QVariant Class Reference

Detailed Description

Because C++ forbids unions from including types that have non-default constructors or destructors, most interesting Qt classes cannot be used in unions. Without QVariant, this would be a problem for QObject::property() and for database work, etc.

A QVariant object holds a single value of a single type() at a time. (Some type()s are multi-valued, for example a string list.) You can find out what type, T, the variant holds, convert it to a different type using one of the asT() functions, e.g. asSize(), get its value using one of the toT() functions, e.g. toSize(), and check whether the type can be converted to a particular type using canCast().

The methods named toT() (for any supported T, see the Type documentation for a list) are const. If you ask for the stored type, they return a copy of the stored object. If you ask for a type that can be generated from the stored type, toT() copies and converts and leaves the object itself unchanged. If you ask for a type that cannot be generated from the stored type, the result depends on the type (see the function documentation for details).

Note that three data types supported by QVariant are explicitly shared, namely QImage, QPointArray, and QCString, and in these cases the toT() methods return a shallow copy. In almost all cases you must make a deep copy of the returned values before modifying them.

The asT() functions are not const. They do conversion like the toT() methods, set the variant to hold the converted value, and return a reference to the new contents of the variant.

Here is some example code to demonstrate the use of QVariant:

    QDataStream out(...);
    QVariant v(123);          // The variant now contains an int
    int x = v.toInt();        // x = 123
    out << v;                 // Writes a type tag and an int to out
    v = QVariant("hello");    // The variant now contains a QCString
    v = QVariant(tr("hello"));// The variant now contains a QString
    int y = v.toInt();        // y = 0 since v cannot be converted to an int
    QString s = v.toString(); // s = tr("hello")  (see QObject::tr())
    out << v;                 // Writes a type tag and a QString to out
    ...
    QDataStream in(...);      // (opening the previously written stream)
    in >> v;                  // Reads an Int variant
    int z = v.toInt();        // z = 123
    qDebug("Type is %s",      // prints "Type is int"
            v.typeName());
    v.asInt() += 100;         // The variant now hold the value 223.
    v = QVariant( QStringList() );
    v.asStringList().append( "Hello" );
    

You can even store QValueLists and QMaps in a variant, so you can easily construct arbitrarily complex data structures of arbitrary types. This is very powerful and versatile, but may prove less memory and speed efficient than storing specific types in standard data structures.

QVariant also supports the notion of NULL values, where you have a defined type with no value set.

    QVariant x, y( QString() ), z( QString("") );
    x.asInt();
    // x.isNull() == TRUE, y.isNull() == TRUE, z.isNull() == FALSE
    

See the Collection Classes.

See also Miscellaneous Classes and Object Model.

Member Type Documentation

QVariant::Type

This enum type defines the types of variable that a QVariant can contain.

• QVariant::Invalid - no type
• QVariant::BitArray - a QBitArray
• QVariant::ByteArray - a QByteArray
• QVariant::Bitmap - a QBitmap
• QVariant::Bool - a bool
• QVariant::Brush - a QBrush
• QVariant::Color - a QColor
• QVariant::ColorGroup - a QColorGroup
• QVariant::Cursor - a QCursor
• QVariant::Date - a QDate
• QVariant::DateTime - a QDateTime
• QVariant::Double - a double
• QVariant::Font - a QFont
• QVariant::IconSet - a QIconSet
• QVariant::Image - a QImage
• QVariant::Int - an int
• QVariant::KeySequence - a QKeySequence
• QVariant::List - a QValueList
• QVariant::LongLong - a long long
• QVariant::ULongLong - an unsigned long long
• QVariant::Map - a QMap
• QVariant::Palette - a QPalette
• QVariant::Pen - a QPen
• QVariant::Pixmap - a QPixmap
• QVariant::Point - a QPoint
• QVariant::PointArray - a QPointArray
• QVariant::Rect - a QRect
• QVariant::Region - a QRegion
• QVariant::Size - a QSize
• QVariant::SizePolicy - a QSizePolicy
• QVariant::String - a QString
• QVariant::CString - a QCString
• QVariant::StringList - a QStringList
• QVariant::Time - a QTime
• QVariant::UInt - an unsigned int

Note that Qt's definition of bool depends on the compiler. qglobal.h has the system-dependent definition of bool.

