QT中的元对象系统（一）：QVariant的简单说明

QT中的元对象系统（一）：QVariant的简单说明 收藏
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作者：清林，博客名：飞空静渡

QVariant可以表示QT中的大部分类型，它和pascal中的variant类型或c中的void类型有点相似，不过它的使用和c中的union类似，其实现也是用union，在qvariant.h头文件中，我们可以看到这样定义：

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class Q_CORE_EXPORT QVariant
{
public:
enum Type {
Invalid = 0,

Bool = 1,
Int = 2,
UInt = 3,
LongLong = 4,
ULongLong = 5,
......
UserType = 127,
#ifdef QT3_SUPPORT
IconSet = Icon,
CString = ByteArray,
PointArray = Polygon,
#endif
LastType = 0xffffffff // need this so that gcc >= 3.4 allocates 32 bits for Type
};

inline QVariant();
...............
}
class Q_CORE_EXPORT QVariant
{
public:
enum Type {
Invalid = 0,

Bool = 1,
Int = 2,
UInt = 3,
LongLong = 4,
ULongLong = 5,
......
UserType = 127,
#ifdef QT3_SUPPORT
IconSet = Icon,
CString = ByteArray,
PointArray = Polygon,
#endif
LastType = 0xffffffff // need this so that gcc >= 3.4 allocates 32 bits for Type
};

inline QVariant();
...............
}

我们可以用type()成员函数来返回它的类型：Type QVariant::type () const 。

为了取得它的值，我们可以用类似的toT()这样的函数来取得，如toInt()、toString()等。

基本的使用如下：

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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());
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());

QViriant支持空值的使用，你可以定义一个空的QViriant，并在后面才给它赋值。

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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 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是定义在QtCore库中的，前面说的qvariant.h就在QtCore目录下，因此它不提供类似于toInt()、toString()这样的函数去转化QtGui中的类型，如QColor、QImage和QPixmap等。但你可以使用 QVariant::value()或者 qVariantValue ()模板函数转化。

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

相反的赋值就可以这样：

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

你可以使用 canConvert() 确定是否可以转化。

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/fjb2080/archive/2009/12/09/4972915.aspx