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QVariant Class Reference
[QtCore module]

The QVariant class acts like a union for the most common Qt data types. More...

 #include <QVariant>

Inherited by QDBusVariant.

• List of all members, including inherited members
• Qt 3 support members

Public Types

Public Functions

Static Public Members

Related Non-Members

Detailed Description

The QVariant class acts like a union for the most common Qt data types.

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 convert(), 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 canConvert().

The methods named toT() (e.g., toInt(), toString()) 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.

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 QByteArray
 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 = v.toInt() + 100;            // The variant now hold the value 223
 v = QVariant(QStringList());

You can even store QList<QVariant> and QMap<QString, QVariant> values 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 can have a defined type with no value set. However, note that QVariant types can only be cast when they have had a value set.

 QVariant x, y(QString()), z(QString(""));
 x.convert(QVariant::Int);
 // x.isNull() == true
 // y.isNull() == true, z.isNull() == false
 // y.isEmpty() == true, z.isEmpty() == true

QVariant can be extended to support other types than those mentioned in the Type enum. See the QMetaType documentation for details.

A Note on GUI Types

Because QVariant is part of the QtCore library, it cannot provide conversion functions to data types defined in QtGui, such as QColor, QImage, and QPixmap. In other words, there is no toColor() function. Instead, you can use the QVariant::value() or the qVariantValue() template function. For example:

 QVariant variant;
 ...
 QColor color = variant.value<QColor>();

The inverse conversion (e.g., from QColor to QVariant) is automatic for all data types supported by QVariant, including GUI-related types:

 QColor color = palette().background().color();
 QVariant variant = color;

Using canConvert() and convert() Consecutively

When using canConvert() and convert() consecutively, it is possible for canConvert() to return true, but convert() to return false. This is typically because canConvert() only reports the general ability of QVariant to convert between types given suitable data; it is still possible to supply data which cannot actually be converted.

For example, canConvert() would return true when called on a variant containing a string because, in principle, QVariant is able to convert strings of numbers to integers. However, if the string contains non-numeric characters, it cannot be converted to an integer, and any attempt to convert it will fail. Hence, it is important to have both functions return true for a successful conversion.

See also QMetaType.

Member Type Documentation

enum QVariant::Type

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

Member Function Documentation

QVariant::QVariant ()

Constructs an invalid variant.

QVariant::QVariant ( const QLocale & l )

Constructs a new variant with a locale value, l.

QVariant::QVariant ( const QRegExp & regExp )

Constructs a new variant with the regexp value regExp.

QVariant::QVariant ( Qt::GlobalColor color )

Constructs a new variant of type QVariant::Color and initializes it with color.

This is a convenience constructor that allows QVariant(Qt::blue); to create a valid QVariant storing a QColor.

Note: This constructor will assert if the application does not link to the Qt GUI library.

This function was introduced in Qt 4.2.

QVariant::QVariant ( Type type )

Constructs a null variant of type type.

QVariant::QVariant ( int typeOrUserType, const void * copy )

Constructs variant of type typeOrUserType, and initializes with copy if copy is not 0.

Note that you have to pass the address of the variable you want stored.

Usually, you never have to use this constructor, use qVariantFromValue() instead to construct variants from the pointer types represented by QMetaType::VoidStar, QMetaType::QObjectStar and QMetaType::QWidgetStar.

See also qVariantFromValue() and Type.

QVariant::QVariant ( const QVariant & p )

Constructs a copy of the variant, p, passed as the argument to this constructor.

QVariant::QVariant ( QDataStream & s )

Reads the variant from the data stream, s.

QVariant::QVariant ( int val )

Constructs a new variant with an integer value, val.

QVariant::QVariant ( uint val )

Constructs a new variant with an unsigned integer value, val.

QVariant::QVariant ( qlonglong val )

Constructs a new variant with a long long integer value, val.

QVariant::QVariant ( qulonglong val )

Constructs a new variant with an unsigned long long integer value, val.

QVariant::QVariant ( bool val )

Constructs a new variant with a boolean value, val.

QVariant::QVariant ( double val )

Constructs a new variant with a floating point value, val.

QVariant::QVariant ( float val )

Constructs a new variant with a floating point value, val.

