13 R6

13.1 Introduction

This chapter describes the R6 OOP system. R6 has two special properties:

  • It uses the encapsulated OOP paradigm, which means that methods belong to objects, not generics, and you call them like object$method().

  • R6 objects are mutable, which means that they are modified in place, and hence have reference semantics.

If you’ve learned OOP in another programming language, it’s likely that R6 will feel very natural, and you’ll be inclined to prefer it over S3. Resist the temptation to follow the path of least resistance: in most cases R6 will lead you to non-idiomatic R code. We’ll come back to this theme in Section 15.3.

R6 is very similar to a base OOP system called reference classes, or RC for short. I describe why I teach R6 and not RC in Section 13.5.

Outline

  • Section 13.2 introduces R6::R6class(), the one function that you need to know to create R6 classes. You’ll learn about the constructor method, $new(), which allows you to create R6 objects, as well as other important methods like $initialize() and $print().

  • Section 13.3 discusses the access mechanisms of R6: private and active fields. Together, these allow you to hide data from the user, or expose private data for reading but not writing.

  • Section 13.4 explores the consequnces of R6’s reference semantics. You’ll learn about the use of finalizers to automatically clean up any operations performed in the intializer, and a common gotcha if you use an R6 object as a field in another R6 object.

  • Section 13.5 describes why I cover R6, rather than the base RC system.

Prerequisites

Because R6 is not built into base R, you’ll need to install and load the R6 package to use it:

R6 objects have reference semantics which means that they are modified in-place, not copied-on-modify. If you’re not familiar with these terms, brush up your vocab by reading Section 2.5.

13.2 Classes and methods

R6 only needs a single function call to create both the class and its methods: R6::R6Class(). This is the only function from the package that you’ll ever use!47

The following example shows the two most important arguments to R6Class():

  • The first argument is the classname. It’s not strictly needed, but it improves error messages and makes it possible to use R6 objects with S3 generics. By convention, R6 classes have UpperCamelCase names.

  • The second argument, public, supplies a list of methods (functions) and fields (anything else) that make up the public interface of the object. By convention, methods and fields use snake_case. Methods can access the methods and fields of the current object via self$.48

You should always assign the result of R6Class() into a variable with the same name as the class, because R6Class() returns an R6 object that defines the class:

You construct a new object from the class by calling the new() method. In R6, methods belong to objects, so you use $ to access new():

You can then call methods and access fields with $:

In this class, the fields and methods are public, which means that you can get or set the value of any field. Later, we’ll see how to use private fields and methods to prevent casual access to the internals of your class.

To make it clear when we’re talking about fields and methods as opposed to variables and functions, I’ll prefix their names with $. For example, the Accumulate class has field $sum and method $add().

13.2.1 Method chaining

$add() is called primarily for its side-effect of updating $sum.

Side-effect R6 methods should always return self invisibly. This returns the “current” object and makes it possible to chain together multiple method calls:

For, readability, you might put one method call on each line:

This technique is called method chaining and is commonly used in languages like Python and JavaScript. Method chaining is deeply related to the pipe, and we’ll discuss the pros and cons of each approach in Section 15.3.3.

13.2.2 Important methods

There are two important methods that should be defined for most classes: $initialize() and $print(). They’re not required, but providing them will make your class easier to use.

$initialize() overrides the default behaviour of $new(). For example, the following code defines an Person class with fields $name and $age. To ensure that that $name is always a single string, and $age is always a single number, I placed checks in $initialize().

If you have more expensive validation requirements, implement them in a separate $validate() and only call when needed.

Defining $print() allows you to override the default printing behaviour. As with any R6 method called for its side effects, $print() should return invisible(self).

This code illustrates an important aspect of R6. Because methods are bound to individual objects, the previously created hadley object does not get this new method:

From the perspective of R6, there is no relationship between hadley and hadley2; they just coincidentally share the same class name. This doesn’t cause problems when using already developed R6 problems but can make interactive experimentation confusing. If you’re changing the code and can’t figure out why the results of method calls aren’t any different, make sure you’ve re-constructed R6 objects with the new class.

13.2.3 Adding methods after creation

Instead of continuously creating new classes, it’s also possible to modify the fields and methods of an existing class. This is useful when exploring interactively, or when you have a class with many functions that you’d like to break up into pieces. Add new elements to an existing class with $set(), supplying the visibility (more on in Section 13.3), the name, and the component.

