Backwards-compatible QDataStream?

[tmladek]

Hi, is it possible to make QDataStream's output "backwards-compatible"?

I mean something like Google's protobuf, where when you add a field, it doesn't affect any of the apps that don't know how to deal with it. I would like to be able to add (or even remove fields) without making the whole file unusable for other/older applications.

Thanks!

[THE_MASTER]

Use QDataStream::setVersion to set fixed minimal version

[VRonin]

Unfortunately there is no visioning implemented out of the box. it's up to you to make it work with old versions.

To be clear what I mean is if you have this old code:

void write(QIODevice* destination)
{
qint32 a=0;
qint32 c=3;
QDataStram stream(destination);
stream << a <<c
}
void read(QIODevice* source){
qint32 a;
qint32 c;
QDataStram stream(source);
stream >> a >> c;
}

and you want to change it into

void write(QIODevice* destination)
{
qint32 a=0;
qint32 b=1;
qint32 c=3;
QDataStram stream(destination);
stream << a << b<< c;
}
void read(QIODevice* source){
qint32 a;
qint32 b;
qint32 c;
QDataStram stream(source);
stream >> a >> b >> c;
}

There is no out-of-the-box solution to make it backward compatible

[raven-worx]

@tmladek
normally you write your own custom version number (magic-number) at the beginning of the data stream so you know how the data structure looks like before starting reading.

[mtrch]

You can use JSON or XML and applications will process only needed tags/values.

[mtrch]

Another solution: Store all your data as key-value pairs in QMap<QString, QVariant> and write it to QDataStream. Next, applications will read full QMap contents from stream, then access needed values by their names.