C++ QVariant转换为基本类型
我有这样的课:C++ QVariant转换为基本类型,c++,qt4,C++,Qt4,我有这样的课: class QObjectDerived : public QObject { Q_OBJECT // ... }; Q_DECLARE_METATYPE(QObjectDerived*) 当将此类存储到QVariant时,就会发生这种行为 QObjectDerived *object = new QObjectDerived(this); QVariant variant = QVariant::fromValue(object); qDebug(
class QObjectDerived : public QObject
{
Q_OBJECT
// ...
};
Q_DECLARE_METATYPE(QObjectDerived*)
当将此类存储到QVariant时,就会发生这种行为
QObjectDerived *object = new QObjectDerived(this);
QVariant variant = QVariant::fromValue(object);
qDebug() << variant; // prints QVariant(QObjectDerived*, )
qDebug() << variant.value<QObject*>(); // prints QObject(0x0)
qDebug() << variant.value<QObjectDerived*>(); // QObjectDerived(0x8c491c8)
variant = QVariant::fromValue(static_cast<QObject*>(object));
qDebug() << variant; // prints QVariant(QObject*, QObjectDerived(0x8c491c8) )
qDebug() << variant.value<QObject*>(); // prints QObjectDerived(0x8c491c8)
qDebug() << variant.value<QObjectDerived*>(); // QObject(0x0)
QObjectDerived*object=新的QObjectDerived(此);
QVariant=QVariant::fromValue(对象);
qDebug()仅通过写入
QObject *value = variant.value<QObjectDerived*>();
QObject*value=variant.value();
对于您的类型,可以部分专门化qvariant\u cast
,但这不是一个有文档支持的用例,我不愿意依赖它。qvariant.h(Qt 4.8.6):
模板
内联T值()常量
{返回qvariant_cast(*this);}
模板内联T qvariant_cast(常量qvariant&v)
{
const int vid=qMetaTypeId(静态_cast(0));
if(vid==v.userType())
return*reinterpret_cast(v.constData());
if(vid
QObject*
存储为内置的QMetaType::QObjectStar
类型,而QObjectDerived
是用户定义的id类型,由元类型系统定义。也就是说,您必须手动强制转换它
template<typename T>
inline T value() const
{ return qvariant_cast<T>(*this); }
template<typename T> inline T qvariant_cast(const QVariant &v)
{
const int vid = qMetaTypeId<T>(static_cast<T *>(0));
if (vid == v.userType())
return *reinterpret_cast<const T *>(v.constData());
if (vid < int(QMetaType::User)) {
T t;
if (qvariant_cast_helper(v, QVariant::Type(vid), &t))
return t;
}
return T();
}