As above, new methods and fields are only available to new objects; they are not retrospectively added to existing objects.

13.2.4 Inheritance

To inherit behaviour from an existing class, provide the class object to the inherit argument:

Note that $add() overrides the implementation in the superclass, but we can delegate to the superclass implementation through super$. (This is analagous to NextMethod() in S3, as discussed in Section 12.6.) Any methods which are not overridden will use the implementation in the parent class.

13.2.5 Introspection

Every R6 object has an S3 class that reflects its hierarchy of R6 classes. This means that the easiest way to determine the class (and all classes it inherits from) is to use class():

The S3 hierarchy includes the base “R6” class. This provides common behaviour, including a print.R6() method which calls $print(), as described above.

You can list all methods and fields with names():

We defined $name, $age, $print, and $initialize. As suggested by the name .__enclos_env__ is an internal implementation detail that you shouldn’t touch; we’ll come back to $clone() in Section 13.4.

13.2.6 Exercises

  1. Create a bank account R6 class that stores a balance and allows you to deposit and withdraw money. Create a subclass that throws an error if you attempt to go into overdraft. Create another subclass that allows you to go into overdraft, but charges you a fee.

  2. Create an R6 class that represents a shuffled deck of cards. You should be able to draw cards from the deck with $draw(n), and return all cards to the deck and reshuffle with $reshuffle(). Use the following code to make a vector of cards.

  3. Why can’t you model a bank account or a deck of cards with an S3 class?

  4. Create an R6 class that allows you to get and set the current timezone. You can access the current timezone with Sys.timezone() and set it with Sys.setenv(TZ = "newtimezone"). When setting the time zone, make sure the new time zone is in the list provided by OlsonNames().

  5. Create an R6 class that manages the current working directory. It should have $get() and $set() methods.

  6. Why can’t you model the time zone or current working directory with an S3 class?

  7. What base type are R6 objects built on top of? What attributes do they have?

13.3 Controlling access

R6Class() has two other arguments that work similarly to public:

  • private allows you to create fields and methods that are only available from within the class, not outside of it.

  • active allows you to use accessor functions to define dynamic, or active, fields.

These are described in the following sections.

13.3.1 Privacy

With R6 you can define private fields and methods, elements that can only be accessed from within the class, not from the outside49. There are two things that you need to know to take advantage of private elements:

  • The private argument to R6Class works in the same way as the public argument: you give it a named list of methods (functions) and fields (everything else).

  • Fields and methods defined in private are available within the methods using private$ instead of self$. You cannot access private fields or methods outside of the class.

To make this concrete, we could make $age and $name fields of the Person class private. With this definition of Person we can only set $age and $name during object creation, and we cannot access their values from outside of the class.

The distinction between public and private fields is important when you create complex networks of classes, and you want to make it as clear as possible what it’s ok for others to access. Anything that’s private can be more easily refactored because you know others aren’t relying on it. Private methods tend to be less important in R compared to other programming languages because the object hierarchies in R tend to be simpler.

13.3.2 Active fields

Active fields allow you to define components that look like fields from the outside, but are defined with functions, like methods. Active fields are implemented using active bindings (see Section 6.2.7 for details). Each active binding is a function that takes a single argument: value. If the argument is missing(), the value is being retrieved; otherwise it’s being modified.

For example, you could make an active field random that returns a different value every time you access it:

Active fields are particularly useful in conjunction with private fields, because they make it possible to implement components that look like fields from the outside but provide additional checks. For example, we can use then make a read-only age field, and to ensure that name is a length 1 character vector.

13.3.3 Exercises

  1. Create a bank account class that prevents you from directly setting the account balance, but you can still withdraw from and deposit to. Throw an error if you attempt to go into overdraft.

  2. Create an class with a write-only $password field. It should have $check_password(password) method that returns TRUE or FALSE, but there should be no way to view the complete password.

  3. Extend the Rando class with another active binding that allows you to access the previous random value. Ensure that active binding is the only way to access the value.

  4. Can subclasses access private fields/methods from their parent? Perform an experiment to find out.

13.4 Reference semantics

One of the big differences between R6 and most other objects is that they have reference semantics. The primary consequence of reference semantics is that objects are not copied when modified:

Instead, if you want a copy, you’ll need to explicitly $clone() the object:

(Note that $clone() does not recursively clone nested R6 objects. If you want that, you’ll need to use $clone(deep = TRUE).)

