Const-correctness

[genemerewether]

Hi, is there a policy / upcoming work on const versions of API functions, where reasonable?

i.e. const accessors for internal::Unique, const pointers in function prototypes where the underlying data is not modified, or where a const flavor of e.g. ArrowArrayView could be used instead?

[paleolimbot]

I honestly just haven't thought about it! Most current usage/testing is in C right now where const-ness isn't quite as baked in to the language.

One first step is could be to go through the C API functions and declare pointers that are not modified as const...there are quite a few of them but mostly that lives in documentation at this point and not as const declarations.

[paleolimbot]

Just a note that I'm bumping this to 0.4 because it would result in a lot of compiler warnings for any project that was using a development version between 0.2 and 0.3 (and because the 0.3 release is forthcoming!).

This has come up a few times in existing usage...declaring input pointers (e.g., ArrowSchema*) in the C API as const ArrowSchema* is also a helpful way to communicate that the function will not take ownership of the schema.