Can't get my object back from QVariant

[ronM71]

This is how encode my object (ContactPtr) as QVariant

@QVariant ContactsModel::data(const QModelIndex &index, int role) const
{
if (!index.isValid())
return QVariant();

if (index.row() >= this->contacts.size())

return QVariant();

if (role == Qt::DisplayRole)
{

ContactPtr ptr = const_cast<Contact*>(&this->contacts[index.row()]);
return QVariant(QVariant::UserType, &ptr);
}
else
return QVariant();
}@

At some point in the rendering delegate, I am trying to get the ContactPtr back from the QVariant. I cannot get past the compilation errors for this. Everything I try fail. The ContactPtr is registered with:

@typedef Contact * ContactPtr;
Q_DECLARE_METATYPE(ContactPtr);@

Can someone please help to point me in the right direction? I'd like to:

// qv is my QVariant.
@ContactPtr ptr = qv.value<ContactPtr>;@

but it fails compilation. How do I do this??

[giesbert]

youz are using the wrong functions, if you look at "the docs":http://doc.qt.nokia.com/4.7/qvariant.html#QVariant-3 . Use "qVariantFromValue":http://doc.qt.nokia.com/4.7/qvariant.html#qVariantFromValue instead.

The point is with Q_DECLARE_METATYPE, you define an id for your type, which migth be QVariant::UserType, but also something else.

To get it back, use "qVariantValue":http://doc.qt.nokia.com/4.7/qvariant.html#qVariantValue

[goetz]

What type is ContactPtr? Just a

@
typedef Contact* ContactPtr
@

or something else?

In the return line you put a pointer to the ptr (which is of type ContactPtr) object, i.e. you put a ContactPtr* into the QVariant. ptr obviously goes out of scope, once you leave the data() method, so you have a dangling pointer in your QVariant.

[ronM71]

Ok. I've cancelled the ContactPtr. There was no real need for it. Now I do:

@Q_DECLARE_METATYPE(Contact*);@

I encode into QVariant like this:

@return qVariantFromValue(const_cast<Contact*>(&this->contacts[index.row()]));@

But I still cannot decode:

@
void ContactDelegate::paint(QPainter * painter, const QStyleOptionViewItem & option, const QModelIndex &index) const
{
painter->save();
Contact * contact = qVariantValue(index.data()); // <---- error:
@

Error:
ContactDelegate.cpp:15: error: no matching function for call to 'qVariantValue(QVariant)'

[goetz]

qVariantValue is a template function. You must call:

@
Contact * contact = qVariantValue<Contact *>(index.data());
@

[ronM71]

That did the job. I missed that little detail as the documentation's title is: "T qVariantValue ( const QVariant & value )"

Thanks Volker and Gerolf