There are three other less obvious consequences:

  • It is harder to reason about code that uses R6 objects because you need to understand more context.

  • It makes sense to think about when an R6 object is deleted, and you can write a $finalize() to complement the $initialize().

  • If one of the fields is an R6 object, you must create it inside $initialize(), not R6Class().

These consequences are described in more detail below.

13.4.1 Reasoning

Generally, reference semantics makes code harder to reason about. Take this very simple example:

For the vast majority of functions, you know that the final line only modifies z.

Take a similar example that uses an imaginary List reference class:

The final line is much harder to reason about: if f() calls methods of x or y, it might modify them as well as z. This is the biggest potential downside of R6 and you should take care to avoid it by writing functions that either return a value, or modify their R6 inputs, but not both. That said, doing both can lead to substantially simpler code in some cases, and we’ll discuss further in Section 15.3.2.

13.4.2 Finalizer

One useful property of reference semantics is that it makes sense to think about when an R6 object is finalized, i.e. when it’s deleted. This doesn’t make sense for most objects because copy-on-modify semantics mean that there may be many transient versions of an object, as alluded to in Section 2.6. For example, the following creates two two factor objects: the second is created when the levels are modified, leaving the first to be destroyed by the garbage collector.

Since R6 objects are not copied-on-modify they are only deleted once, and it makes sense to think about $finalize() as a complement to $initialize(). Finalizers usually play a similar role to on.exit() (as described in Section 5.7.4), cleaning up any resources created by the initializer. For example, the following class wraps up a temporary file, automatically deleting it when the class is finalized.

The finalize method will be run when the object is deleted (or more precisely, by the first garbage collection after the object has been unbound from all names) or when R exits. This means that the finalizer can called effectively anywhere in your R code, and therefore it’s almost impossible to reason about finalizer code that touches shared data structures. Avoid these potential problems by only using the finalizer to clean up private resources allocated by initializer.

13.4.3 R6 fields

A final consequence of reference semantics can crop up where you don’t expect it. If you use a R6 class as the default value of a field, it will be shared across all instances of the object! Take the following code: we want to create a temporary database every time we call TemporaryDatabase$new(), but the current code always uses the same path.

(If you’re familiar with Python, this is very similar to the “mutable default argument” problem.)

The problem arises because TemporaryFile$new() is called only once when the TemporaryDatabase class is defined. To fix the problem, we need to make sure it’s called every time that TemporaryDatabase$new() is called, i.e. we need to put it in $initialize():

13.4.4 Exercises

  1. Create a class that allows you to write a line to a specified file. You should open a connection to the file in $initialize(), append a line using cat() in $append_line(), and close the connection in $finalize().

13.5 Why R6?

R6 is very similar to a built-in OO system called reference classes, or RC for short. I prefer R6 to RC because:

  • R6 is much simpler. Both R6 and RC are built on top of environments, but while R6 uses S3, RC uses S4. This means to fully understand RC, you need to understand how the more complicated S4 works.

  • R6 has comprehensive online documentation at https://r6.r-lib.org.

  • R6 has a simpler mechnaism for cross-package subclassing, which just works without you having to think about. For RC, read the details in the “External Methods; Inter-Package Superclasses” section of ?setRefClass.

  • RC mingles variables and fields in the same stack of environments so that you get (field) and set (field <<- value) fields like regular values. R6 puts fields in a separate environment so you get (self$field) and set (self$field <- value) with a prefix. The R6 approach is more verbose but I like it because it is more explicit.

  • R6 is much faster than RC. Generally, the speed of method dispatch is not important outside of microbenchmarks. However, RC is quite slow, and switching from RC to R6 had a substantial performance improvements in the shiny package. For more details, see vignette("Performance", "R6").

  • RC is tied to R. That means if any bugs are fixed, you can only take advantage of the fixes by requiring a newer version of R. This makes it difficult for packages (like those in the tidyverse) that need to work across many R versions.

  • Finally, because the ideas that underlie R6 and RC are similar, it will only require a small amount of additional effort to learn RC if you need to.


  1. That means if you’re creating R6 in a package, you only need to make sure it’s listed in the Imports field of the DESCRIPTION. There’s no need to import the package into the NAMESPACE.

  2. Unlike in this in python, the self variable is automatically provided by R6, and does not form part of the method signature.

  3. Because R is such a flexible language, it’s technically still possible to access private values, but you’ll have to try much harder, spelunking in to the details of R6’s implementation.