R – 继承

继承是面向对象编程中的概念之一，通过它可以从现有或基类派生新类，有助于代码的可重用性。派生类可以与基类相同，也可以具有扩展功能，从而在编程环境中创建类的层次结构。在本文中，我们将讨论 R 编程中三种不同类型的类是如何继承继承的。

S3 类中的继承

R 编程语言中的 S3 类没有正式和固定的定义。在 S3 对象中，具有其类属性的列表设置为类名。 S3 类对象仅从其基类继承方法。

例子：

# Create a function to create an object of class
student <- function(n, a, r){
  value <- list(name = n, age = a, rno = r)
  attr(value, "class") <- student
  value
}
  
# Method for generic function print()
print.student <- function(obj){
  cat(obj$name, "\n")
  cat(obj$age, "\n")
  cat(obj$rno, "\n")
}
  
# Create an object which inherits class student
s <- list(name = "Utkarsh", age = 21, rno = 96,
          country = "India")
  
# Derive from class student
class(s) <- c("InternationalStudent", "student")
  
cat("The method print.student() is inherited:\n")
print(s)
  
# Overwriting the print method
print.InternationalStudent <- function(obj){
cat(obj$name, "is from", obj$country, "\n")
}
  
cat("After overwriting method print.student():\n")
print(s)
  
# Check imheritance
cat("Does object 's' is inherited by class 'student' ?\n")
inherits(s, "student")

输出：

The method print.student() is inherited:
Utkarsh 
21 
96 
After overwriting method print.student():
Utkarsh is from India
Does object 's' is inherited by class 'student' ?
[1] TRUE

S4类中的继承

R 编程中的 S4 类具有适当的定义，派生类将能够从其基类继承属性和方法。

例子：

# Define S4 class
setClass("student",
         slots = list(name = "character", 
                      age = "numeric", rno = "numeric") 
)
   
# Defining a function to display object details
setMethod("show", "student",
          function(obj){
            cat(obj@name, "\n")
            cat(obj@age, "\n")
            cat(obj@rno, "\n")
          } 
)
   
# Inherit from student
setClass("InternationalStudent",
slots = list(country = "character"),
contains = "student"
)
   
# Rest of the attributes will be inherited from student
s <- new("InternationalStudent", name = "Utkarsh", 
         age = 21, rno = 96, country="India")
show(s)

输出：

Utkarsh 
21 
96 

引用类中的继承

引用类中的继承与 S4 类几乎相似，使用setRefClass()函数进行继承。

例子：

# Define class
student <- setRefClass("student",
   fields = list(name = "character",
                 age = "numeric", rno = "numeric"),
   methods = list(
     inc_age <- function(x) {
       age <<- age + x
     },
     dec_age <- function(x) {
       age <<- age - x
     }
   )
)
  
# Inheriting from Reference class
InternStudent <- setRefClass("InternStudent", 
   fields = list(country = "character"), 
   contains = "student",
   methods = list(
   dec_age <- function(x) {
     if((age - x) < 0)  stop("Age cannot be negative")
     age <<- age - x
   }
   ) 
)
  
# Create object
s <- InternStudent(name = "Utkarsh",
                   age = 21, rno = 96, country = "India")
  
cat("Decrease age by 5\n")
s$dec_age(5)
s$age
  
cat("Decrease age by 20\n")
s$dec_age(20) 
s$age

输出：

[1] 16 
Error in s$dec_age(20) : Age cannot be negative
[1] 16