Explore changes to object.size in R-devel about lobstr

[hadley]

size.c hasn't changed since 2015, so base R clearly isn't doing anything yet.

From ALTREP vignette:

ALTREP objects are allocated as CONS objects and identified by having the altrep bit set in the header. The GC checks for the bit and scans the fields as for a CONS cell. The TAG field holds class information; the CAR and CDR hold SEXP values that are used for instance data; they are considered invisible outside of methods for the specific class.

So an altrep object has two additional SEXPS; this is more easily seen in the constructor

Can tell if object uses altrep with ALTREP (which I'm pretty sure is not in internals)