QDataStream and QByteArray as a buffer

  • Can you explain why this code:
    @int main(int argc, char *argv[])
    QByteArray buffer;
    QHash< QString, QByteArray > data;
    data.insert( "123", "qwe" );
    qDebug() << "Data:" << data;
    QDataStream stream( &buffer, QIODevice::ReadWrite );
    stream << data;
    qDebug() << "Buffer's size:" << buffer.size();
    stream >> data;
    qDebug() << "Data:" << data;
    qDebug() << "Buffer's size:" << buffer.size();
    qDebug() << "Buffer:" << buffer.toHex();
    gives this output:
    @Data: QHash(("123", "qwe"))
    Buffer's size: 21
    Data: QHash()
    Buffer's size: 21
    Buffer: "000000010000000600310032003300000003717765" @

    As i can understand from http://doc.qt.nokia.com/stable/datastreamformat.html streaming from data to buffer is correct (the last line of the output). But why then there is no streaming from buffer to data?

  • Stream's status becomes QDataStream::ReadPastEnd so i think i just misunderstood logic of streaming.

  • Indeed. You have to seek() back to the beginning of the buffer.

  • Do you mean QDataStream::device().seek()?

  • Yes (remember that QByteArray isn't a QIODevice, thus QDataStream creates a QBuffer internally).

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