How to save QList into QSettings file

Question

I have a large amount of data stored in a variable of type QList<QMap<QString,float>> and I need to save the variable to easily retrieve it. I want to save it into a file with QSettings but with no luck. How can I do this?

score 2 · Accepted Answer · answered Jan 09 '19 at 13:48

In order to save some type of data QSettings uses QDataStream so you must implement it for QMap <QString, float>, in addition to registering it using qRegisterMetaTypeStreamOperators.

#include <QCoreApplication>
#include <QSettings>
#include <QDebug>
#include <QDataStream>

#ifndef QT_NO_DATASTREAM
QDataStream &operator<<(QDataStream &stream, const QMap<QString,float> &map)
{
    QMap<QString, float>::const_iterator i = map.constBegin();
    while (i != map.constEnd()) {
        stream << i.key() << i.value();
        ++i;
    }
    return stream;
}
QDataStream &operator>>(QDataStream &stream, QMap<QString,float> &map)
{
    QString key;
    float value;
    stream >> key;
    stream >> value;
    map[key] = value;
    return stream;
}
#endif

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    qRegisterMetaTypeStreamOperators<QMap<QString,float>>("QMap<QString,float>");
    qRegisterMetaTypeStreamOperators<QList<QMap<QString,float>>>("QList<QMap<QString,float>>");
    {
        QSettings settings;
        QList<QMap<QString,float>> l;
        for(int i=0; i<3; i++){
            QMap<QString, float> map;
            map["a"] = 0.1*(i+1);
            map["b"] = 0.2*(i+1);
            map["c"] = 0.3*(i+1);
            l<< map;
        }
        QVariant v = QVariant::fromValue(l);
        settings.setValue("val", v);
    }
    {
        QSettings settings;
        QVariant v = settings.value("val");
        QList<QMap<QString,float>> l = v.value<QList<QMap<QString,float>>>();
        int j=0;
        for(const QMap<QString, float> &map: l ){
             qDebug()<< "index: " << j;
            QMap<QString, float>::const_iterator i = map.constBegin();
            while (i != map.constEnd()) {
                qDebug() << i.key() << ": " << i.value();
                ++i;
            }
            j++;
        }
    }
    return 0;
}

Output:

index:  0
"a" :  0.1
"b" :  0.2
"c" :  0.3
index:  1
"a" :  0.2
"b" :  0.4
"c" :  0.6
index:  2
"a" :  0.3
"b" :  0.6
"c" :  0.9

score 0 · Answer 2 · answered Jan 09 '19 at 12:52

0

QSettings settings("name" "key");
foreach(QMap<QString, float> element, list)
    settings.value(element.key,element.value);

This would be my approach. What is not working?

answered Jan 09 '19 at 12:52

RoQuOTriX

2,871
14
25

this one will not work with me as the keys of every map object inside the list are the same. – Alaa Agwa Jan 09 '19 at 12:54
So I would say this is a design problem and not a QSettings problem? Why using a map with always the same key? – RoQuOTriX Jan 09 '19 at 12:56
it's not a design problem of course, let's assume that these maps are some signals and it's values during some certain amount of time. the values are changing from a map to another. – Alaa Agwa Jan 09 '19 at 12:58
Then maybe at a timestamp to the QStrings? When I consider it right, all QMaps are one measurement of different singals at the same time? Or consider using an own class inheriting from QMap, where you add a timestamp member, that you can use while saving – RoQuOTriX Jan 09 '19 at 13:04

How to save QList into QSettings file

2 Answers2