How to know that QDataStream cannot deserialize something?

Please, consider the following code fragment from the QDataStream documentation:

QFile file("file.dat");
file.open(QIODevice::ReadOnly);
QDataStream in(&file);    // read the data serialized from the file
QString str;
qint32 a;
in >> str >> a;           // extract "the answer is" and 42

Is there a way to know that QDataStream cannot deserialize the content of the file to QString and qint32 and how to handle such deserialization errors in QDatastream?

According to official documentation, you can (and should) use Read Transactions:

  in.startTransaction();

  QString str;
  qint32 a;

  in >> str >> a; // try to read packet atomically

  if(in.commitTransaction())
  {
      // read is ok, check the data you had read
  } 
  else
  {
      // wait for more data or show unknown error
  }

If you have a file as an IO device, you can read without transactions, but you have to check manually whether the necessary amount of data is available. When using QDataStream you shall be sure about a sequence and composition of data.