Uses on Qvariant in the address book example

[thefatladysingsopera]

Hello,i am studying the address book example on Qt Assistant and on the address book example i have a come across usages of Qvariant which i don't understand.

Here is the code:

QVariant TableModel::data(const QModelIndex &index, int role) const
 {
     if (!index.isValid())
         return QVariant();//what does the use of Qvariant here mean - 1
 
     if (index.row() >= listOfPairs.size() || index.row() < 0)
         return QVariant();//what does the use of Qvariant here mean - 2
 
     if (role == Qt::DisplayRole) {
         QPair<QString, QString> pair = listOfPairs.at(index.row());
 
         if (index.column() == 0)
             return pair.first;
         else if (index.column() == 1)
             return pair.second;
     }
     return QVariant();//what does the use of Qvariant here mean - 3
 }

QVariant TableModel::data(const QModelIndex &index, int role) const
 {
     if (!index.isValid())
         return QVariant();//what does the use of Qvariant here mean - 1
      
     if (index.row() >= listOfPairs.size() || index.row() < 0)
         return QVariant();//what does the use of Qvariant here mean - 2

     if (role == Qt::DisplayRole) {
         QPair<QString, QString> pair = listOfPairs.at(index.row());

         if (index.column() == 0)
             return pair.first;
         else if (index.column() == 1)
             return pair.second;
     }
     return QVariant();//what does the use of Qvariant here mean - 3
 }

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I have commented on the code next to Qvariant usages.Someone help me understand what it means.

[mcosta]

The default QVariant::QVariant() constructor creates an invalid object.

You can test for valid QVariant with code

QVariant obj = model->data(index, role);
if (obj.isValid()) {
    // Do Something with valid data
}
else {
    // Handle invalid data
}

QVariant obj = model->data(index, role);
if (obj.isValid()) {
    // Do Something with valid data
}
else {
    // Handle invalid data
}

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[thefatladysingsopera]

I don't understand,could you explain in relation to the code i've given?.

Thanks.

[mcosta]

The code you posted check if there are the condition to return a "valid" result.
If don't it returs an invalid QVariant, in this way the caller may say if the result is valid.