S7 basics

Classes and objects

S7 classes have a formal definition that you create with new_class(). There are two arguments that you’ll use with almost every class:

• The name of the class, supplied in the first argument.
• The class properties, the data associated with each instance of the class. The easiest way to define properties is to supply a named list where the values define the valid types of the property.

The following code defines a simple dog class with two properties: a character name and a numeric age.

Dog <- new_class("Dog", properties = list(
  name = class_character,
  age = class_numeric
))
Dog
#> <Dog> class
#> @ parent     : <S7_object>
#> @ constructor: function(name, age) {...}
#> @ validator  : <NULL>
#> @ properties :
#>  $ name: <character>          
#>  $ age : <integer> or <double>

S7 provides a number of built-in definitions that allow you to refer to existing base types that are not S7 classes. You can recognize these definitions because they all start with class_.

Note that I’ve assigned the return value of new_class() to an object with the same name as the class. This is important! That object represents the class and is what you use to construct instances of the class:

lola <- Dog(name = "Lola", age = 11)
lola
#> <Dog>
#>  @ name: chr "Lola"
#>  @ age : num 11

Once you have an S7 object, you can get and set properties using @:

lola@age <- 12
lola@age
#> [1] 12

S7 automatically validates the type of the property using the type supplied in new_class():

lola@age <- "twelve"
#> Error: <Dog>@age must be <integer> or <double>, not <character>

Given an object, you can retrieves its class S7_class():

S7_class(lola)
#> <Dog> class
#> @ parent     : <S7_object>
#> @ constructor: function(name, age) {...}
#> @ validator  : <NULL>
#> @ properties :
#>  $ name: <character>          
#>  $ age : <integer> or <double>

S7 objects also have an S3 class(). This is used for compatibility with existing S3 generics and you can learn more about it in vignette("compatibility").

class(lola)
#> [1] "Dog"       "S7_object"

If you want to learn more about the details of S7 classes and objects, including validation methods and more details of properties, please see vignette("classes-objects").

Generics and methods

S7, like S3 and S4, is built around the idea of generic functions, or generics for short. A generic defines an interface, which uses a different implementation depending on the class of one or more arguments. The implementation for a specific class is called a method, and the generic finds that appropriate method by performing method dispatch.

Use new_generic() to create a S7 generic. In its simplest form, it only needs two arguments: the name of the generic (used in error messages) and the name of the argument used for method dispatch:

speak <- new_generic("speak", "x")

Like with new_class(), you should always assign the result of new_generic() to a variable with the same name as the first argument.

Once you have a generic, you can register methods for specific classes with method(generic, class) <- implementation.

method(speak, Dog) <- function(x) {
  "Woof"
}

Once the method is registered, the generic will use it when appropriate:

speak(lola)
#> [1] "Woof"

Let’s define another class, this one for cats, and define another method for speak():

Cat <- new_class("Cat", properties = list(
  name = class_character,
  age = class_double
))
method(speak, Cat) <- function(x) {
  "Meow"
}

fluffy <- Cat(name = "Fluffy", age = 5)
speak(fluffy)
#> [1] "Meow"

You get an error if you call the generic with a class that doesn’t have a method:

speak(1)
#> Error: Can't find method for `speak(<double>)`.

Method dispatch and inheritance

The cat and dog classes share the same properties, so we could use a common parent class to extract out the duplicated specification. We first define the parent class:

Pet <- new_class("Pet",
  properties = list(
    name = class_character,
    age = class_numeric
  )
)

Then use the parent argument to new_class:

Cat <- new_class("Cat", parent = Pet)
Dog <- new_class("Dog", parent = Pet)

Cat
#> <Cat> class
#> @ parent     : <Pet>
#> @ constructor: function(name, age) {...}
#> @ validator  : <NULL>
#> @ properties :
#>  $ name: <character>          
#>  $ age : <integer> or <double>
Dog
#> <Dog> class
#> @ parent     : <Pet>
#> @ constructor: function(name, age) {...}
#> @ validator  : <NULL>
#> @ properties :
#>  $ name: <character>          
#>  $ age : <integer> or <double>

Because we have created new classes, we need to recreate the existing lola and fluffy objects:

lola <- Dog(name = "Lola", age = 11)
fluffy <- Cat(name = "Fluffy", age = 5)

Method dispatch takes advantage of the hierarchy of parent classes: if a method is not defined for a class, it will try the method for the parent class, and so on until it finds a method or gives up with an error. This inheritance is a powerful mechanism for sharing code across classes.

describe <- new_generic("describe", "x")
method(describe, Pet) <- function(x) {
  paste0(x@name, " is ", x@age, " years old")
}
describe(lola)
#> [1] "Lola is 11 years old"
describe(fluffy)
#> [1] "Fluffy is 5 years old"

method(describe, Dog) <- function(x) {
  paste0(x@name, " is a ", x@age, " year old dog")
}
describe(lola)
#> [1] "Lola is a 11 year old dog"
describe(fluffy)
#> [1] "Fluffy is 5 years old"

You can define a fallback method for any S7 object by registering a method for S7_object:

method(describe, S7_object) <- function(x) {
  "An S7 object"
}

Cocktail <- new_class("Cocktail",
  properties = list(
    ingredients = class_character
  )
)
martini <- Cocktail(ingredients = c("gin", "vermouth"))
describe(martini)
#> [1] "An S7 object"

Printing a generic will show you which methods are currently defined:

describe
#> <S7_generic> describe(x, ...) with 3 methods:
#> 1: method(describe, Dog)
#> 2: method(describe, S7_object)
#> 3: method(describe, Pet)

And you can use method() to retrieve the implementation of one of those methods:

method(describe, Pet)
#> <S7_method> method(describe, Pet)
#> function (x) 
#> {
#>     paste0(x@name, " is ", x@age, " years old")
#> }
#> <bytecode: 0x11631e0b0>

Learn more about method dispatch in vignette("generics-methods").