The QVariant class acts like a union for the most common Qt data 类型。 更多...
Any Python object may be used whenever a QVariant is expected. None will be interpreted as an invalid QVariant.
The QVariant class acts like a union for the most common Qt data 类型。
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 枚举。 见 QMetaType documentation for details.
Because QVariant is part of the QtCore library, it cannot provide conversion functions to data types defined in QtGui ，譬如 QColor , QImage ，和 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>();
反向转换 (如，从 QColor to QVariant) is automatic for all data types supported by QVariant, including GUI-related types:
QColor color = palette().background().color(); QVariant variant = color;
当使用 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.
例如， 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.
This enum type defines the types of variable that a QVariant can contain.
| 常量 | 值 | 描述 |
|---|---|---|
| QVariant.Invalid | 0 | no type |
| QVariant.BitArray | 13 | a QBitArray |
| QVariant.Bitmap | 73 | a QBitmap |
| QVariant.Bool | 1 | a bool |
| QVariant.Brush | 66 | a QBrush |
| QVariant.ByteArray | 12 | a QByteArray |
| QVariant.Char | 7 | a QChar |
| QVariant.Color | 67 | a QColor |
| QVariant.Cursor | 74 | a QCursor |
| QVariant.Date | 14 | a QDate |
| QVariant.DateTime | 16 | a QDateTime |
| QVariant.Double | 6 | a double |
| QVariant.EasingCurve | 29 | a QEasingCurve |
| QVariant.Font | 64 | a QFont |
| QVariant.Hash | 28 | a QVariantHash |
| QVariant.Icon | 69 | a QIcon |
| QVariant.Image | 70 | a QImage |
| QVariant.Int | 2 | an int |
| QVariant.KeySequence | 76 | a QKeySequence |
| QVariant.Line | 23 | a QLine |
| QVariant.LineF | 24 | a QLineF |
| QVariant.List | 9 | a QVariantList |
| QVariant.Locale | 18 | a QLocale |
| QVariant.LongLong | 4 | a qlonglong |
| QVariant.Map | 8 | a QVariantMap |
| QVariant.Matrix | 80 | a QMatrix (obsolete) |
| QVariant.Transform | 81 | a QTransform |
| QVariant.Matrix4x4 | 82 | a QMatrix4x4 |
| QVariant.Palette | 68 | a QPalette |
| QVariant.Pen | 77 | a QPen |
| QVariant.Pixmap | 65 | a QPixmap |
| QVariant.Point | 25 | a QPoint |
| QVariant.PointArray | Polygon | a QPointArray |
| QVariant.PointF | 26 | a QPointF |
| QVariant.Polygon | 71 | a QPolygon |
| QVariant.Quaternion | 86 | a QQuaternion |
| QVariant.Rect | 19 | a QRect |
| QVariant.RectF | 20 | a QRectF |
| QVariant.RegExp | 27 | a QRegExp |
| QVariant.Region | 72 | a QRegion |
| QVariant.Size | 21 | a QSize |
| QVariant.SizeF | 22 | a QSizeF |
| QVariant.SizePolicy | 75 | a QSizePolicy |
| QVariant.String | 10 | a QString |
| QVariant.StringList | 11 | a QStringList |
| QVariant.TextFormat | 79 | a QTextFormat |
| QVariant.TextLength | 78 | a QTextLength |
| QVariant.Time | 15 | a QTime |
| QVariant.UInt | 3 | a uint |
| QVariant.ULongLong | 5 | a qulonglong |
| QVariant.Url | 17 | a QUrl |
| QVariant.Vector2D | 83 | a QVector2D |
| QVariant.Vector3D | 84 | a QVector3D |
| QVariant.Vector4D | 85 | a QVector4D |
| QVariant.UserType | 127 | Base value for user-defined types. |
Constructs an invalid variant.
Constructs a null variant of type type .
Constructs variant of type typeOrUserType ，和 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 QVariant.fromValue () instead to construct variants from the pointer types represented by QMetaType.VoidStar , QMetaType.QObjectStar and QMetaType.QWidgetStar .
