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.