Peter Dimov wrote:
Joel de Guzman wrote:
Peter Dimov wrote:
Hi,
Here's a use case that I can't seem to find a way to do using boost.serialization.
Say with version 1, I want to serialize X Y and Z. These are hierarchical and expensive data structures. Now, with version 2, I do not need to create Y objects anymore. Yet, I still have to open version 1 files for backward compatibility. So, when loading version 1 files, I want to skip reading Y objects. How do I do that without having to create a temporary Y object that will be discarded later? You need to define a proxy Y object that can deserialize itself into
Joel de Guzman wrote: thin air. I thought about that. But that would mean that blank_proxy<Y> knows all about Y -- its composition down to the leaves -- and wrap them all as blank_proxy<T>s. A hairy proposition, IMO.
This is the only way that works reliably with any archive. :-) (And, since the serialization library does not document its external format, the only way that works reliably with it.)
The option of actually reading a temporary Y and destroying it afterwards is also feasible. You don't necessarily have to optimize the new version so that it reads v1 files faster than v1 itself. :-)
There might be a possibility that Y is already obsolete at version 2. Sure you can "emulate" it through the proxy thing, but then again that's less than ideal. I simply want to skip some data and I can't do it with Boost.Serialization. I can do it with simple streams, for example, by prepending the data with a length (in bytes). I can also do it with XML by ignoring everything in between the current tag and its matching end tag. Rene (Rivera) noted that some archive types also allow it. For example, IFF (http://en.wikipedia.org/wiki/Interchange_File_Format) allows it through chunking. "Because the spec includes explicit lengths for each chunk, it is possible for a parser to skip over chunks which it either can't or doesn't care to process." IMO, this is a valid use case that Boost.Serialization can address, at least, for archive forms that allow it. Perhaps a SkippableArchive concept? IMO, the ability to skip is a prerequisite for transparent versioning. Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net