QSettings sqlite format

main.cpp

#include <QtWidgets>
#include <QtSql>

//to ensure the database connection is closed and removed at the end
//of the write/read func call
class SQLConnectionRAIIWrapper{
public:
    SQLConnectionRAIIWrapper(QString fileName){
        //use a unique connection name, to ensure thread safety
        connectionName = QString("SQLSETTINGS%1").arg((int)QThread::currentThreadId());
        QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE", connectionName);
        db.setDatabaseName(fileName);
        db.open();
    }
    ~SQLConnectionRAIIWrapper(){
        getDb().close();
        QSqlDatabase::removeDatabase(connectionName);
    }
    QSqlDatabase getDb(){ return QSqlDatabase::database(connectionName); }
private:
    QString connectionName;
};

//write function creates a temporary file, stores
//map in it (using sqlite), writes data from the temp file into the device
bool writeSqliteFunc(QIODevice& device, const QSettings::SettingsMap& map){
    QTemporaryFile tempFile;
    if(!tempFile.open()) return false;
    QString tempFileName = tempFile.fileName();
    tempFile.close();
    {
        SQLConnectionRAIIWrapper connectionWrapper(tempFileName);
        QSqlDatabase db = connectionWrapper.getDb();
        if(!db.isOpen()) return false;
        QSqlQuery query(db);
        if(!query.exec("CREATE TABLE IF NOT EXISTS settings(key TEXT PRIMARY KEY, value TEXT);"))
            return false;
        query.prepare("REPLACE INTO settings(key, value) VALUES(?, ?)");
        QVariantList keys;
        for(QVariant key: map.keys()) keys.append(key);
        query.addBindValue(keys);
        query.addBindValue(map.values());
        if(!query.execBatch()) return false;
    }
    if(!tempFile.open()) return false;
    auto bytesWritten = device.write(tempFile.readAll());
    tempFile.close();
    return bytesWritten > 0;
}

//uses a similar strategy to read the data from a temporary file into the map
bool readSqliteFunc(QIODevice& device, QSettings::SettingsMap& map){
    QTemporaryFile tempFile;
    if(!tempFile.open()) return false;
    QString tempFileName = tempFile.fileName();
    tempFile.write(device.readAll());
    tempFile.close();

    SQLConnectionRAIIWrapper connectionWrapper(tempFileName);
    QSqlDatabase db = connectionWrapper.getDb();
    if(!db.isOpen()) return false;
    QSqlQuery query(db);
    if(!query.exec("SELECT key, value FROM settings;"))
        return false;
    while(query.next()){
        QString key = query.record().value("key").toString();
        QVariant value = query.record().value("value");
        map[key]= value;
    }
    return true;
}

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

    const QSettings::Format sqliteFormat = QSettings::registerFormat("sqlite", &readSqliteFunc, &writeSqliteFunc);

    QWidget w;
    QFormLayout layout(&w);
    QLineEdit lineEditA;
    QLineEdit lineEditB;
    QPushButton buttonSave("Save");
    layout.addRow("KeyA:", &lineEditA);
    layout.addRow("KeyB:", &lineEditB);
    layout.addRow(&buttonSave);

    //read settings
    QSettings settings("settings.sqlite", sqliteFormat);
    lineEditA.setText(settings.value("KeyA").toString());
    lineEditB.setText(settings.value("KeyB").toString());
    QObject::connect(&buttonSave, &QPushButton::clicked, [&]{
        //save settings
        QSettings settings("settings.sqlite", sqliteFormat);
        settings.setValue("KeyA", lineEditA.text());
        settings.setValue("KeyB", lineEditB.text());
    });
    w.show();

    return a.exec();
}