QTextStream to fill a QString

HoMa

Hi all - I am afraid I have a big missconception here.

I tried to format a QString by piping stuff in via QTextStream. However, after some hundret characters everything gets messed up and the resulting QString ends in unreadable characters. The code looks like this:

    QString all;
    QTextStream sout(&all);
    sout << Qt::endl;
    sout << qsl("Some String :") << Qt::endl << someObject[0].toString() << Qt::endl;
    sout << qsl("Some String ") << Qt::endl << someObject[1].toString() << Qt::endl;
    sout << qsl("Some String ") << Qt::endl << someObject[2].toString() << Qt::endl;

    sout << qsl("Some String :") << Qt::endl << someObject[0].toString() << Qt::endl;
    sout << qsl("Some String :") << Qt::endl << someObject[1].toString() << Qt::endl;
    sout << qsl("Some String :") << Qt::endl << someObject[2].toString() << Qt::endl;
(some more)

    sout.flush();
    return all;

Is this not a valid use of QTextStream? Is the QString not auto-resizing? ... Who can give me a hint?

regards
Holger

HoMa

Oh, wow - this is not about QTextStream at all - sorry for misleading. I can recreate the effect by just writing a long QString to qDebug()!
And this was reported before: https://forum.qt.io/topic/72244/limitation-of-string-length-for-qdebug-output/7

Best regards
Holger

PS: this was enough to reproduce:

#include "mainwindow.h"

#include <QDebug>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    qDebug() <<
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------";

    return a.exec();
}

mrjj

Hi
What does someObject[0].toString() return ?
Do you have any std:: strings you call c_str() on or anything else that could become invalid later?

HoMa

the .toString() functions return QStrings. Actually it is the output of some .arg(...) call:

    QString out =
          "#nbr :%1"
        "; #nbr      :%2"
        "; float   (e):%3";

    return out.arg(QString::number(nbrCreditors), QString::number(nbrContracts), QString::number(value, 'f', 2));

mrjj

@HoMa
Hmm. I cant really make it do odd things.
Can you try

   QString all;
    QTextStream sout(&all);
    for (int c=0; c < 1000; c++ )
     sout << "this is a test" << Qt::endl;

HoMa

I initialized the QString like this

QString all(2048, '\0');

and the problem does not occure ...
Strange world. However: This should work, right?

Holger

mrjj

@HoMa
Hi
Yes it should work as i just tried in a small test app and it
show the 1000 lines fine in a PlainTextEdit.
(Qt 5.15, 64 bit)
What Qt version do you use ? and 32 or 64 bit ?

this worked fine.. ?

 QString all;
    QTextStream sout(&all);
    for (int c=0; c < 1000; c++ )
     sout << "this is a test" << Qt::endl;

    ui->plainTextEdit->appendPlainText(all);

HoMa

Thx.

I am on Qt Creator 4.13.0, just currently updated. This was the first update since the fresh install (I ... think).

Best Regards
Holger

HoMa

Oh, wow - this is not about QTextStream at all - sorry for misleading. I can recreate the effect by just writing a long QString to qDebug()!
And this was reported before: https://forum.qt.io/topic/72244/limitation-of-string-length-for-qdebug-output/7

Best regards
Holger

PS: this was enough to reproduce:

#include "mainwindow.h"

#include <QDebug>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    qDebug() <<
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------"
        "----------------------------------------------------------------------------------------------------";

    return a.exec();
}

aha_1980

Just as follow up: this seems to be a limitation of GDB on Windows.

See QTCREATORBUG-24649 for the continuation discussion.

Regards