另请参阅 QVariant.fromValue () 和 Type .
Constructs a copy of the variant, p , passed as the argument to this constructor.
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:
| Type | Automatically Cast To |
|---|---|
| Bool | Char , Double , Int , LongLong , 字符串 , UInt , ULongLong |
| ByteArray | Double , Int , LongLong , 字符串 , UInt , ULongLong |
| Char | Bool , Int , UInt , LongLong , ULongLong |
| Color | 字符串 |
| Date | DateTime , 字符串 |
| DateTime | Date , 字符串 , Time |
| Double | Bool , Int , LongLong , 字符串 , UInt , ULongLong |
| Font | 字符串 |
| Int | Bool , Char , Double , LongLong , 字符串 , UInt , ULongLong |
| KeySequence | Int , 字符串 |
| List | StringList (if the list's items can be converted to strings) |
| LongLong | Bool , ByteArray , Char , Double , Int , 字符串 , UInt , ULongLong |
| Point | PointF |
| Rect | RectF |
| 字符串 | Bool , ByteArray , Char , Color , Date , DateTime , Double , Font , Int , KeySequence , LongLong , StringList , Time , UInt , ULongLong |
| StringList | List , 字符串 (if the list contains exactly one item) |
| Time | 字符串 |
| UInt | Bool , Char , Double , Int , LongLong , 字符串 , ULongLong |
| ULongLong | Bool , Char , Double , Int , LongLong , 字符串 , UInt |
另请参阅 convert ().
Convert this variant to type Invalid and free up any resources used.
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.
警告： 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.
另请参阅 canConvert () 和 clear ().
Returns true if this is a NULL variant, false otherwise.
Returns true if the storage type of this variant is not QVariant.Invalid ；否则返回 false.
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 .
Swaps variant other with this variant. This operation is very fast and never fails.
该函数在 Qt 4.8 引入。
Returns the variant as a QBitArray if the variant has type () BitArray ; otherwise returns an empty bit array.
另请参阅 canConvert () 和 convert ().
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 ，或 ULongLong and the value is non-zero, or if the variant has type 字符串 or ByteArray and its lower-case content is not empty, "0" or "false"; otherwise returns false.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QByteArray if the variant has type () ByteArray or 字符串 (converted using QString.fromAscii ()); otherwise returns an empty byte array.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QChar 若 variant has type () Char , Int ，或 UInt ; otherwise returns an invalid QChar .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QDate 若 variant has type () Date , DateTime ，或 字符串 ; otherwise returns an invalid date.
若 type () 是 字符串 , an invalid date will be returned if the string cannot be parsed as a Qt.ISODate format date.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QDateTime if the variant has type () DateTime , Date ，或 字符串 ; otherwise returns an invalid date/time.
若 type () 是 字符串 , an invalid date/time will be returned if the string cannot be parsed as a Qt.ISODate format date/time.
另请参阅 canConvert () 和 convert ().
Returns the variant as a double if the variant has type () Double , QMetaType.Float , Bool , ByteArray , Int , LongLong , 字符串 , UInt ，或 ULongLong ; otherwise returns 0.0.
若 ok is non-null: * ok is set to true if the value could be converted to a double; otherwise * ok 被设为 false。
另请参阅 canConvert () 和 convert ().
Returns the variant as a QEasingCurve if the variant has type () EasingCurve ; otherwise returns a default easing curve.
该函数在 Qt 4.7 引入。
另请参阅 canConvert () 和 convert ().
Returns the variant as a float if the variant has type () Double , QMetaType.Float , Bool , ByteArray , Int , LongLong , 字符串 , UInt ，或 ULongLong ; otherwise returns 0.0.
若 ok is non-null: * ok is set to true if the value could be converted to a double; otherwise * ok 被设为 false。
该函数在 Qt 4.6 引入。
另请参阅 canConvert () 和 convert ().
Returns the variant as a QHash < QString , QVariant > if the variant has type () Hash ; otherwise returns an empty map.
另请参阅 canConvert () 和 convert ().
