为了简化起见，S3 对象是具有特殊属性“class”的列表，用于调用正确的通用函数。print是一个通用函数，当 R 输出时被调用tibble object.
library(tibble)
tb <- tibble(x = 1:5, y = 1, z = x ^ 2 + y)
dput(tb)
#structure(list(x = 1:5, y = c(1, 1, 1, 1, 1), z = c(2, 5, 10,
#17, 26)), row.names = c(NA, -5L), class = c("tbl_df", "tbl",
#"data.frame"))
attributes(tb)
#$`names`
#[1] "x" "y" "z"
#
#$row.names
#[1] 1 2 3 4 5
#
#$class
#[1] "tbl_df" "tbl" "data.frame"
当您使用 S3 父类创建 S4 类时，R 仅将列表存储在.Data投币口。 R 仍然保留 S3 对象的属性，但不在.Data投币口。当您打印TestClass，您将获得 tibble 输出加上 S4 插槽。如果只想要 S3 对象，您可以使用as(object,"S3").
setOldClass(c('tbl_df', 'tbl', 'data.frame'))
setClass(Class = 'TestClass', slots = c(name = 'character'), contains = 'tbl_df')
tmp1 <- new('TestClass', tibble(x = 1:5, y = 1, z = x ^ 2 + y), name = 'firsttest1')
tmp1
#Object of class "TestClass"
## A tibble: 5 x 3
# x y z
#* <int> <dbl> <dbl>
#1 1 1 2
#2 2 1 5
#3 3 1 10
#4 4 1 17
#5 5 1 26
#Slot "name":
#[1] "firsttest1"
attributes(tmp1)
#$`names`
#[1] "x" "y" "z"
#
#$row.names
#[1] 1 2 3 4 5
#
#$.S3Class
#[1] "tbl_df" "tbl" "data.frame"
#
#$name
#[1] "firsttest1"
#
#$class
#[1] "TestClass"
#attr(,"package")
#[1] ".GlobalEnv"
as(tmp1,"S3")
## A tibble: 5 x 3
# x y z
#* <int> <dbl> <dbl>
#1 1 1 2
#2 2 1 5
#3 3 1 10
#4 4 1 17
#5 5 1 26
