Serializing QVariant through QDataStream

Question

I'm possibly writing it wrong but here's the code that i'm trying to execute and it fails to do what's expected:

#include <QDataStream>
#include <QByteArray>
#include <QVariant>
#include <iostream>


int main(){

    QByteArray data;
    QDataStream ds(&data,QIODevice::WriteOnly);
    QVariant v(123);
    ds << v;
    std::cout <<"test: " <<  data.data() << std::endl; // "test: 123"


    return 0;
}

Given the example in the documentation of the QVariant class :

http://qt-project.org/doc/qt-5.1/qtcore/qvariant.html#type

It should serialize the value 123 correctly to the QByteArray but doesn't do so,instead it just writes out:

test: 

Anybody has an idea how to fix this ?

EDIT

Well, maybe it was not clear but here is the original problem:

I have possibly any QVariant built-in type stored in the QVariant such as QStringList, QString , double, int , etc....

What I want is a way to serialize the QVariant to a string and restoring it without having to do it myself for each type. As far as I know the QVariant::toString() method does not work with all types that are accepted through QVariant, and I was thinking that passing by a QDataStream could pass me a serialized version of the QVariant.

EDIT 2

Thanks to the answer of piotruś I was able to answer my problem. Here's the program:

int main(){
    QString str;
    {
        //serialization
        QByteArray data;
        QDataStream ds(&data,QIODevice::WriteOnly);
        QVariant v(123);
        ds << v;
        data = data.toBase64();
        str = QString(data);
        cout << str.toStdString() << endl;

    }
    {
        //deserialization
        QByteArray data = str.toLatin1();
        data = QByteArray::fromBase64(data);
        QDataStream ds(&data,QIODevice::ReadOnly);
        QVariant v;
        ds >> v;
        cout << v.toInt() << endl;
    }

    return 0;
}

Answer 1

QDataStream does not write text, it writes binary data in the form specified here. So you should not expect to see any printable characters in the byte array, and certainly not the string representation of the number.

QTextStream is used for writing out text, however it is not directly compatible with QVariant. You would need to write your own code to test what type the variant contains and write out the appropriate string.

Answer 2

QByteArray data;
QDataStream dsw(&data,QIODevice::WriteOnly);
QVariant v(123);
dsw << v;

QDataStream dsr(&data,QIODevice::ReadOnly);
QVariant qv;
dsr>>qv;
qDebug()<<qv.toString();

However your data 123 is written to QByteArray already. The point is QByteArray being stored as array of char in memory will not show you "123" if you try to debug this but will show "{" instead which is ASCII character of code 123. You can still print this QByteArray from the raw memory. You will do it this way:

const char *dataPtr = data.operator const char *();
     int i=0;
     while (i<9) {
         std::cout << "[" << *dataPtr << "]";
         ++dataPtr;
         ++i;
     }

and the output is: [] [] [] [] [] [] [] [] [{]

you might want to do `

data.setByteOrder( QDataStream::LittleEndian );

to print bytes from stream stored in little endian order.