Caching reference objects

[rhijmans]

Is there a way, or could support be added, to tell knitr how to serialize objects of a particular class before caching, and how to restore them after caching? See this and this questions on stackoverflow for more context

The "terra" package uses S4 objects with one slot that has a reference to a C++ object (via a Rcpp module). These S4 objects can be serialized and unserialized with very little effort. For example, one can do

library(terra)
f <- system.file("ex/elev.tif", package="terra")
r <- terra::rast(f)  # S4 object that has a reference class
x <- serialize(r, NULL) 
# recreate the object 
s <- terra::rast( unserialize(x) )  
s
#class       : SpatRaster 
#dimensions  : 90, 95, 1  (nrow, ncol, nlyr)
#resolution  : 0.008333333, 0.008333333  (x, y)
#extent      : 5.741667, 6.533333, 49.44167, 50.19167  (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (EPSG:4326) 
#source      : memory 
#name        : elevation 
#min value   :       141 
#max value   :       547 

But can one use that mechanism with knitr caching? I suppose that it could be possible, in principle, to specify that objects of class "SpatRaster" should be passed to serialize before caching, and that terra::rast should be used when restoring the object. Whether this is easy to do in practice, I do not know.

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