How to deserialize a QMap<int,QVariant> ?

Mike Fidel

Hello!

I need help deserializing a map that has QVariants as values...

I use the following code to serialize it:

void MyClass::SetMessageDataFromMap(QMap<int, QVariant> &myMap, QByteArray &myData)
{

    QDataStream oMessageDataStream(&myData,QIODevice::WriteOnly);

    oMessageDataStream << myMap;    

}

I say it seems to work because I seem to see some data in the corresponding variable when I look at it in the debugger after serialization. However, I'm not sure if the data is correct, or is it just gibberish.

I use the following code to deserialze the map:

void MyClass::GetMessageDatatoMap(QMap<int, QVariant> &myMap, QByteArray &myData)
{

  QDataStream in (&myData,QIODevice::ReadOnly);
  in >> myMap;

}

When I searched Google for the answer, I found that QVariant serialization isn't supported 'out of the box'. So I would appreciate any pointers to how I should go about serializing and deserializing QMap<int, QVariant> or even if such practice is advisable at all.

Taz742

@Mike-Fidel Hi.
Hm... Its work for me.

#include "mainwindow.h"
#include <QApplication>
#include "QDebug"

template<typename T>
void writeData(T &val, QByteArray &out) {
    QDataStream in(&out, QIODevice::WriteOnly);
    in << val;
}

template<typename T>
void readData(T &val, QByteArray &data) {
    QDataStream out(&data, QIODevice::ReadOnly);
    out >> val;
}

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

    QMap<int, QVariant> mpWrite;
    mpWrite.insert(1, "taz");
    mpWrite.insert(2, 2);
    mpWrite.insert(3, 3.2);

    QByteArray writeBuff;
    writeData(mpWrite, writeBuff);

    QMap<int, QVariant> mpRead;
    readData(mpRead, writeBuff);

    for (const auto &it : mpRead) {
        qDebug() << it;
    }
    //Result is
    //QVariant(QString, "taz")
    //QVariant(int, 2)
    //QVariant(double, 3.2)

    MainWindow w;
    w.show();

    return a.exec();
}

You can use QVariant here to deserialize it.
What types are in QVariant back? Only primitive types(int, double, QString etc..) or your custom Class?

JonB

@Mike-Fidel said in How to deserialize a QMap<int,QVariant> ?:

When I searched Google for the answer, I found that QVariant serialization isn't supported 'out of the box'.

? http://doc.qt.io/qt-5/datastreamformat.html describes how QVariant is supported for serialization, so correspondingly you can deserialize.

Alternatively, you can also do a QVariant via JSON.

Mike Fidel

@Taz742 said in How to deserialize a QMap<int,QVariant> ?:

Thank you! It does work indeed. Turns out, I had a bug elsewhere... Thank you for your time.

For anyone else wondering, This works exactly like @Taz742 said!

Taz742

@Mike-Fidel Good!
If you have a number of functions that you use to write or read different types of data, use the template <> as you have seen in the example. Because it will save you a lot of time!

Something like static function.

#include "mainwindow.h"
#include <QApplication>
#include "QDebug"

class MyStreamOperators {
    public:
        template<typename T>
        static void writeData(T &val, QByteArray &out) {
            QDataStream in(&out, QIODevice::WriteOnly);
            in << val;
        }

        template<typename T>
        static void readData(T &val, QByteArray &data) {
            QDataStream out(&data, QIODevice::ReadOnly);
            out >> val;
        }
};

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

    QMap<int, QVariant> mpWrite;
    mpWrite.insert(1, "taz");
    mpWrite.insert(2, 2);
    mpWrite.insert(3, 3.2);

    QByteArray writeBuff;
    MyStreamOperators::writeData(mpWrite, writeBuff);

    QMap<int, QVariant> mpRead;
    MyStreamOperators::readData(mpRead, writeBuff);

    for (const auto &it : mpRead) {
        qDebug() << it;
    }
    //Map example End. ///////////////////////////////////////////////////////


    QVector<int> vectorToWrite = {1, 2, 3, 4, 5};
    QByteArray writeVectorBuff;
    MyStreamOperators::writeData(vectorToWrite, writeVectorBuff);

    QVector<int> vectorToRead;
    MyStreamOperators::readData(vectorToRead, writeVectorBuff);

    for (const auto &it : vectorToRead) {
        qDebug() << it;
    }
    //Vector example End. ///////////////////////////////////////////////////////


    return a.exec();
}

And dont forget to mark this topic to SOLVED. Thank :)

Mike Fidel

@Taz742 Thank you for the tip!