Reference class in R programming is similar to the object oriented programming we are used to seeing in common languages like C++, Java, Python etc.
Unlike S3 and S4 classes, methods belong to class rather than generic functions. Reference class are internally implemented as S4 classes with an environment added to it.
How to defined a reference class?
Defining reference class is similar to defining a S4 class. Instead of setClass() we use the setRefClass() function.
> setRefClass("student")
Member variables of a class, if defined, need to be included in the class definition. Member variables of reference class are called fields (analogous to slots in S4 classes).
Following is an example to define a class called student with 3 fields, name, age and GPA.
> setRefClass("student", fields = list(name = "character", age = "numeric", GPA = "numeric"))
How to create a reference objects?
The function setRefClass() returns a generator function which is used to create objects of that class.
> student <- setRefClass("student",
fields = list(name = "character", age = "numeric", GPA = "numeric"))
> # now student() is our generator function which can be used to create new objects
> s <- student(name = "John", age = 21, GPA = 3.5)
> s
Reference class object of class "student"
Field "name":
[1] "John"
Field "age":
[1] 21
Field "GPA":
[1] 3.5
How to access and modify fields?
Fields of the object can be accessed using the $ operator.
> s$name
[1] "John"
> s$age
[1] 21
> s$GPA
[1] 3.5
Similarly, it is modified by reassignment.
> s$name <- "Paul"
> s
Reference class object of class "student"
Field "name":
[1] "Paul"
Field "age":
[1] 21
Field "GPA":
[1] 3.5
Warning Note:
In R programming, objects are copied when assigned to new variable or passed to a function (pass by value). For example.
> # create list a and assign to b
> a <- list("x" = 1, "y" = 2)
> b <- a
> # modify b
> b$y = 3
> # a remains unaffected
> a
$x
[1] 1
$y
[1] 2
> # only b is modified
> b
$x
[1] 1
$y
[1] 3
But this is not the case with reference objects. Only a single copy exist and all variables reference to the same copy. Hence the name, reference.
> # create reference object a and assign to b
> a <- student(name = "John", age = 21, GPA = 3.5)
> b <- a
> # modify b
> b$name <- "Paul"
> # a and b both are modified
> a
Reference class object of class "student"
Field "name":
[1] "Paul"
Field "age":
[1] 21
Field "GPA":
[1] 3.5
> b
Reference class object of class "student"
Field "name":
[1] "Paul"
Field "age":
[1] 21
Field "GPA":
[1] 3.5
This can cause some unwanted change in values and be the source of strange bugs. We need to keep this in mind while working with reference objects. To make a copy, we can use the copy() method made availabe to us.
> # create reference object a and assign a’s copy to b
> a <- student(name = "John", age = 21, GPA = 3.5)
> b <- a$copy()
> # modify b
> b$name <- "Paul"
> # a remains unaffected
> a
Reference class object of class "student"
Field "name":
[1] "John"
Field "age":
[1] 21
Field "GPA":
[1] 3.5
> # only b is modified
> b
Reference class object of class "student"
Field "name":
[1] "Paul"
Field "age":
[1] 21
Field "GPA":
[1] 3.5
Reference Methods
Methods are defined for a reference class and do not belong to generic functions as in S3 and S4 classes.
All reference class have some methods predefined because they all are inherited from the superclass envRefClass.
> student
Generator for class "student":
Class fields:
Name: name age GPA
Class: character numeric numeric
Class Methods:
"callSuper", "copy", "export", "field", "getClass", "getRefClass",
"import", "initFields", "show", "trace", "untrace", "usingMethods"
Reference Superclasses:
"envRefClass"
We can see class methods like copy(), field() and show() in the above list. We can create our own methods for the class.
This can be done during the class definition by passing a list of function definitions to methods argument of setRefClass().
student <- setRefClass("student",
fields = list(name = "character", age = "numeric", GPA = "numeric"),
methods = list(
inc_age = function(x) {
age <<- age + x
},
dec_age = function(x) {
age <<- age - x
}
)
)
In the above section of our code, we defined two methods called inc_age() and dec_age(). These two method modify the field age.
Note that we have to use the non-local assignment operator <<- since age isn't in the method's local environment. This is important.
Using the simple assignment operator <- would have created a local variable called age, which is not what we want. R will issue a warning in such case.
Here is a sample run where we use the above defined methods.
> s <- student(name = "John", age = 21, GPA = 3.5)
> s$inc_age(5)
> s$age
[1] 26
> s$dec_age(10)
> s$age
[1] 16