Remove unclass call from [.sfc

[dbaston]

References #1666. This does not replace the solution in 4911f5a but greatly improves runtime in other cases where you need to index into an sfc. There appear to be other cases that could could benefit from the same technique.

[edzer]

Makes total sense, I'm amazed that this matters so much. Thanks!

[dbaston]

From ?unclass, "unclass returns (a copy of) its argument with its class attribute removed. (It is not allowed for objects which cannot be copied, namely environments and external pointers.)"

So if you loop over 50,000 geometries, you copy 50,000 geometries 50,000 times :)

[dbaston]

@edzer So apparently this commit broke the following:

x <- st_as_sfc('POINT (3 8)')
x[1]   # works
x[1, ] # used to work, now gives an "incorrect number of dimensions" error

Was the second usage supposed to work? Is the behavior change a problem?

[edzer]

No, that was not supposed to work. sfc is a list, and needs a single index. Let's see what the revdep checks give, it may be necessary to postpone this change if things are bad.