[SOLVED] QTextStream messing its position pointer

QTextStream messing its position pointer

I'm writing a parser for a text file, but QTextStream seems to be missing something. I'm using Qt 5.4.1, and the code is single-threaded, if that matters.

This is the method:

const Event* AviLogFile::nextEvent() {                                          
    qDebug() << "Entering nextEvent()";                                         
    qDebug() << "m_file.pos() before QTextStream: " << m_file.pos();            
    QTextStream in(&m_file);                                                    
    qDebug() << "m_file.pos() after QTextStream: " << m_file.pos();             
    qDebug() << "in.pos(): " << in.pos();                                       
    Event* ev = 0;                                                              

    while (!in.atEnd() && ev == 0) {                                            
        QString line = in.readLine();                                           
        qDebug() << "(inside loop) m_file.pos(): " << m_file.pos();             
        qDebug() << "(inside loop) in.pos(): " << in.pos();                     
        ev = parseEvent(line);                                                  
    }                                                                           

    m_currentEvent = ev;                                                        
    if (!ev) {                                                                  
        qDebug() << "in.AtEnd: " << in.atEnd() << ". file.atEnd: " << m_file.atEnd();
    }                                                                           
    return ev;                                                                  
}

I'm calling it in a loop. The first call works fine, but at the second one, I get this output:

Entering nextEvent()
m_file.pos() before QTextStream:  2244
m_file.pos() after QTextStream:  2244
in.pos():  2244
(inside loop) m_file.pos():  18628
(inside loop) in.pos():  65

As you can see, the QTextStream and QFile internal pointers are fine before entering the loop, but completely messed up inside the loop.

Any idea on what is happening here?

Solution

There are several standing bugs regarding using QTextStream::pos() with an IO device underneath. Many of them are "unresolved" or "won't fix" and their reproducer case looks exactly as your problem.

I suggest to avoid relying on that function (plus, citing from the documentation of QTextStream:

Because QTextStream is buffered, this function may have to seek the device to reconstruct a valid device position.

it would also render things slow during your loop).