Returns the variant as an int if the variant has type () Int , Bool , ByteArray , Char , Double , LongLong , 字符串 , UInt ，或 ULongLong ; otherwise returns 0.
若 ok is non-null: * ok is set to true if the value could be converted to an int; otherwise * ok 被设为 false。
警告： 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.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QLine 若 variant has type () Line ; otherwise returns an invalid QLine .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QLineF 若 variant has type () LineF ; otherwise returns an invalid QLineF .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QVariantList 若 variant has type () List or StringList ; otherwise returns an empty list.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QLocale if the variant has type () Locale ; otherwise returns an invalid QLocale .
另请参阅 canConvert () 和 convert ().
Returns the variant as a long long int if the variant has type () LongLong , Bool , ByteArray , Char , Double , Int , 字符串 , UInt ，或 ULongLong ; otherwise returns 0.
若 ok is non-null: * ok is set to true if the value could be converted to an int; otherwise * ok 被设为 false。
另请参阅 canConvert () 和 convert ().
Returns the variant as a QMap < QString , QVariant > if the variant has type () Map ; otherwise returns an empty map.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QPoint 若 variant has type () Point or PointF ; otherwise returns a null QPoint .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QPointF if the variant has type () Point or PointF ; otherwise returns a null QPointF .
另请参阅 canConvert () 和 convert ().
Returns the variant as a qreal if the variant has type () Double , QMetaType.Float , Bool , ByteArray , Int , LongLong , 字符串 , UInt ，或 ULongLong ; otherwise returns 0.0.
若 ok is non-null: * ok is set to true if the value could be converted to a double; otherwise * ok 被设为 false。
该函数在 Qt 4.6 引入。
另请参阅 canConvert () 和 convert ().
Returns the variant as a QRect 若 variant has type () Rect ; otherwise returns an invalid QRect .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QRectF 若 variant has type () Rect or RectF ; otherwise returns an invalid QRectF .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QRegExp if the variant has type () RegExp ; otherwise returns an empty QRegExp .
该函数在 Qt 4.1 引入。
另请参阅 canConvert () 和 convert ().
Returns the variant as a QSize 若 variant has type () Size ; otherwise returns an invalid QSize .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QSizeF 若 variant has type () SizeF ; otherwise returns an invalid QSizeF .
另请参阅 canConvert () 和 convert ().
Returns the variant as a QString if the variant has type () 字符串 , Bool , ByteArray , Char , Date , DateTime , Double , Int , LongLong , StringList , Time , UInt ，或 ULongLong ; otherwise returns an empty string.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QStringList if the variant has type () StringList , 字符串 ，或 List of a type that can be converted to QString ; otherwise returns an empty list.
另请参阅 canConvert () 和 convert ().
Returns the variant as a QTime 若 variant has type () Time , DateTime ，或 字符串 ; otherwise returns an invalid time.
若 type () 是 字符串 , an invalid time will be returned if the string cannot be parsed as a Qt.ISODate format time.
另请参阅 canConvert () 和 convert ().
Returns the variant as an unsigned int if the variant has type () UInt , Bool , ByteArray , Char , Double , Int , LongLong , 字符串 ，或 ULongLong ; otherwise returns 0.
若 ok is non-null: * ok is set to true if the value could be converted to an unsigned int; otherwise * ok 被设为 false。
警告： 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 代码。
另请参阅 canConvert () 和 convert ().
Returns the variant as as an unsigned long long int if the variant has type () ULongLong , Bool , ByteArray , Char , Double , Int , LongLong , 字符串 ，或 UInt ; otherwise returns 0.
若 ok is non-null: * ok is set to true if the value could be converted to an int; otherwise * ok 被设为 false。
另请参阅 canConvert () 和 convert ().
Returns the variant as a QUrl 若 variant has type () Url ; otherwise returns an invalid QUrl .
另请参阅 canConvert () 和 convert ().
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 返回 QVariant.Char , which 如同 QMetaType.QChar , but for a variant of type char ，此函数返回 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* ，和 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 ().
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.
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.
Returns the storage type of the value stored in the variant. For non-user types, this is the same as type ().
另请参阅 type ().
