design pattern for "save" - c++

Question

I'm currently working on a "save" mechanism, which allows a user to save the project his working on on hard disc. The output will be a XML file containing all kinds of data.
Now our project structure is about to change and we need to write a new xml file (create a new save method).
So now here comes the challenge: When saving I want the user to be able to choose which file format he will be creating (version1 (old) or version2 (new)).
Does anyone now how to achieve that? Is there a suitable design pattern around?
Remarks:
- The data we are saving can be seen as unrelated blocks, so it would actually be easy to exchange an old block with a new one.
- The whole goal of the thing is, it should be readable again when loading an old project. (I assume this can be done by tags, and just react on tags when loading?)

Answer 1

This sounds like a good application for the Strategy pattern.
You would create an abstract base class FileFormat (the Strategy interface) with two virtual functions, projectToXml and xmlToProject, which are supposed to turn your internal project representation into XML or vice versa.
Then you create two implementing subclasses FileFormatNew and FileFormatLegacy (these are the concrete strategies).
Your save functions would then additionally require an instance of FileFormat, and call the corresponding method of that object to do the data conversion. Your load function could choose the strategy to use by examining the XML tree for something which tells it which version it is.
And when you ever need to support another file format, you just have to create a new class which is a subclass of FileFormat.
Addendum after the exchange in the comments
When you are going to have a lot of versions with very small differences and you still want to use the Strategy pattern, you could make the FileFormat a composite of multiple strategies: A CircleStragegy, a RectangleStrategy, a LineStrategy etc.. In that case I wouldn't use different classes for different versions of the FileFormat. I would create a static factory function for each version which returns a FileFormat with the Strategy objects used in that version.
FileFormat FileFormat::createVersion1_0() {
return new FileFormat(
new LineStrategyOld(),
new CircleStrategyOld(),
new RectangleStragegyOld()
);
}
FileFormat FileFormat::createVersion1_1() {
// the 1.1 version introduced the new way to save lines
return new FileFormat(
new LineStrategyNew(),
new CircleStrategyOld(),
new RectangleStragegyOld()
);
}
FileFormat FileFormat::createVersion1_2() {
// 1.2 uses the new format to save circles
return new FileFormat(
new LineStrategyNew(),
new CircleStrategyNew(),
new RectangleStragegyOld()
);
}
FileFormat FileFormat::createVersion1_3() {
// 1.3 uses a new format to save rectangles, but we realized that
// the new way to save lines wasn't that good after all, so we
// returned to the old way.
return new FileFormat(
new LineStrategyOld(),
new CircleStrategyNew(),
new RectangleStragegyNew()
);
}
Note: In real code you would of course use more descriptive suffixes than "Old" and "New" for your strategy class names.