I am looking for simple syntax for data.table's cartesian join e.g. I am using as below to get 1024 row in c_join but it requires to convert data.table to data.frame
d1 <- data.table(mtcars)
d2 <- data.table(mtcars)
c_join <- setDT(merge(x=as.data.frame(d1), y=as.data.frame(d2), by=NULL))