Member Function Documentation

QVariant::QVariant ()

QVariant::QVariant ( bool val, int )

QVariant::QVariant ( double val )

QVariant::QVariant ( QSizePolicy val )

QVariant::QVariant ( const QVariant & p )

QVariant::QVariant ( QDataStream & s )

QVariant::QVariant ( const QString & val )

QVariant::QVariant ( const QCString & val )

If you want to modify the QCString after you've passed it to this constructor, we recommend passing a deep copy (see QCString::copy()).

QVariant::QVariant ( const char * val )

If val is null, the resulting variant has type Invalid.

QVariant::QVariant ( const QStringList & val )

QVariant::QVariant ( const QFont & val )

QVariant::QVariant ( const QPixmap & val )

QVariant::QVariant ( const QImage & val )

Because QImage is explicitly shared, you may need to pass a deep copy to the variant using QImage::copy(), e.g. if you intend changing the image you've passed later on.

QVariant::QVariant ( const QBrush & val )

QVariant::QVariant ( const QPoint & val )

QVariant::QVariant ( const QRect & val )

QVariant::QVariant ( const QSize & val )

QVariant::QVariant ( const QColor & val )

QVariant::QVariant ( const QPalette & val )

QVariant::QVariant ( const QColorGroup & val )

QVariant::QVariant ( const QIconSet & val )

QVariant::QVariant ( const QPointArray & val )

Because QPointArray is explicitly shared, you may need to pass a deep copy to the variant using QPointArray::copy(), e.g. if you intend changing the point array you've passed later on.

QVariant::QVariant ( const QRegion & val )

QVariant::QVariant ( const QBitmap & val )

QVariant::QVariant ( const QCursor & val )

QVariant::QVariant ( const QDate & val )

QVariant::QVariant ( const QTime & val )

QVariant::QVariant ( const QDateTime & val )

QVariant::QVariant ( const QByteArray & val )

QVariant::QVariant ( const QBitArray & val )

QVariant::QVariant ( const QKeySequence & val )

QVariant::QVariant ( const QPen & val )

QVariant::QVariant ( const QValueList<QVariant> & val )

QVariant::QVariant ( const QMap<QString, QVariant> & val )

QVariant::QVariant ( int val )

QVariant::QVariant ( uint val )

QVariant::QVariant ( Q_LLONG val )

QVariant::QVariant ( Q_ULLONG val )

QVariant::~QVariant ()

Note that subclasses that reimplement clear() should reimplement the destructor to call clear(). This destructor calls clear(), but because it is the destructor, QVariant::clear() is called rather than a subclass's clear().

QBitArray & QVariant::asBitArray ()

Tries to convert the variant to hold a QBitArray value. If that is not possible then the variant is set to an empty bitarray.

Returns a reference to the stored bitarray.

See also toBitArray().

QBitmap & QVariant::asBitmap ()

Tries to convert the variant to hold a bitmap value. If that is not possible the variant is set to a null bitmap.

Returns a reference to the stored bitmap.

See also toBitmap().

bool & QVariant::asBool ()

QBrush & QVariant::asBrush ()

Tries to convert the variant to hold a brush value. If that is not possible the variant is set to a default black brush.

Returns a reference to the stored brush.

See also toBrush().

QByteArray & QVariant::asByteArray ()

Tries to convert the variant to hold a QByteArray value. If that is not possible then the variant is set to an empty bytearray.

Returns a reference to the stored bytearray.

See also toByteArray().

QCString & QVariant::asCString ()

Tries to convert the variant to hold a string value. If that is not possible the variant is set to an empty string.

Returns a reference to the stored string.

See also toCString().

QColor & QVariant::asColor ()

Tries to convert the variant to hold a QColor value. If that is not possible the variant is set to an invalid color.

Returns a reference to the stored color.

See also toColor() and QColor::isValid().

QColorGroup & QVariant::asColorGroup ()

Tries to convert the variant to hold a QColorGroup value. If that is not possible the variant is set to a color group of all black colors.

Returns a reference to the stored color group.

See also toColorGroup().

QCursor & QVariant::asCursor ()

Tries to convert the variant to hold a QCursor value. If that is not possible the variant is set to a default arrow cursor.

Returns a reference to the stored cursor.

