class<- vs. as()

Dear all,

I would appreciate a lot, if someone could explain to me in a simple
way, why the assignment class<- is not always working and one has to
take as() like in the example below.

(v <- matrix(1:9, 3))

     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

class(v)

[1] "matrix"

class(v) <- "integer"
class(v)

[1] "matrix"

v

     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

(vi <- as(v, "integer"))

[1] 1 2 3 4 5 6 7 8 9

class(vi)

[1] "integer"


With many thanks,

Stefan
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