This function was introduced in Qt 4.6.

QVariant::QVariant ( const char * val )

Constructs a new variant with a string value of val. The variant creates a deep copy of val, using the encoding set by QTextCodec::setCodecForCStrings().

Note that val is converted to a QString for storing in the variant and QVariant::type() will return QMetaType::QString for the variant.

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications.

See also QTextCodec::setCodecForCStrings().

QVariant::QVariant ( const QByteArray & val )

Constructs a new variant with a bytearray value, val.

QVariant::QVariant ( const QBitArray & val )

Constructs a new variant with a bitarray value, val.

QVariant::QVariant ( const QString & val )

Constructs a new variant with a string value, val.

QVariant::QVariant ( const QLatin1String & val )

Constructs a new variant with a string value, val.

QVariant::QVariant ( const QStringList & val )

Constructs a new variant with a string list value, val.

QVariant::QVariant ( const QChar & c )

Constructs a new variant with a char value, c.

QVariant::QVariant ( const QDate & val )

Constructs a new variant with a date value, val.

QVariant::QVariant ( const QTime & val )

Constructs a new variant with a time value, val.

QVariant::QVariant ( const QDateTime & val )

Constructs a new variant with a date/time value, val.

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

Constructs a new variant with a list value, val.

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

Constructs a new variant with a map of QVariants, val.

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

Constructs a new variant with a hash of QVariants, val.

QVariant::QVariant ( const QSize & val )

Constructs a new variant with a size value of val.

QVariant::QVariant ( const QSizeF & val )

Constructs a new variant with a size value of val.

QVariant::QVariant ( const QPoint & val )

Constructs a new variant with a point value of val.

QVariant::QVariant ( const QPointF & val )

Constructs a new variant with a point value of val.

QVariant::QVariant ( const QLine & val )

Constructs a new variant with a line value of val.

QVariant::QVariant ( const QLineF & val )

Constructs a new variant with a line value of val.

QVariant::QVariant ( const QRect & val )

Constructs a new variant with a rect value of val.

QVariant::QVariant ( const QRectF & val )

Constructs a new variant with a rect value of val.

QVariant::QVariant ( const QUrl & val )

Constructs a new variant with a url value of val.

QVariant::~QVariant ()

Destroys the QVariant and the contained object.

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().

bool QVariant::canConvert ( Type t ) const

Returns true if the variant's type can be cast to the requested type, t. Such casting is done automatically when calling the toInt(), toBool(), ... methods.

The following casts are done automatically:

See also convert().

bool QVariant::canConvert () const

Returns true if the variant can be converted to the template type T, otherwise false.

Example:

 QVariant v = 42;

 v.canConvert<int>();              // returns true
 v.canConvert<QString>();          // returns true

 MyCustomStruct s;
 v.setValue(s);

 v.canConvert<int>();              // returns false
 v.canConvert<MyCustomStruct>();   // returns true

Warning: This function is not available with MSVC 6. Use qVariantCanConvert() instead if you need to support that version of the compiler.

See also convert().

void QVariant::clear ()

Convert this variant to type Invalid and free up any resources used.

bool QVariant::convert ( Type t )

Casts the variant to the requested type, t. If the cast cannot be done, the variant is cleared. Returns true if the current type of the variant was successfully cast; otherwise returns false.

Warning: For historical reasons, converting a null QVariant results in a null value of the desired type (e.g., an empty string for QString) and a result of false.

See also canConvert() and clear().

QVariant QVariant::fromValue ( const T & value )   [static]

Returns a QVariant containing a copy of value. Behaves exactly like setValue() otherwise.

Example:

 MyCustomStruct s;
 return QVariant::fromValue(s);

Note: If you are working with custom types, you should use the Q_DECLARE_METATYPE() macro to register your custom type.

Warning: This function is not available with MSVC 6. Use qVariantFromValue() instead if you need to support that version of the compiler.

See also setValue() and value().

bool QVariant::isNull () const

Returns true if this is a NULL variant, false otherwise.

bool QVariant::isValid () const

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

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

Converts the string representation of the storage type given in name, to its enum representation.

