QVariant Class

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

Header: #include <QVariant>
qmake: QT += core

Public Functions

QVariant(QDataStream &s)
QVariant &operator=(const QVariant &variant)
bool canConvert(int targetTypeId) const
void clear()
bool convert(int targetTypeId)
bool isNull() const
QBitArray toBitArray() const
bool toBool() const
double toDouble(bool *ok = nullptr) const
float toFloat(bool *ok = nullptr) const
QHash<QString, QVariant> toHash() const
int toInt(bool *ok = nullptr) const
QJsonArray toJsonArray() const
QJsonDocument toJsonDocument() const
QJsonObject toJsonObject() const
QJsonValue toJsonValue() const
QList<QVariant> toList() const
qlonglong toLongLong(bool *ok = nullptr) const
QMap<QString, QVariant> toMap() const
QModelIndex toModelIndex() const
QPersistentModelIndex toPersistentModelIndex() const
qreal toReal(bool *ok = nullptr) const
QString toString() const
uint toUInt(bool *ok = nullptr) const
qulonglong toULongLong(bool *ok = nullptr) const
QUuid toUuid() const
QVariant::Type type() const
const char *typeName() const
int userType() const

Static Public Members

QVariant::Type nameToType(const char *name)
const char *typeToName(int typeId)
typedef QVariantHash
typedef QVariantList
typedef QVariantMap

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 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

QVariant can be extended to support other types than those mentioned in the Type enum. See Creating Custom Qt Types for details.

A Note on GUI Types

Because QVariant is part of the Qt Core module, it cannot provide conversion functions to data types defined in Qt GUI, such as QColor, QImage, and QPixmap. In other words, there is no toColor() function. Instead, you can use the QVariant::value() or the qvariant_cast() 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(Int) 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 Function Documentation

QVariant::QVariant(QDataStream &s)

Reads the variant from the data stream, s.

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

Assigns the value of the variant variant to this variant.

bool QVariant::canConvert(int targetTypeId) const

Returns true if the variant's type can be cast to the requested type, targetTypeId. Such casting is done automatically when calling the toInt(), toBool(), ... methods.

The following casts are done automatically:

TypeAutomatically Cast To
QMetaType::BoolQMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, QMetaType::ULongLong
QMetaType::QByteArrayQMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, QMetaType::ULongLong, QMetaType::QUuid
QMetaType::QCharQMetaType::Bool, QMetaType::Int, QMetaType::UInt, QMetaType::LongLong, QMetaType::ULongLong
QMetaType::QColorQMetaType::QString
QMetaType::QDateQMetaType::QDateTime, QMetaType::QString
QMetaType::QDateTimeQMetaType::QDate, QMetaType::QString, QMetaType::QTime
QMetaType::DoubleQMetaType::Bool, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, QMetaType::ULongLong
QMetaType::QFontQMetaType::QString
QMetaType::IntQMetaType::Bool, QMetaType::QChar, QMetaType::Double, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, QMetaType::ULongLong
QMetaType::QKeySequenceQMetaType::Int, QMetaType::QString
QMetaType::QVariantListQMetaType::QStringList (if the list's items can be converted to QStrings)
QMetaType::LongLongQMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::QString, QMetaType::UInt, QMetaType::ULongLong
QMetaType::QPointQMetaType::QPointF
QMetaType::QRectQMetaType::QRectF
QMetaType::QStringQMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::QColor, QMetaType::QDate, QMetaType::QDateTime, QMetaType::Double, QMetaType::QFont, QMetaType::Int, QMetaType::QKeySequence, QMetaType::LongLong, QMetaType::QStringList, QMetaType::QTime, QMetaType::UInt, QMetaType::ULongLong, QMetaType::QUuid
QMetaType::QStringListQMetaType::QVariantList, QMetaType::QString (if the list contains exactly one item)
QMetaType::QTimeQMetaType::QString
QMetaType::UIntQMetaType::Bool, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::ULongLong
QMetaType::ULongLongQMetaType::Bool, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt
QMetaType::QUuidQMetaType::QByteArray, QMetaType::QString

A QVariant containing a pointer to a type derived from QObject will also return true for this function if a qobject_cast to the type described by targetTypeId would succeed. Note that this only works for QObject subclasses which use the Q_OBJECT macro.

