QVariant是一種可以存儲不同類型的數據結構，在很多場合這是很有用得
為了達到這種目的，可以想象，該對象應該存儲對象的類型信息，數據信息以及其他輔助詳細
考慮用途，這種對象必須支持對不同對象的存儲，對存儲類型的檢測以及取對象三個功能
1.對象的存儲
代碼見下：
QVariant(Type type);
QVariant(int typeOrUserType, const void *copy);
QVariant(int typeOrUserType, const void *copy, uint flags);
QVariant(const QVariant &other);
#ifndef QT_NO_DATASTREAM
QVariant(QDataStream &s);
#endif
QVariant(int i);
QVariant(uint ui);
QVariant(qlonglong ll);
QVariant(qulonglong ull);
QVariant(bool b);
QVariant(double d);
QVariant(float f) { d.is_null = false; d.type = QMetaType::Float; d.data.f = f; }
#ifndef QT_NO_CAST_FROM_ASCII
QT_ASCII_CAST_WARN_CONSTRUCTOR QVariant(const char *str);
#endif
QVariant(const QByteArray &bytearray);
QVariant(const QBitArray &bitarray);
QVariant(const QString &string);
QVariant(const QLatin1String &string);
QVariant(const QStringList &stringlist);
QVariant(const QChar &qchar);
QVariant(const QDate &date);
QVariant(const QTime &time);
QVariant(const QDateTime &datetime);
QVariant(const QList<QVariant> &list);
QVariant(const QMap<QString,QVariant> &map);
QVariant(const QHash<QString,QVariant> &hash);
#ifndef QT_NO_GEOM_VARIANT
QVariant(const QSize &size);
QVariant(const QSizeF &size);
QVariant(const QPoint &pt);
QVariant(const QPointF &pt);
QVariant(const QLine &line);
QVariant(const QLineF &line);
QVariant(const QRect &rect);
QVariant(const QRectF &rect);
#endif
QVariant(const QUrl &url);
QVariant(const QLocale &locale);
#ifndef QT_NO_REGEXP
QVariant(const QRegExp ®Exp);
#endif
#ifndef QT_BOOTSTRAPPED
QVariant(const QEasingCurve &easing);
#endif
QVariant(Qt::GlobalColor color);
2.QVariant Type
在該對象中type負責記錄對象的類型，這對于正確取出對象是很用得
3.成員函數
Type type() const;
int userType() const;
const char *typeName() const;
bool canConvert(Type t) const;
bool convert(Type t);
#ifdef QT3_SUPPORT
inline QT3_SUPPORT bool canCast(Type t) const
{ return canConvert(t); }
inline QT3_SUPPORT bool cast(Type t)
{ return convert(t); }
#endif
這幾個函數的用途很顯然，看看其中一個的實現吧
bool QVariant::canConvert(Type t) const
{
//we can treat floats as double
//the reason for not doing it the "proper" way is that QMetaType::Float's value is 135,
//which can't be handled by qCanConvertMatrix
//In addition QVariant::Type doesn't have a Float value, so we're using QMetaType::Float
const uint currentType = ((d.type == QMetaType::Float) ? QVariant::Double : d.type);
if (uint(t) == uint(QMetaType::Float)) t = QVariant::Double;
if (currentType == uint(t))
return true;
if (currentType > QVariant::LastCoreType || t > QVariant::LastCoreType) {
switch (uint(t)) {
case QVariant::Int:
return currentType == QVariant::KeySequence
|| currentType == QMetaType::ULong
|| currentType == QMetaType::Long
|| currentType == QMetaType::UShort
|| currentType == QMetaType::UChar
|| currentType == QMetaType::Char
|| currentType == QMetaType::Short;
case QVariant::Image:
return currentType == QVariant::Pixmap || currentType == QVariant::Bitmap;
case QVariant::Pixmap:
return currentType == QVariant::Image || currentType == QVariant::Bitmap
|| currentType == QVariant::Brush;
case QVariant::Bitmap:
return currentType == QVariant::Pixmap || currentType == QVariant::Image;