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

void QVariant::setValue ( const T & value )

Stores a copy of value. If T is a type that QVariant doesn't support, QMetaType is used to store the value. A compile error will occur if QMetaType doesn't handle the type.

Example:

 QVariant v;

 v.setValue(5);
 int i = v.toInt();         // i is now 5
 QString s = v.toString()   // s is now "5"

 MyCustomStruct c;
 v.setValue(c);

 ...

 MyCustomStruct c2 = v.value<MyCustomStruct>();

Warning: This function is not available with MSVC 6. Use qVariantSetValue() instead if you need to support that version of the compiler.

See also value(), fromValue(), and canConvert().

QBitArray QVariant::toBitArray () const

Returns the variant as a QBitArray if the variant has type() BitArray; otherwise returns an empty bit array.

See also canConvert() and convert().

bool QVariant::toBool () const

Returns the variant as a bool if the variant has type() Bool.

Returns true if the variant has type() Bool, Char, Double, Int, LongLong, UInt, or ULongLong and the value is non-zero, or if the variant has type String or ByteArray and its lower-case content is not empty, "0" or "false"; otherwise returns false.

See also canConvert() and convert().

QByteArray QVariant::toByteArray () const

Returns the variant as a QByteArray if the variant has type() ByteArray or String (converted using QString::fromAscii()); otherwise returns an empty byte array.

See also canConvert() and convert().

QChar QVariant::toChar () const

Returns the variant as a QChar if the variant has type() Char, Int, or UInt; otherwise returns an invalid QChar.

See also canConvert() and convert().

QDate QVariant::toDate () const

Returns the variant as a QDate if the variant has type() Date, DateTime, or String; otherwise returns an invalid date.

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 canConvert() and convert().

QDateTime QVariant::toDateTime () const

Returns the variant as a QDateTime if the variant has type() DateTime, Date, or String; otherwise returns an invalid date/time.

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 canConvert() and convert().

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

Returns the variant as a double if the variant has type() Double, QMetaType::Float, Bool, ByteArray, Int, LongLong, String, UInt, or ULongLong; otherwise returns 0.0.

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 canConvert() and convert().

float QVariant::toFloat ( bool * ok = 0 ) const

Returns the variant as a float if the variant has type() Double, QMetaType::Float, Bool, ByteArray, Int, LongLong, String, UInt, or ULongLong; otherwise returns 0.0.

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.

This function was introduced in Qt 4.6.

See also canConvert() and convert().

QHash<QString, QVariant> QVariant::toHash () const

Returns the variant as a QHash<QString, QVariant> if the variant has type() Hash; otherwise returns an empty map.

See also canConvert() and convert().

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

Returns the variant as an int if the variant has type() Int, Bool, ByteArray, Char, Double, LongLong, String, UInt, or ULongLong; otherwise returns 0.

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.

Warning: If the value is convertible to a LongLong but is too large to be represented in an int, the resulting arithmetic overflow will not be reflected in ok. A simple workaround is to use QString::toInt(). Fixing this bug has been postponed to Qt 5 in order to avoid breaking existing code.

See also canConvert() and convert().

QLine QVariant::toLine () const

Returns the variant as a QLine if the variant has type() Line; otherwise returns an invalid QLine.

See also canConvert() and convert().

QLineF QVariant::toLineF () const

Returns the variant as a QLineF if the variant has type() LineF; otherwise returns an invalid QLineF.

See also canConvert() and convert().

QList<QVariant> QVariant::toList () const

Returns the variant as a QVariantList if the variant has type() List or StringList; otherwise returns an empty list.

See also canConvert() and convert().

QLocale QVariant::toLocale () const

Returns the variant as a QLocale if the variant has type() Locale; otherwise returns an invalid QLocale.

See also canConvert() and convert().

qlonglong QVariant::toLongLong ( bool * ok = 0 ) const

Returns the variant as a long long int if the variant has type() LongLong, Bool, ByteArray, Char, Double, Int, String, UInt, or ULongLong; otherwise returns 0.

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 canConvert() and convert().

QMap<QString, QVariant> QVariant::toMap () const

Returns the variant as a QMap<QString, QVariant> if the variant has type() Map; otherwise returns an empty map.

