If you're calling C functions that take your member variables by non-const pointer, those wrapper functions should probably not be const. If they're only observing state and not modifying it, you can make your methods const--even if the C API is not const-correct you can use const_cast<>
or mutable
on your member variables as needed.
Think about it in terms of the semantics--if a method doesn't alter the state of the world, make it const. If it does alter the state of the world, it probably shouldn't be const, unless the altered state is something like a cache where it's only an optimization that results in the alteration and not something essential.