Count number of bytes written to QDataStream in Qt5

Question

In my code I am preparing some data for sending over a network using QDataStream. In the middle of the preparations I would like to know exactly how many bytes that were prepared at that point. How can I do that?

Example:

int myFunc(QDataStream &ds){
   ds << "hello"
   ds << 123;
   ds << 456.7f;
   //...
   //How can I do this:
   return ds.numberOfBytesPipedIntoThisQDataStreamAtThisTime();
}

What you mean by "preparing"? Are you streaming data to some buffer in memory and once all data is ready, send it over network? — Oleg Andriyanov, Jan 24 '16 at 23:22
Yes. My QDataStream wraps a QByteArray that is later sent over the wire. — Mr. Developerdude, Jan 25 '16 at 00:01

Oleg Andriyanov · Answer 1 · 2016-01-24T23:46:45.723

2

This could work (although it looks not very elegant).

int myFunc(QDataStream& ds){
    QBuffer buf;
    buf.open(QBuffer::WriteOnly);
    QDataStream tmpStream(&buf);

    tmpStream << "hello";
    tmpStream << 123;
    tmpStream << 456.7f;
    int written = buf.size();
    if(ds.device()->write(buf.data()) != buf.data().size()) {
        // handle error
    }
    return written;
}

edited Jan 24 '16 at 23:46

answered Jan 24 '16 at 23:28

Oleg Andriyanov

5,069
1
22
36

1

`ds << buf.data();` - this code will writes **4 bytes** + data - https://doc.qt.io/qt-5/datastreamformat.html – Vladimir Bershov Jan 24 '16 at 23:35
Is there any reason why I would prefer QBuffer over say QByteArray? – Mr. Developerdude Jan 25 '16 at 00:02
@LennartRolland No. `QBuffer` uses `QByteArray` as storage, so they are similar. – Oleg Andriyanov Jan 25 '16 at 00:06

Vladimir Bershov · Answer 2 · 2016-01-24T23:38:32.440

2

Usual way to know the number of bytes:

QByteArray buf;
QDataStream stream( &buf, QIODevice::ReadWrite );
stream << someData;
int count = buf.size();

edited Jan 24 '16 at 23:38

answered Jan 24 '16 at 23:30

Vladimir Bershov

2,701
2
21
51

This is actually what I am doing now. – Mr. Developerdude Jan 25 '16 at 00:01

Count number of bytes written to QDataStream in Qt5

2 Answers2