    /MyCompany/MyApplication/recent files/2
    /MyCompany/MyApplication/recent files/3
    
    QStringList keys = subkeyList( "/MyCompany/MyApplication" );
    
keys contains 'geometry' and 'recent files'. It does not contain 'background color' or 'foreground color' because they are keys which contain entries not keys. To get a list of keys that have values rather than subkeys use entryList().

See also entryList().

bool QSettings::writeEntry ( const QString & key, bool value )

Writes the boolean entry value into key key. The key is created if it doesn't exist. Any previous value is overwritten by value.

If an error occurs the settings are left unchanged and FALSE is returned; otherwise TRUE is returned.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), and removeEntry().

Example: chart/chartform.cpp.

bool QSettings::writeEntry ( const QString & key, double value )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes the double entry value into key key. The key is created if it doesn't exist. Any previous value is overwritten by value.

If an error occurs the settings are left unchanged and FALSE is returned; otherwise TRUE is returned.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), and removeEntry().

bool QSettings::writeEntry ( const QString & key, int value )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes the integer entry value into key key. The key is created if it doesn't exist. Any previous value is overwritten by value.

If an error occurs the settings are left unchanged and FALSE is returned; otherwise TRUE is returned.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), and removeEntry().

bool QSettings::writeEntry ( const QString & key, const QString & value )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes the string entry value into key key. The key is created if it doesn't exist. Any previous value is overwritten by value. If value is an empty string or a null string the key's value will be an empty string.

If an error occurs the settings are left unchanged and FALSE is returned; otherwise TRUE is returned.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), and removeEntry().

bool QSettings::writeEntry ( const QString & key, const QStringList & value )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes the string list entry value into key key. The key is created if it doesn't exist. Any previous value is overwritten by value.

If an error occurs the settings are left unchanged and FALSE is returned; otherwise returns TRUE.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), and removeEntry().

bool QSettings::writeEntry ( const QString & key, const QStringList & value, const QChar & separator )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.

Writes the string list entry value into key key. The key is created if it doesn't exist. Any previous value is overwritten by value. The list is stored as a sequence of strings separated by separator (using QStringList::join()), so none of the strings in the list should contain the separator. If the list is empty or null the key's value will be an empty string.

Warning: The list should not contain empty or null strings, as readListEntry() will use QStringList::split() to recreate the list. As the documentation states, QStringList::split() will omit empty strings from the list. Because of this, it is impossible to retrieve identical list data that is stored with this function. We recommend using the writeEntry() and readListEntry() overloads that do not take a separator argument.

If an error occurs the settings are left unchanged and FALSE is returned; otherwise returns TRUE.

See also readListEntry(), readNumEntry(), readDoubleEntry(), readBoolEntry(), removeEntry(), and QStringList::join().


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