See also toCursor().

QDate & QVariant::asDate ()

Tries to convert the variant to hold a QDate value. If that is not possible then the variant is set to an invalid date.

Returns a reference to the stored date.

See also toDate().

QDateTime & QVariant::asDateTime ()

Tries to convert the variant to hold a QDateTime value. If that is not possible then the variant is set to an invalid date/time.

Returns a reference to the stored date/time.

See also toDateTime().

double & QVariant::asDouble ()

QFont & QVariant::asFont ()

Tries to convert the variant to hold a QFont. If that is not possible the variant is set to the application's default font.

Returns a reference to the stored font.

See also toFont().

QIconSet & QVariant::asIconSet ()

Tries to convert the variant to hold a QIconSet value. If that is not possible the variant is set to an empty iconset.

Returns a reference to the stored iconset.

See also toIconSet().

QImage & QVariant::asImage ()

Tries to convert the variant to hold an image value. If that is not possible the variant is set to a null image.

Returns a reference to the stored image.

See also toImage().

int & QVariant::asInt ()

QKeySequence & QVariant::asKeySequence ()

Tries to convert the variant to hold a QKeySequence value. If that is not possible then the variant is set to an empty key sequence.

Returns a reference to the stored key sequence.

See also toKeySequence().

QValueList<QVariant> & QVariant::asList ()

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

    QValueList<QVariant> list = myVariant.asList();
    QValueList<QVariant>::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }
    

Q_LLONG & QVariant::asLongLong ()

QMap<QString, QVariant> & QVariant::asMap ()

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

    QMap<QString, QVariant> map = myVariant.asMap();
    QMap<QString, QVariant>::Iterator it = map.begin();
    while( it != map.end() ) {
        myProcessing( *it );
        ++it;
    }
    

QPalette & QVariant::asPalette ()

Tries to convert the variant to hold a QPalette value. If that is not possible the variant is set to a palette of black colors.

Returns a reference to the stored palette.

See also toString().

QPen & QVariant::asPen ()

Tries to convert the variant to hold a QPen value. If that is not possible then the variant is set to an empty pen.

Returns a reference to the stored pen.

See also toPen().

QPixmap & QVariant::asPixmap ()

Tries to convert the variant to hold a pixmap value. If that is not possible the variant is set to a null pixmap.

Returns a reference to the stored pixmap.

See also toPixmap().

QPoint & QVariant::asPoint ()

Tries to convert the variant to hold a point value. If that is not possible the variant is set to a (0, 0) point.

Returns a reference to the stored point.

See also toPoint().

QPointArray & QVariant::asPointArray ()

Tries to convert the variant to hold a QPointArray value. If that is not possible the variant is set to an empty point array.

Returns a reference to the stored point array.

See also toPointArray().

QRect & QVariant::asRect ()

Tries to convert the variant to hold a rectangle value. If that is not possible the variant is set to an empty rectangle.

Returns a reference to the stored rectangle.

See also toRect().

QRegion & QVariant::asRegion ()

Tries to convert the variant to hold a QRegion value. If that is not possible the variant is set to a null region.

Returns a reference to the stored region.

See also toRegion().

QSize & QVariant::asSize ()

Tries to convert the variant to hold a QSize value. If that is not possible the variant is set to an invalid size.

Returns a reference to the stored size.

See also toSize() and QSize::isValid().

QSizePolicy & QVariant::asSizePolicy ()

Tries to convert the variant to hold a QSizePolicy value. If that fails, the variant is set to an arbitrary (valid) size policy.

QString & QVariant::asString ()

Tries to convert the variant to hold a string value. If that is not possible the variant is set to an empty string.

Returns a reference to the stored string.

See also toString().

QStringList & QVariant::asStringList ()

Tries to convert the variant to hold a QStringList value. If that is not possible the variant is set to an empty string list.

Returns a reference to the stored string list.

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

    QStringList list = myVariant.asStringList();
    QStringList::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }
    

See also toStringList().

QTime & QVariant::asTime ()

Tries to convert the variant to hold a QTime value. If that is not possible then the variant is set to an invalid time.

Returns a reference to the stored time.

