Your issue boils down to the fact that QByteArray != QString combined with how
QDataStream serializes data. According to the documentation for serializing
data through a QDataStream (http://doc.qt.io/qt-5/datastreamformat.html) and
your code, you have these two things involved:
QString If the string is null: 0xFFFFFFFF (quint32) Otherwise: The string
length in bytes (quint32) followed by the data in UTF-16
QByteArray If the byte array is null: 0xFFFFFFFF (quint32) Otherwise: the array
size (quint32) followed by the array bytes, i.e. size bytes
> QDataStream out(&bai, QIODevice::WriteOnly);
> out << QString("A QString"); // writes some information on it
So at this point, the QByteArray "bai" contains 4 bytes for the size of the
QString in UTF-16 format (this value is 18, 9 characters at 2 bytes per
character), followed by the string itself in UTF-16 format.
> QByteArray bai;
> out2 << bai.size();
> out2 << bai;
Here you wrote out an int and a QByteArray, so the file contains 4 bytes for
the int "bai.size()" (value of 22), and then because of how QDataStream
serializes QByteArrays, another 4 bytes for the size of the QByteArray (still a
value of 22), plus the contents of the QByteArray itself. At this point,
remember that the contents of the QByteArray are 4 bytes for the size of the
QString (value of 18) plus the QString in UTF-16 format.
> QString str;
> int a; // extracts size information
> in >> a >> str; // extracts QString
But here you're reading in an int and a QString. So you've read in the int for
the size that you explicitly wrote out (the first value of 22), but because
you're now reading in a QString, not a QByteArray, you're grabbing the 4 bytes
that were supposed to be the size of a QByteArray and treating that as the size
of a QString, so the string extraction operation reads in a 22, and then uses
the next 22 bytes to build up a QString.
So you need to ensure you are reading and writing the same things. You put an
int and a QByteArray in, so you should read an int and a QByteArray out. Then
since you know what the contents of the QByteArray are, you can extract that
back to a QString...
Sean
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