[Qt-interest] QTextStrem.pos() incredible slow

[Malyushytsky, Alex]

To me it does not look like a bug since QT documentation states that it is slow:

"
qint64 QTextStream::pos () const
Returns the device position corresponding to the current position of the stream, or -1 if an error occurs (e.g., if there is no device or string, or if there's a device error).

Because QTextStream is buffered, this function may have to seek the device to reconstruct a valid device position. This operation can be expensive, so you may want to avoid calling this function in a tight loop.
"

Regards,
    Alex


-----Original Message-----
From: qt-interest-bounces at trolltech.com [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Niklaus Hersche
Sent: Thursday, October 28, 2010 5:55 AM
To: Qt-interest at trolltech.com
Subject: [Qt-interest] QTextStrem.pos() incredible slow

Hi

I scan a tag in a utf16 text file with QTextStrem.readLine().
Once I have found the tag, I'd like to keep the file position in memory for
further use.

Unfortunately QTextStrem.pos() is so slow, that it's almost of useless.

To find the tag it takes 281 ms.
The subsequent call of QTextStrem.pos() takes 14078 ms.

This is incredible, isn't it?

To me this looks like a bug. Has this issue been fixed in a later version,
or am I doing something  wrong?

Has anybody a clue?

Thanks
Nik



- QTextStrem.pos() returns: 6037728
- OS: WinXP
- Qt: 4.5.2

Simple code to reproduce the issue:
//-----------------------------------------------------------------------
bool test(){

        qint64 filePos = -1;
        QFile f(c_ProjectFile);
        QTextStream ts(&f);

        if(!f.open( QFile::ReadOnly | QFile::Text)){
                return false;
        }

        ts.setCodec("UTF-16LE");

        QTime tm1;
        tm1.start();

        for(QString line; !(line = ts.readLine()).isNull(); ){

                if(line.startsWith( c_StartTag, Qt::CaseInsensitive)){
                        int elapsed = tm1.elapsed();
                        qDebug() << "search: " << elapsed << " ms";

                        tm1.restart();
                        filePos = ts.pos();
                        elapsed = tm1.elapsed();
                        qDebug() << "pos: " << elapsed << " ms";
                        return true;
                }

        } // for

        return false;
}
//-----------------------------------------------------------------------






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