See also toTime().

uint & QVariant::asUInt ()

Q_ULLONG & QVariant::asULongLong ()

bool QVariant::canCast ( Type t ) const

The following casts are done automatically:

bool QVariant::cast ( Type t )

See also canCast().

void QVariant::clear ()

bool QVariant::isNull () const

bool QVariant::isValid () const

Returns TRUE if the storage type of this variant is not QVariant::Invalid; otherwise returns FALSE.

QValueListConstIterator<QVariant> QVariant::listBegin () const

Returns an iterator to the first item in the list if the variant's type is appropriate; otherwise returns a null iterator.

QValueListConstIterator<QVariant> QVariant::listEnd () const

Returns the end iterator for the list if the variant's type is appropriate; otherwise returns a null iterator.

QMapConstIterator<QString, QVariant> QVariant::mapBegin () const

Returns an iterator to the first item in the map, if the variant's type is appropriate; otherwise returns a null iterator.

QMapConstIterator<QString, QVariant> QVariant::mapEnd () const

Returns the end iterator for the map, if the variant's type is appropriate; otherwise returns a null iterator.

QMapConstIterator<QString, QVariant> QVariant::mapFind ( const QString & key ) const

Returns an iterator to the item in the map with key as key, if the variant's type is appropriate and key is a valid key; otherwise returns a null iterator.

Type QVariant::nameToType ( const char * name ) [static]

If the string representation cannot be converted to any enum representation, the variant is set to Invalid.

bool QVariant::operator!= ( const QVariant & v ) const

QVariant & QVariant::operator= ( const QVariant & variant )

This is a deep copy of the variant, but note that if the variant holds an explicitly shared type such as QImage, a shallow copy is performed.

bool QVariant::operator== ( const QVariant & v ) const

QValueListConstIterator<QString> QVariant::stringListBegin () const

Returns an iterator to the first string in the list if the variant's type is StringList; otherwise returns a null iterator.

QValueListConstIterator<QString> QVariant::stringListEnd () const

Returns the end iterator for the list if the variant's type is StringList; otherwise returns a null iterator.

const QBitArray QVariant::toBitArray () const

See also asBitArray().

const QBitmap QVariant::toBitmap () const

See also asBitmap().

bool QVariant::toBool () const

Returns TRUE if the variant has type Int, UInt or Double and its value is non-zero, or if the variant has type String and its lower-case content is not empty, "0" or "false"; otherwise returns FALSE.

See also asBool().

const QBrush QVariant::toBrush () const

See also asBrush().

const QByteArray QVariant::toByteArray () const

See also asByteArray().

const QCString QVariant::toCString () const

See also asCString().

const QColor QVariant::toColor () const

See also asColor().

const QColorGroup QVariant::toColorGroup () const

See also asColorGroup().

const QCursor QVariant::toCursor () const

See also asCursor().

const QDate QVariant::toDate () const

Note that if the type() is String an invalid date will be returned if the string cannot be parsed as a Qt::ISODate format date.

See also asDate().

const QDateTime QVariant::toDateTime () const

Note that if the type() is String an invalid date/time will be returned if the string cannot be parsed as a Qt::ISODate format date/time.

See also asDateTime().

double QVariant::toDouble ( bool * ok = 0 ) const

If ok is non-null: *ok is set to TRUE if the value could be converted to a double; otherwise *ok is set to FALSE.

See also asDouble().

const QFont QVariant::toFont () const

See also asFont().

const QIconSet QVariant::toIconSet () const

See also asIconSet().

const QImage QVariant::toImage () const

See also asImage().

int QVariant::toInt ( bool * ok = 0 ) const

If ok is non-null: *ok is set to TRUE if the value could be converted to an int; otherwise *ok is set to FALSE.

See also asInt() and canCast().

const QKeySequence QVariant::toKeySequence () const

Note that not all Ints and Strings are valid key sequences and in such cases an empty key sequence will be returned.

See also asKeySequence().

const QValueList<QVariant> QVariant::toList () const

    QValueList<QVariant> list = myVariant.toList();
    QValueList<QVariant>::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }
    

    QMap<QString, QVariant> map = myVariant.toMap();
    QMap<QString, QVariant>::Iterator it = map.begin();
    while( it != map.end() ) {
        myProcessing( *it );
        ++it;
    }
    

    QStringList list = myVariant.toStringList();
    QStringList::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }