Determine why size of S4 objects is different about lobstr

[hadley]

• Off by 24 bytes (= 3 * 8)

• Base R code breaks down S4SXP in same way as me: https://github.com/wch/r-source/blob/master/src/library/utils/src/size.c#L119

• Doesn't appear to be because an object is referenced twice

• Size of attributes() is equal

[hadley]

Z <- methods::setClass("Z", slots = c(x = "integer", y = "character"))
z <- Z(x = 1L, y= "a")
  
obj_size(z)
object.size(z)

obj_size(node_tag(z))
object.size(node_tag(z))

obj_size(node_attrib(z))
object.size(node_attrib(z))

The difference seems to be the size of the S4 object itself, not its children.

May because I always start with 4 * ptr_size, and object.size() uses sizeof(SEXPREC_ALIGN) for vectors, and sizeof(SEXPREC) for everything else.