Use of qvariant

One 、 Introduce

QT According to the official document of ：The QVariant class acts like a union for the most common Qt data types.QVariant It can store all kinds of data types ,QVariant Behavior similar to C/C++ Of union, But in Qt Middle ratio union A lot of powerful .

QVariant Built in support of all QMetaType::Type Classes declared in, such as :int,QString,QFont,QColor etc. , even to the extent that QList,QMap<QString, QVariant> Any complex type composed of .

Of course , If the supported type doesn't have what you want , No problem ,QVariant You can also support custom data types . By QVariant The stored data type needs a default constructor and a copy constructor . In order to achieve this function , First of all, you must use Q_DECLARE_METATYPE() macro . The header of this class is usually placed in the following file ：

Q_DECLARE_METATYPE(MyClass)
 

Two 、 Use

Common variables

    QVariant var1;
    var1.setValue(12);
    int data1 = var1.toInt();

    qDebug() << data1;

    QVariant var2 = 14;
    int data2 = var2.toInt();
    qDebug() << data2;

Print

 

Structure

#include <QtCore/QCoreApplication>
#include <QString>
#include <QtDebug>
#include <QVariant>

struct MyClass {
    int id;
    QString name;
};
Q_DECLARE_METATYPE(MyClass)
    
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    MyClass myClass;
    myClass.id = 0;
    myClass.name = QString("LiMing");

    QVariant var3;
    var3.setValue(myClass); // Set up QVariant Value 
    qDebug() << "var3:" << var3;

    MyClass stu; // This part of the code is mainly about QVariant Class reconverted to MyClass class , other QVariant Class can also be transformed into other classes in this way 
    stu = var3.value<MyClass>();
    qDebug() << "stu:" << stu.id << stu.name;

    return a.exec();
} 

 

Save pointer

    // preservation 
    QVariant var = QVariant::fromValue((void*)event);
    
    // obtain 
    QPaintEvent* pointer = (QPaintEvent*)var.value<void*>();

3、 ... and 、 Source code

open QVariant The definition of , We saw it use C++ Union

    struct Private
    {
        union Data
        {
            char c;
            uchar uc;
            short s;
            signed char sc;
            ushort us;
            int i;
            uint u;
            long l;
            ulong ul;
            bool b;
            double d;
            float f;
            qreal real;
            qlonglong ll;
            qulonglong ull;
            QObject *o;
            void *ptr;
            PrivateShared *shared;
        } data;
        uint type : 30;
        uint is_shared : 1;
        uint is_null : 1;
    };

Types of support

    enum Type {
        Invalid = QMetaType::UnknownType,
        Bool = QMetaType::Bool,
        Int = QMetaType::Int,
        UInt = QMetaType::UInt,
        LongLong = QMetaType::LongLong,
        ULongLong = QMetaType::ULongLong,
        Double = QMetaType::Double,
        Char = QMetaType::QChar,
        Map = QMetaType::QVariantMap,
        List = QMetaType::QVariantList,
        String = QMetaType::QString,
        StringList = QMetaType::QStringList,
        ByteArray = QMetaType::QByteArray,
        BitArray = QMetaType::QBitArray,
        Date = QMetaType::QDate,
        Time = QMetaType::QTime,
        DateTime = QMetaType::QDateTime,
        Url = QMetaType::QUrl,
        Locale = QMetaType::QLocale,
        Rect = QMetaType::QRect,
        RectF = QMetaType::QRectF,
        Size = QMetaType::QSize,
        SizeF = QMetaType::QSizeF,
        Line = QMetaType::QLine,
        LineF = QMetaType::QLineF,
        Point = QMetaType::QPoint,
        PointF = QMetaType::QPointF,
        RegExp = QMetaType::QRegExp,
        RegularExpression = QMetaType::QRegularExpression,
        Hash = QMetaType::QVariantHash,
        EasingCurve = QMetaType::QEasingCurve,
        Uuid = QMetaType::QUuid,
        ModelIndex = QMetaType::QModelIndex,
        PersistentModelIndex = QMetaType::QPersistentModelIndex,
        LastCoreType = QMetaType::LastCoreType,

        Font = QMetaType::QFont,
        Pixmap = QMetaType::QPixmap,
        Brush = QMetaType::QBrush,
        Color = QMetaType::QColor,
        Palette = QMetaType::QPalette,
        Image = QMetaType::QImage,
        Polygon = QMetaType::QPolygon,
        Region = QMetaType::QRegion,
        Bitmap = QMetaType::QBitmap,
        Cursor = QMetaType::QCursor,
        KeySequence = QMetaType::QKeySequence,
        Pen = QMetaType::QPen,
        TextLength = QMetaType::QTextLength,
        TextFormat = QMetaType::QTextFormat,
        Matrix = QMetaType::QMatrix,
        Transform = QMetaType::QTransform,
        Matrix4x4 = QMetaType::QMatrix4x4,
        Vector2D = QMetaType::QVector2D,
        Vector3D = QMetaType::QVector3D,
        Vector4D = QMetaType::QVector4D,
        Quaternion = QMetaType::QQuaternion,
        PolygonF = QMetaType::QPolygonF,
        Icon = QMetaType::QIcon,
        LastGuiType = QMetaType::LastGuiType,

        SizePolicy = QMetaType::QSizePolicy,

        UserType = QMetaType::User,
        LastType = 0xffffffff // need this so that gcc >= 3.4 allocates 32 bits for Type
    };

  Constructors

 

Reference resources ：

Qt And QVariant usage _ Snail lx The blog of -CSDN Blog _qvariant

QVariant Usage of _ Few blogs in ten years -CSDN Blog _qvariant