See also canConvert() and convert().

QPoint QVariant::toPoint () const

Returns the variant as a QPoint if the variant has type() Point or PointF; otherwise returns a null QPoint.

See also canConvert() and convert().

QPointF QVariant::toPointF () const

Returns the variant as a QPointF if the variant has type() Point or PointF; otherwise returns a null QPointF.

See also canConvert() and convert().

qreal QVariant::toReal ( bool * ok = 0 ) const

Returns the variant as a qreal if the variant has type() Double, QMetaType::Float, Bool, ByteArray, Int, LongLong, String, UInt, or ULongLong; otherwise returns 0.0.

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.

This function was introduced in Qt 4.6.

See also canConvert() and convert().

QRect QVariant::toRect () const

Returns the variant as a QRect if the variant has type() Rect; otherwise returns an invalid QRect.

See also canConvert() and convert().

QRectF QVariant::toRectF () const

Returns the variant as a QRectF if the variant has type() Rect or RectF; otherwise returns an invalid QRectF.

See also canConvert() and convert().

QRegExp QVariant::toRegExp () const

Returns the variant as a QRegExp if the variant has type() RegExp; otherwise returns an empty QRegExp.

This function was introduced in Qt 4.1.

See also canConvert() and convert().

QSize QVariant::toSize () const

Returns the variant as a QSize if the variant has type() Size; otherwise returns an invalid QSize.

See also canConvert() and convert().

QSizeF QVariant::toSizeF () const

Returns the variant as a QSizeF if the variant has type() SizeF; otherwise returns an invalid QSizeF.

See also canConvert() and convert().

QString QVariant::toString () const

Returns the variant as a QString if the variant has type() String, Bool, ByteArray, Char, Date, DateTime, Double, Int, LongLong, StringList, Time, UInt, or ULongLong; otherwise returns an empty string.

See also canConvert() and convert().

QStringList QVariant::toStringList () const

Returns the variant as a QStringList if the variant has type() StringList, String, or List of a type that can be converted to QString; otherwise returns an empty list.

See also canConvert() and convert().

QTime QVariant::toTime () const

Returns the variant as a QTime if the variant has type() Time, DateTime, or String; otherwise returns an invalid time.

If the type() is String, an invalid time will be returned if the string cannot be parsed as a Qt::ISODate format time.

See also canConvert() and convert().

uint QVariant::toUInt ( bool * ok = 0 ) const

Returns the variant as an unsigned int if the variant has type() UInt, Bool, ByteArray, Char, Double, Int, LongLong, String, or ULongLong; otherwise returns 0.

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

Warning: If the value is convertible to a ULongLong but is too large to be represented in an unsigned int, the resulting arithmetic overflow will not be reflected in ok. A simple workaround is to use QString::toUInt(). Fixing this bug has been postponed to Qt 5 in order to avoid breaking existing code.

See also canConvert() and convert().

qulonglong QVariant::toULongLong ( bool * ok = 0 ) const

Returns the variant as as an unsigned long long int if the variant has type() ULongLong, Bool, ByteArray, Char, Double, Int, LongLong, String, or UInt; otherwise returns 0.

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 canConvert() and convert().

QUrl QVariant::toUrl () const

Returns the variant as a QUrl if the variant has type() Url; otherwise returns an invalid QUrl.

See also canConvert() and convert().

Type QVariant::type () const

Returns the storage type of the value stored in the variant. Although this function is declared as returning QVariant::Type, the return value should be interpreted as QMetaType::Type. In particular, QVariant::UserType is returned here only if the value is equal or greater than QMetaType::User.

Note that return values in the ranges QVariant::Char through QVariant::RegExp and QVariant::Font through QVariant::Transform correspond to the values in the ranges QMetaType::QChar through QMetaType::QRegExp and QMetaType::QFont through QMetaType::QQuaternion.

Pay particular attention when working with char and QChar variants. Note that there is no QVariant constructor specifically for type char, but there is one for QChar. For a variant of type QChar, this function returns QVariant::Char, which is the same as QMetaType::QChar, but for a variant of type char, this function returns QMetaType::Char, which is not the same as QVariant::Char.

