void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{
    if( m_Block )
        return;

    m_Block = true;

    switch( index.column() )
    {
         case 7:
         {
                /* update column 8 */
         }
         break;
         case 8:
         {
                /* update column 7 */
         }
         break;
   }

   m_Block = false;
}

    if( m_Block )
        return;
    m_Block = true;

    // altering code ...
    if(index.column() == 8){
       mModCc->setData(mModCc->index(index.row(),7),ui->dsbGTotal->value()*_Percent/100,Qt::EditRole);
    }

   m_Block = false;

Where do I have to write the code to calculate column 8 after editing column 7?

Thank you!

void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{

    if(index.column() == 8){
       mModCc->setData(mModCc->index(index.row(),7),ui->dsbGTotal->value()*_Percent/100,Qt::EditRole);
    }

    bool m_Block;
    if( m_Block )
        return;
    m_Block = true;

    // altering code ...
    if(index.column() == 7){
        mModCc->setData(mModCc->index(index.row(),8),ui->dsbGTotal->value()/_Value*100,Qt::EditRole);
    }

   m_Block = false;
}

void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{
    if( m_Block )
        return;
    m_Block = true;

    // altering code ...

   m_Block = false;
}

the connect statement:

connect(ui->tblCc->model(), &QSqlRelationalTableModel::dataChanged, this, &BVoucherPosForm::onCcDataChanged);

and then:

void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{
    qDebug() << Q_FUNC_INFO << "COLUMN:" << index.column();
    qDebug() << Q_FUNC_INFO << "ROW:" << index.row();

    if(index.column() == 8){
       mModCc->setData(mModCc->index(index.row(),7),ui->dsbGTotal->value()*_Percent/100,Qt::EditRole);
    }
}

I also get this debug-message on editing cell 1,8:

void BVoucherPosForm::onCcDataChanged(const QModelIndex&) COLUMN: 8
void BVoucherPosForm::onCcDataChanged(const QModelIndex&) ROW: 1
void BVoucherPosForm::onCcDataChanged(const QModelIndex&) COLUMN: 7
void BVoucherPosForm::onCcDataChanged(const QModelIndex&) ROW: 1
void BVoucherPosForm::onCcDataChanged(const QModelIndex&) COLUMN: 0
void BVoucherPosForm::onCcDataChanged(const QModelIndex&) ROW: 1

I don't know, why I get the debug-lines 5 and 6, and why the value in line 5 in 0 and in line 6 still 1 (??), but in works.

But now there is an new problem. I would like to edit both columns, column 8 and column 7. Editing column 8 will calculate column 7 and editing column 7 will calculate column 8. On adding a if-statement to the code to control editing column 7 the way I did it above with column 8, the program crashes after editing one of the columns (the dataCanged-Signal emits endless).

Is there a way to realise this, e.g. to emit the dataChanges-signal exactly on changes in a specific column?

Thank you!

So simply check for the column index and only do your work for the desired column and ignore the calls for the rest of the columns and you're good to go.

void BVoucherPosForm::onCcDataChanged(const QModelIndex&) COLUMN: 8
void BVoucherPosForm::onCcDataChanged(const QModelIndex&) COLUMN: 0

I changed an value in column 8.

Ok, a little bit better. But the "index.column()" gives me not the value of the column.

instead of setting the value to the spinbox try the following and check the console:

void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{
    qDebug() << Q_FUNC_INFO << "COLUMN:" << index.column();
}

Since you set the column on the spinbox, you will only see what was the value of the last call of your slot. That's why i added a check for the column index in my previous post, which you completely left out in your last snippet.

connect(ui->tblCc->model(), &QSqlRelationalTableModel::dataChanged, this, &BVoucherPosForm::onCcDataChanged);

and

void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{
    ui->change->setText("ccc");
    ui->spinBox->setValue(index.column());
}
The QLAbel is "ccc" but the Spinbox is "0".

Thank you!

just to add: you also need a C++11 supported compiler.

Well spotted.

If you use Qt5, you can use the new syntax:

just to add: you also need a C++11 supported compiler.

connect(sender, &Sender::valueChanged, receiver, &Receiver::updateValue);

Which here would be:

connect(ui->tblCc->model(), &QSqlRelationalTableModel::dataChanged, this, &BVoucherPosForm::onCcDataChanged);

This syntax has the advantage to be evaluated at compile time, giving you an explicit error, rather than a silently output at run time.

connect(ui->tblCc->model(), SIGNAL(dataChanged(const QModelIndex&,const QModelIndex&)), this, SLOT(onCcDataChanged()));

and get the error:
"QObject::connect: No such slot BVoucherPosForm::onCcDataChanged()"

    connect(ui->tblCc->model(), SIGNAL(dataChanged(const QModelIndex&,const QModelIndex&)), this, SLOT(onCcDataChanged(const QModelIndex& index)));

and

void BVoucherPosForm::onCcDataChanged(const QModelIndex& index)
{
    ui->change->setText("ccc");


    if(index.column() == 8){
        ui->change->setText("xxx");
    }
}

But I don't get either "ccc" or a "xxx" in my QLabel (change).

I also get the error "QObject::connect: No such signal QSqlRelationalTableModel::dataChanged(const QModelIndex& index)".

What I am doing wrong?

connect(ui->tblCc->model(), SIGNAL(dataChanged(const QModelIndex&,const QModelIndex&)), this, SLOT(onCcDataChanged(const QModelIndex&));
...
void onCcDataChanged(const QModelIndex& index)
{
     if( index.column() != DESIRED_COLUMN )
         return;
    //...
}