Doubts with QtableModel

jss193

Hello everybody,

I am doing this tutorial http://doc.qt.io/archives/qt-4.8/modelview.html#
And I come up some questions regarding to the following code:

#include <QAbstractTableModel>

class MyModel : public QAbstractTableModel
{
    Q_OBJECT
public:
    MyModel(QObject *parent);
    int rowCount(const QModelIndex &parent = QModelIndex()) const ;
    int columnCount(const QModelIndex &parent = QModelIndex()) const;
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const;
};

When we create the constructor MyModel(QObject *parent);
why do we pass to MyModel pointer to QObject named parent, is parent any reserved class referring to parent class from QObject?

Second,

QVariant MyModel::data(const QModelIndex &index, int role) const
{
    if (role == Qt::DisplayRole)
    {
       return QString("Row%1, Column%2")
                   .arg(index.row() + 1)
                   .arg(index.column() +1);
    }
    return QVariant();
}

Are we returning twice 2 different objects? first we return QStringobject and then QVariant. How is possible that a Method returning 2 different objects?

Thank you very much!

JKSH

@jss193 said in Doubts with QtableModel:

When we create the constructor MyModel(QObject *parent);
why do we pass to MyModel pointer to QObject named parent

All QObjects can form parent-child relationships. A parent object manages the memory of its children.

Read more:

QVariant MyModel::data(const QModelIndex &index, int role) const
{
    if (role == Qt::DisplayRole)
    {
       return QString("Row%1, Column%2")
                   .arg(index.row() + 1)
                   .arg(index.column() +1);
    }
    return QVariant();
}
Are we returning twice 2 different objects? first we return QStringobject and then QVariant. How is possible that a Method returning 2 different objects?

Notice the if() statement. It only lets you return one object:

If the role is Qt::DisplayRole, the function returns a QString.
If the role is not Qt::DisplayRole, the function returns an empty QVariant.

I am doing this tutorial http://doc.qt.io/archives/qt-4.8/modelview.html#

Are you using Qt 4? If not, follow the Qt 5 documentation instead:

http://doc.qt.io/qt-5/modelview.html

Christian Ehrlicher

@JKSH said in Doubts with QtableModel:

If the role is Qt::DisplayRole, the function returns a QString.

Which is implicitly converted into a QVariant by the compiler :)

JonB

@jss193
@Christian-Ehrlicher's conversion happens without you doing anything because there is an implicit conversion defined from QString to QVariant. Really you are returning a QVariant::QVariant(QString(...)), http://doc.qt.io/qt-5/qvariant.html#QVariant-16, so you are not returning "two different types".