Also note that the types void*, long, short, unsigned long, unsigned short, unsigned char, float, QObject*, and QWidget* are represented in QMetaType::Type but not in QVariant::Type, and they can be returned by this function. However, they are considered to be user defined types when tested against QVariant::Type.

To test whether an instance of QVariant contains a data type that is compatible with the data type you are interested in, use canConvert().

const char * QVariant::typeName () const

Returns the name of the type stored in the variant. The returned strings describe the C++ datatype used to store the data: for example, "QFont", "QString", or "QVariantList". An Invalid variant returns 0.

const char * QVariant::typeToName ( Type typ )   [static]

Converts the enum representation of the storage type, typ, to its string representation.

Returns a null pointer if the type is QVariant::Invalid or doesn't exist.

int QVariant::userType () const

Returns the storage type of the value stored in the variant. For non-user types, this is the same as type().

See also type().

T QVariant::value () const

Returns the stored value converted to the template type T. Call canConvert() to find out whether a type can be converted. If the value cannot be converted, default-constructed value will be returned.

If the type T is supported by QVariant, this function behaves exactly as toString(), toInt() etc.

Example:

 QVariant v;

 MyCustomStruct c;
 if (v.canConvert<MyCustomStruct>())
     c = v.value<MyCustomStruct>(v);

 v = 7;
 int i = v.value<int>();                        // same as v.toInt()
 QString s = v.value<QString>();                // same as v.toString(), s is now "7"
 MyCustomStruct c2 = v.value<MyCustomStruct>(); // conversion failed, c2 is empty

Warning: This function is not available with MSVC 6. Use qVariantValue() or qvariant_cast() instead if you need to support that version of the compiler.

See also setValue(), fromValue(), and canConvert().

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

Compares this QVariant with v and returns true if they are not equal; otherwise returns false.

Warning: This function doesn't support custom types registered with qRegisterMetaType().

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

Assigns the value of the variant variant to this variant.

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

Compares this QVariant with v and returns true if they are equal; otherwise returns false.

In the case of custom types, their equalness operators are not called. Instead the values' addresses are compared.

Related Non-Members

typedef QVariantHash

Synonym for QHash<QString, QVariant>.

This typedef was introduced in Qt 4.5.

typedef QVariantList

Synonym for QList<QVariant>.

typedef QVariantMap

Synonym for QMap<QString, QVariant>.

bool qVariantCanConvert ( const QVariant & value )

Returns true if the given value can be converted to the template type specified; otherwise returns false.

This function is equivalent to QVariant::canConvert(value). It is provided as a work-around for MSVC 6, which doesn't support member template functions.

See also QVariant::canConvert().

QVariant qVariantFromValue ( const T & value )

Returns a variant containing a copy of the given value with template type T.

This function is equivalent to QVariant::fromValue(value). It is provided as a work-around for MSVC 6, which doesn't support member template functions.

For example, a QObject pointer can be stored in a variant with the following code:

 QObject *object = getObjectFromSomewhere();
 QVariant data = qVariantFromValue(object);

See also QVariant::fromValue().

void qVariantSetValue ( QVariant & variant, const T & value )

Sets the contents of the given variant to a copy of the value with the specified template type T.

This function is equivalent to QVariant::setValue(value). It is provided as a work-around for MSVC 6, which doesn't support member template functions.

See also QVariant::setValue().

T qVariantValue ( const QVariant & value )

Returns the given value converted to the template type T.

This function is equivalent to QVariant::value<T>(value). It is provided as a work-around for MSVC 6, which doesn't support member template functions.

See also QVariant::value() and qvariant_cast().

T qvariant_cast ( const QVariant & value )

Returns the given value converted to the template type T.

This function is equivalent to qVariantValue().

See also qVariantValue() and QVariant::value().

bool operator!= ( const QVariant & v1, const QVariant & v2 )

Returns false if v1 and v2 are equal; otherwise returns true.

Warning: This function doesn't support custom types registered with qRegisterMetaType().

bool operator== ( const QVariant & v1, const QVariant & v2 )

Returns true if v1 and v2 are equal; otherwise returns false.

Warning: This function doesn't support custom types registered with qRegisterMetaType().