What Are Accessor Methods in Ruby?

Accessor methods automatically generate getter/setter methods for object instance variables (i.e. @variable) with a single line of code (via metaprogramming):

class Person
    def initialize(name)
        @name = name
    end

    # creates getter/setter methods for `@name`
    attr_accessor :name
end

person = Person.new("John")
puts person.name #=> "John"

person.name = "Wayne"
puts person.name #=> "Wayne"

The example above is equivalent to the following:

class Person
    def initialize(name)
        @name = name
    end

    def name
        @name
    end

    def name=(val)
        @name = val
    end
end

person = Person.new("John")
puts person.name #=> "John"

person.name = "Wayne"
puts person.name #=> "Wayne"

Ruby has three types of accessor methods:

1. attr_reader: generates only a getter for the given instance variables;
2. attr_writer: generates only a setter for the given instance variables;
3. attr_accessor: generates both, a getter and a setter for the given instance variables.

These are useful because they provide a shorter syntax than creating getter/setter methods manually for class instance variable.

It is possible to combine different accessor methods together in a single class, for example, like so:

class Person
    def initialize(name, age)
        @name = name
        @age = age
    end

    # creates getter for `@age`
    attr_reader :age

    # creates getter/setter methods for `@name`
    attr_accessor :name
end

person = Person.new("John", 29)
puts person.name #=> "John"

person.name = "Wayne"
puts person.name #=> "Wayne"

puts person.age #=> 29

In the example above, the @name instance variable can be read or re-assigned, while the @age instance variable can only be read. If you attempt to re-assign @age, then an error will be thrown because a setter method does not exist for it:

# undefined method `age=' ... (NoMethodError)
person.age = 39