A QVariant containing a sequential container will also return true for this function if the targetTypeId is QVariantList. It is possible to iterate over the contents of the container without extracting it as a (copied) QVariantList:


  QList<int> intList = {7, 11, 42};

  QVariant variant = QVariant::fromValue(intList);
  if (variant.canConvert<QVariantList>()) {
      QSequentialIterable iterable = variant.value<QSequentialIterable>();
      // Can use foreach:
      foreach (const QVariant &v, iterable) {
          qDebug() << v;
      }
      // Can use C++11 range-for:
      for (const QVariant &v : iterable) {
          qDebug() << v;
      }
      // Can use iterators:
      QSequentialIterable::const_iterator it = iterable.begin();
      const QSequentialIterable::const_iterator end = iterable.end();
      for ( ; it != end; ++it) {
          qDebug() << *it;
      }
  }

This requires that the value_type of the container is itself a metatype.

Similarly, a QVariant containing a sequential container will also return true for this function the targetTypeId is QVariantHash or QVariantMap. It is possible to iterate over the contents of the container without extracting it as a (copied) QVariantHash or QVariantMap:


  QHash<int, QString> mapping;
  mapping.insert(7, "Seven");
  mapping.insert(11, "Eleven");
  mapping.insert(42, "Forty-two");

  QVariant variant = QVariant::fromValue(mapping);
  if (variant.canConvert<QVariantHash>()) {
      QAssociativeIterable iterable = variant.value<QAssociativeIterable>();
      // Can use foreach over the values:
      foreach (const QVariant &v, iterable) {
          qDebug() << v;
      }
      // Can use C++11 range-for over the values:
      for (const QVariant &v : iterable) {
          qDebug() << v;
      }
      // Can use iterators:
      QAssociativeIterable::const_iterator it = iterable.begin();
      const QAssociativeIterable::const_iterator end = iterable.end();
      for ( ; it != end; ++it) {
          qDebug() << *it; // The current value
          qDebug() << it.key();
          qDebug() << it.value();
      }
  }

See also convert(), QSequentialIterable, Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(), QAssociativeIterable, and Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE().

void QVariant::clear()

Convert this variant to type QMetaType::UnknownType and free up any resources used.

bool QVariant::convert(int targetTypeId)

Casts the variant to the requested type, targetTypeId. If the cast cannot be done, the variant is still changed to the requested type, but is left in a cleared null state similar to that constructed by QVariant(Type).

Returns true if the current type of the variant was successfully cast; otherwise returns false.

A QVariant containing a pointer to a type derived from QObject will also convert and return true for this function if a qobject_cast to the type described by targetTypeId would succeed. Note that this only works for QObject subclasses which use the Q_OBJECT macro.

Note: converting QVariants that are null due to not being initialized or having failed a previous conversion will always fail, changing the type, remaining null, and returning false.

See also canConvert(int targetTypeId) and clear().

bool QVariant::isNull() const

Returns true if this is a null variant, false otherwise. A variant is considered null if it contains no initialized value, or the contained value is a null pointer or is an instance of a built-in type that has an isNull method, in which case the result would be the same as calling isNull on the wrapped object.

Warning: Null variants is not a single state and two null variants may easily return false on the == operator if they do not contain similar null values.

See also convert(int).

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

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.

QBitArray QVariant::toBitArray() const

Returns the variant as a QBitArray if the variant has userType() QMetaType::QBitArray; otherwise returns an empty bit array.

See also canConvert(int targetTypeId) and convert().

bool QVariant::toBool() const

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

Returns true if the variant has userType() QMetaType::Bool, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::UInt, or QMetaType::ULongLong and the value is non-zero, or if the variant has type QMetaType::QString or QMetaType::QByteArray and its lower-case content is not one of the following: empty, "0" or "false"; otherwise returns false.

See also canConvert(int targetTypeId) and convert().

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

Returns the variant as a double if the variant has userType() QMetaType::Double, QMetaType::Float, QMetaType::Bool, QMetaType::QByteArray, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, or QMetaType::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(int targetTypeId) and convert().

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

Returns the variant as a float if the variant has userType() QMetaType::Double, QMetaType::Float, QMetaType::Bool, QMetaType::QByteArray, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, or QMetaType::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(int targetTypeId) and convert().

