invalid QModelIndex

[Talei]

Hello.
I try to write QAbstractTableModel but i have problem with index.
My test class:

class cmodel : public QAbstractTableModel
{
    Q_OBJECT
 
    public:
        cmodel( QObject *parent = 0);
        ~cmodel();
 
        QVariant data(const QModelIndex &index, int role) const;
        bool setData(const QModelIndex &index, const QVariant &value, int role=Qt::DisplayRole);
        int rowCount(const QModelIndex &parent) const;
        int columnCount(const QModelIndex &parent) const;
        QVariant headerData(int section, Qt::Orientation orientation, int role) const;
        Qt::ItemFlags flags(const QModelIndex &index) const;
 
    private:
        QStringList listaStr, listaUrl;       
};

class cmodel : public QAbstractTableModel
{
    Q_OBJECT

    public:
        cmodel( QObject *parent = 0);
        ~cmodel();

        QVariant data(const QModelIndex &index, int role) const;
        bool setData(const QModelIndex &index, const QVariant &value, int role=Qt::DisplayRole);
        int rowCount(const QModelIndex &parent) const;
        int columnCount(const QModelIndex &parent) const;
        QVariant headerData(int section, Qt::Orientation orientation, int role) const;
        Qt::ItemFlags flags(const QModelIndex &index) const;

    private:
        QStringList listaStr, listaUrl;       
};

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and function

void MainWindow::on_pushButton_clicked()
{
    QStringList str;
    str << "1" << "2" << "3";
    cmodel *m = new cmodel( this );
    QModelIndex index = m->index( 0, 0, QModelIndex() );
    m->setData( index, str, Qt::DisplayRole );
    ui->tableView->setModel( m );
}

void MainWindow::on_pushButton_clicked()
{
    QStringList str;
    str << "1" << "2" << "3";
    cmodel *m = new cmodel( this );
    QModelIndex index = m->index( 0, 0, QModelIndex() );
    m->setData( index, str, Qt::DisplayRole );
    ui->tableView->setModel( m );
}

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but in setData my index is invalid ( index.isValid() is false ).
Why is that?

Thanks for any suggestion and sorry for my "enrgish"
best regards