UseMethod common tasks all S3 classes - r

Question

I am using S3 methods in that way.
First, seek all commonn task between all classes programmed and put this code only once before "Usemethod". Then, I program the rest of each class.
The problem arises when you modify an argument, because they are defined by-reference. Other tasks like check arguments or define sub-functions works well in these schemas.
The next example, I modify an argument:
secure_filter <- function(table, col, value){
if(!is.numeric(table[[col]])) table[[col]] <- as.numeric(table[[col]])
message("converting to numeric")
print(str(table))
UseMethod("secure_filter", table)
}
secure_filter.data.table <- function(
table, col, value
){
return(table[ col == value,])
}
secure_filter.data.frame <- function(
table, col, value
){
return(table[table$col == !!value,])
}
and the result is wrong
df <- data.frame(a=c("a", "b", "c"), column = c("1", "2", "3"))
dt <- as.data.table(df)
> secure_filter(dt, "column", 1)
converting to numeric
Classes ‘data.table’ and 'data.frame': 3 obs. of 2 variables:
$ a : chr "a" "b" "c"
$ column: num 1 2 3
- attr(*, ".internal.selfref")=<externalptr>
NULL
Empty data.table (0 rows and 2 cols): a,column
> secure_filter(df, "column", 1)
converting to numeric
'data.frame': 3 obs. of 2 variables:
$ a : chr "a" "b" "c"
$ column: num 1 2 3
NULL
[1] a column
<0 rows> (or 0-length row.names)
Am I using S3 well? How do I save repeated code between S3 classes?
Any example in a well known R function?
I am using this approach to re-program tidyverse scripts to data.table scripts.

Answer 1

Use NextMethod instead UseMethod.
secure_filter <- function(table, col, value){
if(!is.numeric(table[[col]])) table[[col]] <- as.numeric(table[[col]])
message("converting to numeric")
print(str(table))
NextMethod("secure_filter")
#UseMethod("secure_filter", table)
}
> secure_filter(dt, "column", 1)
converting to numeric
Classes ‘data.table’ and 'data.frame': 3 obs. of 2 variables:
$ a : chr "a" "b" "c"
$ column: num 1 2 3
- attr(*, ".internal.selfref")=<externalptr>
NULL
a column
1: a 1
> secure_filter(df, "column", 1)
converting to numeric
'data.frame': 3 obs. of 2 variables:
$ a : chr "a" "b" "c"
$ column: num 1 2 3
NULL
a column
1 a 1
But I don´t know that answer is well behaved because it doesn't shot the dispatched method neither get the generic.
> sloop::s3_dispatch(secure_filter(dt, "column", 1))
secure_filter.data.table
secure_filter.data.frame
secure_filter.default
> sloop::s3_get_method(secure_filter)
Error: Could not find generic