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:

Qt Code: 
  1. QVariant TableModel::data(const QModelIndex &index, int role) const
  2.  {
  3.      if (!index.isValid())
  4.          return QVariant();//what does the use of Qvariant here mean - 1
  5.  
  6.      if (index.row() >= listOfPairs.size() || index.row() < 0)
  7.          return QVariant();//what does the use of Qvariant here mean - 2
  8.  
  9.      if (role == Qt::DisplayRole) {
  10.          QPair<QString, QString> pair = listOfPairs.at(index.row());
  11.  
  12.          if (index.column() == 0)
  13.              return pair.first;
  14.          else if (index.column() == 1)
  15.              return pair.second;
  16.      }
  17.      return QVariant();//what does the use of Qvariant here mean - 3
  18.  }
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I have commented on the code next to Qvariant usages.Someone help me understand what it means.