case QVariant::ByteArray:
return currentType == QVariant::Color;
case QVariant::String:
return currentType == QVariant::KeySequence || currentType == QVariant::Font
|| currentType == QVariant::Color;
case QVariant::KeySequence:
return currentType == QVariant::String || currentType == QVariant::Int;
case QVariant::Font:
return currentType == QVariant::String;
case QVariant::Color:
return currentType == QVariant::String || currentType == QVariant::ByteArray
|| currentType == QVariant::Brush;
case QVariant::Brush:
return currentType == QVariant::Color || currentType == QVariant::Pixmap;
case QMetaType::Long:
case QMetaType::Char:
case QMetaType::UChar:
case QMetaType::ULong:
case QMetaType::Short:
case QMetaType::UShort:
return qCanConvertMatrix[QVariant::Int] & (1 << currentType) || currentType == QVariant::Int;
default:
return false;
}
}
if(t == String && currentType == StringList)
return v_cast<QStringList>(&d)->count() == 1;
else
return qCanConvertMatrix[t] & (1 << currentType);
}
該函數作用是檢測存儲對象是否可以轉換為輸入類型，具體實現很明了
4.QVariant對象的最后一個主要的功能就是到給定類型的轉換
函數簇如下：
int toInt(bool *ok = 0) const;
uint toUInt(bool *ok = 0) const;
qlonglong toLongLong(bool *ok = 0) const;
qulonglong toULongLong(bool *ok = 0) const;
bool toBool() const;
double toDouble(bool *ok = 0) const;
float toFloat(bool *ok = 0) const;
qreal toReal(bool *ok = 0) const;
QByteArray toByteArray() const;
QBitArray toBitArray() const;
QString toString() const;
QStringList toStringList() const;
QChar toChar() const;
QDate toDate() const;
QTime toTime() const;
QDateTime toDateTime() const;
QList<QVariant> toList() const;
QMap<QString, QVariant> toMap() const;
QHash<QString, QVariant> toHash() const;
其一個實現如下:
/*!
\fn QTime QVariant::toTime() const
Returns the variant as a QTime if the variant has type() \l Time,
\l DateTime, or \l String; otherwise returns an invalid time.
If the type() is \l String, an invalid time will be returned if
the string cannot be parsed as a Qt::ISODate format time.
\sa canConvert(), convert()
*/
QTime QVariant::toTime() const
{
return qVariantToHelper<QTime>(d, Time, handler);
}
使用了模板函數:qVariantToHelper
5.關于qVariantToHelper
/*!
\fn bool QVariant::isValid() const
Returns true if the storage type of this variant is not
QVariant::Invalid; otherwise returns false.
*/
template <typename T>
inline T qVariantToHelper(const QVariant::Private &d, QVariant::Type t,
const QVariant::Handler *handler, T * = 0)
{
if (d.type == t)
return *v_cast<T>(&d);
T ret;
handler->convert(&d, t, &ret, 0);
return ret;
}
該函數根據對象信息和目標類型做轉換工作，如果二者類型一致，則直接做轉換，否則交給函數handler->convert處理
6.關于Handler對象
struct Handler {
f_construct construct;
f_clear clear;
f_null isNull;
#ifndef QT_NO_DATASTREAM
f_load load;
f_save save;
#endif
f_compare compare;
f_convert convert;
f_canConvert canConvert;
f_debugStream debugStream;
};
不過好像沒看出什么門道，那就繼續看看
其實際結構為:
const QVariant::Handler qt_kernel_variant_handler = {
construct,
clear,
isNull,
#ifndef QT_NO_DATASTREAM
0,
0,
#endif
compare,
convert,
0,
#if !defined(QT_NO_DEBUG_STREAM) && !defined(Q_BROKEN_DEBUG_STREAM)
streamDebug
#else
0
#endif
};
再看其中 一個函數實現constuct
