Taking advantage of generic object functions with S3 classes can be done using the following steps:

1. Create a generic function in the plot() method:

plot.SimpleGenome <- function(x){
 barplot(x$chromosome_lengths, main = "Chromosome Lengths")
}

2. Create an object and use it in plot():

athal <- SimpleGenome(nchr = 5, lengths = c(34964571, 22037565, 25499034, 20862711, 31270811 ) )
plot(athal)

3. Create a new method first:

genomeLength <- function(x){
 UseMethod("genomeLength", x)
}

genomeLength.SimpleGenome <- function(x){
 return(sum(x$chromosome_lengths))
}
genomeLength(athal)

4. Modify an existing object's class:

some_data <- iris
summary(some_data)
class(some_data) <- c("my_new_class", class(some_data) )
class(some_data)

5. Create a generic function for the new class:

summary.my_new_class <- function(x){
 col_types <- sapply(x, class)
 return(paste0("object contains ", length(col_types), " columns of classes:", paste (col_types, sep =",", collapse = "," )))
}
summary(some_data)