QByteArray being corrupted in a custom class

TheVancedGamer

Hi all. I am writing a Qt app and I have made a custom class for serializing/deserializing a message to be sent to the device.

class Message {
public:
    enum MessageType {
        ACK       = 0x01,
        COMMAND_1 = 0x0c,
        COMMAND_2 = 0x0e
    };

    MessageType type() const;
    int sequence() const;
    QByteArray data() const;

    QByteArray encode();
    QByteArray encode(int sequenceNumber);

    static quint32 payloadLength(const QByteArray &data);

private:
    MessageType m_type;
    int m_sequence;

    QByteArray m_data;
};

Now I have to encode the message when sending it, so I store these Message's in a QQueue. But when I dequeue the message to encode it, QByteArray is magically corrupted! It loses all of the data but all of the other member variables are fine. And interestingly, if I don't initialize m_data but rather add data to it with append(), the corruption goes away!

I have been pulling my hair out for the past week and I have no idea why this happens, any ideas would be greatly appreciated!

IgKh

@TheVancedGamer No, that's a regular copy constructor - which only increments a reference count as QByteArray is implicitly shared.

The symptom you describe makes me think that the original QByteArray you created and eventually passed to the Message constructor doesn't own its' buffer - that is, was created with something like QByteArray::fromRawData or the like.

Bonnie

Did you init it with another non-const QByteArray variable? According to the doc:

QByteArray::QByteArray(QByteArray &&other)
Move-constructs a QByteArray instance, making it point at the same object that other was pointing to.

TheVancedGamer

@Bonnie I mean, I just initialized it with the variable as in the constructor like this:

Message::Message(MessageType type, int sequenceNumber, QByteArray payload):
  m_type(type),
  m_sequence(sequenceNumber),
  m_data(payload)
{
    //m_data.append(payload);
}

This probably did the move-constructor?

IgKh

@TheVancedGamer No, that's a regular copy constructor - which only increments a reference count as QByteArray is implicitly shared.

The symptom you describe makes me think that the original QByteArray you created and eventually passed to the Message constructor doesn't own its' buffer - that is, was created with something like QByteArray::fromRawData or the like.

Bonnie

@TheVancedGamer I think I was wrong. You probably need to init it with a reference variable to call the move-constructor.
@IgKh Right, this makes sense. In that case, deep copy by append() is the correct way.

Axel Spoerl

@TheVancedGamer
QByteArray uses implicit sharing, with copy-on-write. Your append-trick is one option to force a copy. It's better, however, to make the c'tor argument const-ref, i.e. const QByteArray &payload.

TheVancedGamer

@IgKh That was it! I forgot I was hacking around and that did it! Thank you!