Cast a QVariant to a QList<customObject>

rkadeFR

Hello everyone,

I did a search in the forum but didn't find anything. (If I'm wrong, just link me the threads please).

I have some customs classes, and I need to map them with some QVariant.
What I'm doing is a constructor from a QVariant to instantiate them from the QVariant, and an operator QVariant() const in order to use static_cast to cast the object to a QVariant.

But now I'm facing one problem with QList. I have for example a QVariant containing a QList of QMap<QString, QVariant>. A QMap<QString, QVariant> represent my object A. I'm able to cast a QMap to a A and vice versa.

My question is from a QVariant like this (QList<QMap<QString, QVariant> >), how can I cast it to a QList of my custom type (QList<A>).

Code:
@#include <QDebug>

using namespace std;

class A
{
public:
int value;

public:
    A () {cout << "Default constructor" << endl;}
    A (int arg)
    {   
        this->value = arg;
    }   
    A (QVariant variant) throw (int)
    {   
        cout << "Constructor from variant" << endl;
        if (!variant.canConvert(QVariant::Map))
            throw 1;
        QMap<QString,QVariant> mapVariant = variant.toMap();
        this->value = mapVariant["value"].toInt();
    }   
    A operator=(A& copy)
    {   
        cout << "Constructor by copy" << endl;
        this->value = copy.value;
        return (*this);
    }   
    virtual ~A ()
    {}  

    operator QVariant() const
    {   
        QMap<QString,QVariant> mapAns = QMap<QString,QVariant>();
        mapAns.insert("value", this->value);

        return mapAns;
    }

};
Q_DECLARE_METATYPE(A);

Q_DECLARE_METATYPE(QList<A>);
int main(int argc, const char *argv[])
{
cout << "[Log] Beginning of the prog(" << argv[0] << ")" << endl;

QList<QMap<QString, QVariant> > listVariant = QList<QMap<QString, QVariant> >();

QMap<QString, QVariant> mapA1 = QMap<QString,QVariant>();
QMap<QString, QVariant> mapA2 = QMap<QString,QVariant>();
QMap<QString, QVariant> mapA3 = QMap<QString,QVariant>();
QMap<QString, QVariant> mapA4 = QMap<QString,QVariant>();
mapA1.insert("value", 4); 
mapA2.insert("value", 9); 
mapA3.insert("value", 3); 
mapA4.insert("value", 2); 

listVariant.append(mapA1);
listVariant.append(mapA2);
listVariant.append(mapA3);
listVariant.append(mapA4);

QList<A> listA = static_cast<QList< A > >(listVariant);
qDebug() << "[Debug] listA (" << listA << ")";

cout << "[Log] End of the prog" << endl;
return 0;

}
@

rkadeFR

Little up to clarify my claim. I made an even simpler example without any custom type, and I don't get the cast.

I create a QStringList, I pass it to a QVariant, and then I want to cast it to a QList<QString>.

The SL object is composed by my "test 1" and "test 2" QString. But my listResult is completely empty.
@
Q_DECLARE_METATYPE(QList<QString>);
@

@
QStringList SL = QStringList();
SL.append("test 1");
SL.append("test 2");
qDebug() << "[Debug] SL (" << SL << ")";

QList<QString> listResult = QVariant(SL).value<QList<QString> >();
qDebug() << "[Debug] listResult (" << listResult << ")";
@

SGaist

Hi,

QStringList and QList<QString> are two different thing (Not a typedef in this case).

Try with @QStringList listResult = QVariant(SL).value<QStringList>();@

rkadeFR

Ok, so that was a bad example to show my goal hahahaaaaa :(

I have a QList<QVariant>, and the QVariant inside are mapped to a custom type. (see the code above on the first post).

So from a QList<QVariant> I want to cast it to a QList<CustomType>.

The problem is that it cast without error from the compiler in the below example, but the listA is completely empty.

My current example from the first post:
@
QList<QVariant> listVariant = QList<QVariant>();

QMap<QString, QVariant> mapA1 = QMap<QString,QVariant>();
QMap<QString, QVariant> mapA2 = QMap<QString,QVariant>();
QMap<QString, QVariant> mapA3 = QMap<QString,QVariant>();
QMap<QString, QVariant> mapA4 = QMap<QString,QVariant>();
mapA1.insert("value", 4); 
mapA2.insert("value", 9); 
mapA3.insert("value", 3); 
mapA4.insert("value", 2); 

listVariant.append(QVariant(mapA1));
listVariant.append(QVariant(mapA2));
listVariant.append(QVariant(mapA3));
listVariant.append(QVariant(mapA4));

qDebug() << "[Debug] QVariant(listVariant) (" << QVariant(listVariant) << ")";

//output: [Debug] QVariant(listVariant) ( QVariant(QVariantList, (QVariant(QVariantMap, QMap(("value", QVariant(int, 4) ) ) ) , //QVariant(QVariantMap, QMap(("value", QVariant(int, 9) ) ) ) , QVariant(QVariantMap, QMap(("value", QVariant(int, 3) ) ) ) , //QVariant(QVariantMap, QMap(("value", QVariant(int, 2) ) ) ) ) ) )

QList<A> listA = QVariant(listVariant).value<QList<A> >();


qDebug() << "[Debug] listA (" << listA << ")";

//Output: [Debug] listA ( () )

@

SGaist

static_cast is normally used to cast a pointer to an object to one of it's derivative, or a numeric datatype to another (i.e int to char etc...) without any check.

What you are trying to achieve is to transform a QVariantList containing QVariantHash into a class that does not inherit at all from QVariant.

The QVariant operator is basically used to put your class in a QVariant (it is used where you would otherwise have to use QVariant::fromValue)

The value<T> function will work on a QVariant if the object in contains already is of the type T or can be converted to type T. AFAIK QVariantHash cannot be converted to A.

Hope this helps