Convert QStringList to QList<const QString>

randsfjorden

My problem is that I have a model containing a lot of data and I want to work as memory-efficient as possible.

I want to save the data in my model in QVector<const QString>. I want to use const QStrings, because I don't want Qt to preallocate memory for every single QString. That is not necessary, because the whole model is Read-only.

The data comes from a file, and I use QString.split to devide it. Split returns a QStringList, which is a QList<QString>. I could unfortunately not figure out how to convert the QStringList<QString> to a QStringList<const QString>. Is there a possiblility to do that? Or is the only possibility to save the model data as QVector<QString> (without const), and then call resize and squeeze() for every single entry?

Unfortunatly I could not find any relevant information about this in the Qt Forums, this is why I ask that Question here.

joeQ

@randsfjorden

hi, friend, welcome devnet.
i tred way of your said, it is unallowed convert QStringList to convert QList<const QString> or QVector<const QString> direct.
we know than const variable is unallowed to modify after initalization.
in my sight, i hasve to use the const QVector<QString> strVe = strlt.toVector(), like the following code:


#include <QCoreApplication>

#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QString str = "one,two,three,four";
    QStringList strlt = str.split(QChar(','));

    // QVector<const QString> strVe = strlt.toVector(); // not allow
    const QVector<QString> strVe = strlt.toVector();
    qDebug() << strVe;
    strVe[0] = "five"; // unallowed to modify the data

    /**
    strVe.cbegin();
    strVe.constBegin();
    strVe.constData();
    strVe.constFirst();
    strVe.constLast();
    */

    return a.exec();
}

maybe some one has the better way.

J.Hilk

Hi,
recently I learned something new from @Chris-Kawa .
Namly the existence of

QString::splitRef

it seems, this might be the solution for your situation :)

SplitRef does not return a copy of strings like normal split would do.

jsulm

@randsfjorden "I want to use const QStrings, because I don't want Qt to preallocate memory for every single QString" - what do you mean? You need memory for each string you store in the vector. What does "preallocate" mean in this context?

randsfjorden

Heyy
Thank you very much for replying that quickly. Unfortunately I wasn't notified about your answers, this is why I didn't see them before today.

Experimented a little bit more with const QStrings and now I think that it was no good approach for that purpose, to safe memory in this particular case it was more efficient to call squeeze.

@joeQ I now used your approach. I used qDebug to check the capacities/lengths of the strings inside the QStringList i got from split, and found out that non of the strings in that list had a higher capacity than length. So it was unneccessary to call squeeze on these, but I used it for the resulting vector.

@jsulm Qt automatically allocates some more memory when a QString / QStringList / QVector is created, to avoid needing to copy the object if something gets added to it. If I for eksample initialize a QString with 5 letters, Qt would preallocate space in memory for 10, so that it doesn't need to copy it, when the user adds 5 more letters. This preallocation is what I wanted to avoid, because I know that I won't change the strings in the ReadOnly-model again.

@J-Hilk: I actually checked that possibility out as well :)

Thank you very much for helping me!

jsulm

@randsfjorden From your description it wasn't clear what you mean. Now I understood what your question was about.