[R] Why is R making a copy-on-modification after using str?

[Georg Kindermann]

Dear list members,

I was wondering why R is making a copy-on-modification after using str.

m <- matrix(1:12, 3)
tracemem(m)
#[1] "<0x559df861af28>"
dim(m) <- 4:3
m[1,1] <- 0L
m[] <- 12:1
str(m)
# int [1:4, 1:3] 12 11 10 9 8 7 6 5 4 3 ...
dim(m) <- 3:4  #Here after str a copy is made
#tracemem[0x559df861af28 -> 0x559df838e4a8]:
dim(m) <- 3:4
str(m)
# int [1:3, 1:4] 12 11 10 9 8 7 6 5 4 3 ...
dim(m) <- 3:4 #Here again after str a copy
#tracemem[0x559df838e4a8 -> 0x559df82c9d78]:

Also I was wondering why a copy is made when having a Task Callback.

TCB <- addTaskCallback(function(...) TRUE)
m <- matrix(1:12, nrow = 3)
tracemem(m)
#[1] "<0x559dfa79def8>"
dim(m) <- 4:3  #Copy on modification
#tracemem[0x559dfa79def8 -> 0x559dfa8998e8]:
removeTaskCallback(TCB)
#[1] TRUE
dim(m) <- 4:3  #No copy

I am using R version 4.0.3.

Kind regards,
Georg