How to use QDataStream::readBytes()

Question

According to the documentation for readBytes() (in Qt 5.4's QDataStream), I would expect the following code to copy the input_array into newly allocated memory and point raw at the copy:

QByteArray input_array{"\x01\x02\x03\x04qwertyuiop"};
QDataStream unmarshaller{&input_array, QIODevice::ReadOnly};

char* raw;
uint length;
unmarshaller.readBytes(raw, length);

qDebug() << "raw null? " << (raw == nullptr) << " ; length = " << length << endl;

...but the code prints raw null? true ; length = 0, indicating that no bytes were read from the input array.

Why is this? What am I misunderstanding about readBytes()?

The QByteArray constructor needs a nul terminated char array if you don't specify the size. — cmannett85
@cmannett85 Ah, thanks. But if I read off a series of quint8s using >> first, I get the expected 1, 2, 3, 4, which seems to imply that I am indeed constructing the expected QByteArray....right? — Kyle Strand
@cmannett85 Also, I can print the QByteArray with qDebug(). — Kyle Strand
@cmannett85 Finally, adding \0 at the end of the input_array constructor string doesn't appear to change the code's behavior. — Kyle Strand

hank hank · Accepted Answer · 2015-04-15T08:44:13

The documentation does not describe this clearly enough, but QDataStream::readBytes expects the data to be in a certain format: quint32 part which is the data length and then the data itself.

So to read data using QDataStream::readBytes you should first write it using QDataStream::writeBytes or write it any other way using the proper format.

An example:

QByteArray raw_input = "\x01\x02\x03\x04qwertyuiop";

QByteArray ba;

QDataStream writer(&ba, QIODevice::WriteOnly);
writer.writeBytes(raw_input.constData(), raw_input.length());

QDataStream reader(ba);

char* raw;
uint length;
reader.readBytes(raw, length);

qDebug() << "raw null? " << (raw == NULL) << " ; length = " << length << endl;

Also you can use QDataStream::readRawData and QDataStream::writeRawData to read and write arbitrary data.

How to use QDataStream::readBytes()

1 Answers