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

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

See also canConvert(int targetTypeId) and convert().

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

Returns the variant as an int if the variant has userType() QMetaType::Int, QMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::Double, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, or QMetaType::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 QMetaType::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().

See also canConvert(int targetTypeId) and convert().

QJsonArray QVariant::toJsonArray() const

Returns the variant as a QJsonArray if the variant has userType() QJsonArray; otherwise returns a default constructed QJsonArray.

This function was introduced in Qt 5.0.

See also canConvert(int targetTypeId) and convert().

QJsonDocument QVariant::toJsonDocument() const

Returns the variant as a QJsonDocument if the variant has userType() QJsonDocument; otherwise returns a default constructed QJsonDocument.

This function was introduced in Qt 5.0.

See also canConvert(int targetTypeId) and convert().

QJsonObject QVariant::toJsonObject() const

Returns the variant as a QJsonObject if the variant has userType() QJsonObject; otherwise returns a default constructed QJsonObject.

This function was introduced in Qt 5.0.

See also canConvert(int targetTypeId) and convert().

QJsonValue QVariant::toJsonValue() const

Returns the variant as a QJsonValue if the variant has userType() QJsonValue; otherwise returns a default constructed QJsonValue.

This function was introduced in Qt 5.0.

See also canConvert(int targetTypeId) and convert().

QList<QVariant> QVariant::toList() const

Returns the variant as a QVariantList if the variant has userType() QMetaType::QVariantList or QMetaType::QStringList; otherwise returns an empty list.

See also canConvert(int targetTypeId) and convert().

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

Returns the variant as a long long int if the variant has userType() QMetaType::LongLong, QMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::QString, QMetaType::UInt, or QMetaType::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(int targetTypeId) and convert().

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

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

See also canConvert(int targetTypeId) and convert().

QModelIndex QVariant::toModelIndex() const

Returns the variant as a QModelIndex if the variant has userType() QModelIndex; otherwise returns a default constructed QModelIndex.

This function was introduced in Qt 5.0.

See also canConvert(int targetTypeId), convert(), and toPersistentModelIndex().

QPersistentModelIndex QVariant::toPersistentModelIndex() const

Returns the variant as a QPersistentModelIndex if the variant has userType() QPersistentModelIndex; otherwise returns a default constructed QPersistentModelIndex.

This function was introduced in Qt 5.5.

See also canConvert(int targetTypeId), convert(), and toModelIndex().

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

Returns the variant as a qreal if the variant has userType() QMetaType::Double, QMetaType::Float, QMetaType::Bool, QMetaType::QByteArray, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, or QMetaType::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(int targetTypeId) and convert().

QString QVariant::toString() const

Returns the variant as a QString if the variant has a userType() including, but not limited to:

QMetaType::QString, QMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::QDate, QMetaType::QDateTime, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QStringList, QMetaType::QTime, QMetaType::UInt, or QMetaType::ULongLong.

Calling QVariant::toString() on an unsupported variant returns an empty string.

See also canConvert(int targetTypeId) and convert().

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

Returns the variant as an unsigned int if the variant has userType() QMetaType::UInt, QMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, or QMetaType::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 QMetaType::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().

See also canConvert(int targetTypeId) and convert().

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

Returns the variant as an unsigned long long int if the variant has type() QMetaType::ULongLong, QMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, or QMetaType::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(int targetTypeId) and convert().

QUuid QVariant::toUuid() const

Returns the variant as a QUuid if the variant has type() QMetaType::QUuid, QMetaType::QByteArray or QMetaType::QString; otherwise returns a default-constructed QUuid.

This function was introduced in Qt 5.0.

See also canConvert(int targetTypeId) and convert().

QVariant::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.

[static] const char *QVariant::typeToName(int typeId)

Converts the int representation of the storage type, typeId, to its string representation.

Returns a null pointer if the type is QMetaType::UnknownType 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().

Related Non-Members

typedef QVariant::QVariantHash

Synonym for QHash<QString, QVariant>.

This typedef was introduced in Qt 4.5.

typedef QVariant::QVariantList

Synonym for QList<QVariant>.

typedef QVariant::QVariantMap

Synonym for QMap<QString, QVariant>.