SQLite Forum

Subclassing & private data
> However, I wonder how versioning is supposed to work. Some implementation will put in a size parameter to size the structure as to infer the version of the struct. Without that information, a new version of struct could potentially read private data from the old version and therefore garbage. But I don't see such member in sqlite's structure. Does the member iVersion of the struct sqlite3_module handle this?

I'm reasonably sure that the iVersion member is intended to deal with attempts to use extensions written for SQLite major version 3 with some later version(s). In other words, it addresses possible changes in the extension **host**.

The problem you are wondering about is how to deal with changes in your own code. (I presume you are writing such, or contemplate doing so.) For that, you probably need a mechanism that is available even before attempting to "attach" an extension after it is loaded. And that mechanism will be one you devise or model after similar solutions. This is a non-trivial problem, especially with dynamically loaded code. And the solutions vary according to how much and what kind of change is anticipated. You should not expect a ready-made solution to that general problem in the SQLite library or project.

I say this to perhaps put an end to your wondering how that problem is already solved. It is not.

You may recognize, in the pre-defined structs for various extension types, the polymorphism that is available in C++ and other fully OO languages. Typically, that is utilized purely through variation in what the methods do, without relying on varying data being somehow dealt with at the interface. I suggest that this sort of discipline will simplify how you might insulate your extension design from implementation changes or behavior evolution at the interface.