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.