[R-Forge #2296] Copy-on-:= at column level, DT[,list(...)] shallow copy and add cols to shallow(DT, cols)

[arunsrinivasan]

Submitted by: Matt Dowle; Assigned to: Nobody; R-Forge link

Export shallow().

DT2 = shallow(DT)

would add reference counter attribute. (Both DT2[1,a:=5L] AND DT[1,a:=5L] would need to copy a. Maybe the ExternalPointer destructor could decrement as well, so a rm(DT2) would automatically take DT back to being the sole copy.

Then with this in place,

DT[, cols, with=FALSE]  
# and 
DT[, list(col1,col2,...)]

could do shallow copies, too. See TO DO in source.

In the meantime, allow shallow() to accept a subset of columns and then use it in duplicated.R to save the copies of the by columns there. Upped priority for this part (easier).

[arunsrinivasan]

@mattdowle, the idea we recently discussed is already here (and written here in an even better fashion). Would be great to have this for next release! Also in my wish list for shallow() to accept a set of columns.. We could've shallow() and shallowv() like usual.