static void construct(QVariant::Private *x, const void *copy)
{
x->is_shared = false;
switch (x->type) {
case QVariant::String:
v_construct<QString>(x, copy);
break;
case QVariant::Char:
v_construct<QChar>(x, copy);
break;
case QVariant::StringList:
v_construct<QStringList>(x, copy);
break;
case QVariant::Map:
v_construct<QVariantMap>(x, copy);
break;
case QVariant::Hash:
v_construct<QVariantHash>(x, copy);
break;
case QVariant::List:
v_construct<QVariantList>(x, copy);
break;
case QVariant::Date:
v_construct<QDate>(x, copy);
break;
case QVariant::Time:
v_construct<QTime>(x, copy);
break;
case QVariant::DateTime:
v_construct<QDateTime>(x, copy);
break;
case QVariant::ByteArray:
v_construct<QByteArray>(x, copy);
break;
case QVariant::BitArray:
v_construct<QBitArray>(x, copy);
break;
#ifndef QT_NO_GEOM_VARIANT
case QVariant::Size:
v_construct<QSize>(x, copy);
break;
case QVariant::SizeF:
v_construct<QSizeF>(x, copy);
break;
case QVariant::Rect:
v_construct<QRect>(x, copy);
break;
case QVariant::LineF:
v_construct<QLineF>(x, copy);
break;
case QVariant::Line:
v_construct<QLine>(x, copy);
break;
case QVariant::RectF:
v_construct<QRectF>(x, copy);
break;
case QVariant::Point:
v_construct<QPoint>(x, copy);
break;
case QVariant::PointF:
v_construct<QPointF>(x, copy);
break;
#endif
case QVariant::Url:
v_construct<QUrl>(x, copy);
break;
case QVariant::Locale:
v_construct<QLocale>(x, copy);
break;
#ifndef QT_NO_REGEXP
case QVariant::RegExp:
v_construct<QRegExp>(x, copy);
break;
#endif
#ifndef QT_BOOTSTRAPPED
case QVariant::EasingCurve:
v_construct<QEasingCurve>(x, copy);
break;
#endif
case QVariant::Int:
x->data.i = copy ? *static_cast<const int *>(copy) : 0;
break;
case QVariant::UInt:
x->data.u = copy ? *static_cast<const uint *>(copy) : 0u;
break;
case QVariant::Bool:
x->data.b = copy ? *static_cast<const bool *>(copy) : false;
break;
case QVariant::Double:
x->data.d = copy ? *static_cast<const double*>(copy) : 0.0;
break;
case QMetaType::Float:
x->data.f = copy ? *static_cast<const float*>(copy) : 0.0f;
break;
case QMetaType::QObjectStar:
x->data.o = copy ? *static_cast<QObject *const*>(copy) : 0;
break;
case QVariant::LongLong:
x->data.ll = copy ? *static_cast<const qlonglong *>(copy) : Q_INT64_C(0);
break;
case QVariant::ULongLong:
x->data.ull = copy ? *static_cast<const qulonglong *>(copy) : Q_UINT64_C(0);
break;
case QVariant::Invalid:
case QVariant::UserType:
break;
default:
void *ptr = QMetaType::construct(x->type, copy);
if (!ptr) {
x->type = QVariant::Invalid;
} else {
x->is_shared = true;
x->data.shared = new QVariant::PrivateShared(ptr);
}
break;
}
x->is_null = !copy;
}
繼續看v_construct
該函數模板實現和平臺有關，其中一個平臺的實現如下:
template <class T>
inline void v_construct(QVariant::Private *x, const void *copy, T * = 0)
{
if (sizeof(T) > sizeof(QVariant::Private::Data)) {
x->data.shared = copy ? new QVariantPrivateSharedEx<T>(*static_cast<const T *>(copy))
: new QVariantPrivateSharedEx<T>;
x->is_shared = true;
} else {
if (copy)
new (&x->data.ptr) T(*static_cast<const T *>(copy));
else
new (&x->data.ptr) T;
}
}
這里主要是把傳入對象指針導入為本身的數據指針data.ptr
QVariant大致就這個樣子