What does mean the class attribute in structure() and how it works ?

[mikrom]

Dear R-Gurus,
I needed to create in R a function similar to build-in acf() (= Auto Correlation Function).
First I looked at the source of of build-in acf() using the command

getAnywhere(plot.acf)

Then I created my function named my_acf() with almost same result structure as the original acf() has, i.e.:

> str(my_acf(x))
List of 6
 $ acf   : num [1:4, 1, 1] 1 0.25 -0.3 -0.45
 $ type  : chr "correlation"
 $ n.used: int 4
 $ lag   : num [1:4, 1, 1] 0 1 2 3
 $ series: chr "x"
 $ snames: NULL

> str(acf(x))
List of 6
 $ acf   : num [1:4, 1, 1] 1 0.25 -0.3 -0.45
 $ type  : chr "correlation"
 $ n.used: int 4
 $ lag   : num [1:4, 1, 1] 0 1 2 3
 $ series: chr "x"
 $ snames: NULL
 - attr(*, "class")= chr "acf"

But I had no luck with that:
Although the original function acf() returns on this vector

> x <- c(1, 2, 3, 4)

the nice output

> y <- acf(x)
> y

Autocorrelations of series ‘x’, by lag

    0     1     2     3 
 1.00  0.25 -0.30 -0.45

my function my_acf() returned this ugly output

> y <- my_acf(x)
> y

$acf
, , 1

      [,1]
[1,]  1.00
[2,]  0.25
[3,] -0.30
[4,] -0.45


$type
[1] "correlation"

$n.used
[1] 4

$lag
, , 1

     [,1]
[1,]    0
[2,]    1
[3,]    2
[4,]    3


$series
[1] "x"

$snames
NULL

Then I noticed that I don't have the class="acf" attribute in my function result.
I added it and now it's OK:

> y <- my_acf(x)
> y

Autocorrelations of series ‘x’, by lag

    0     1     2     3 
 1.00  0.25 -0.30 -0.45 
> y <- acf(x)
> y

Autocorrelations of series ‘x’, by lag

    0     1     2     3 
 1.00  0.25 -0.30 -0.45

Now the output of my function my_acf() matches the output of the built-in acf() function.

But still, I don't understand how R knows that it should make a special output for the class "acf".
Where and how is it coded?

 

[mikrom]

These sources helped me to answer my question:

https://www.datamentor.io/r-programming/s3-class

https://stackoverflow.com/questions/8691812/get-object-methods-r

https://stackoverflow.com/questions/47563538/creating-s3-methods-in-r

https://stackoverflow.com/questions/6583265/what-does-s3-methods-mean-in-r