What is an accessor method?

A method that reads and writes the value of an instance variable. Ruby is designed so that instance variables cannot be accessed as it is.

(Example)

class Animal
  @name = ""
end

animal = Animal.new
animal.name = "cat"
puts animal.name # => ruby.rb:6:in `<main>': undefined method `name=' for #<Animal:0x00007fce4a8b4b40> (NoMethodError)

According to this code definition method, @name is defined as a different variable called "class instance variable", which is inaccessible from the instance.

How should I define the instance variables that can be accessed from the instance?

So we use the "accessor method".

First, there are three types of accessor methods.

attr_reader
attr_writer
attr_accessor

attr_reader attr_reader is a getter for accessing instance variables.

(Example)

class Animal
  attr_reader :name 
  def initialize(name)
    @name = name
  end
end

animal = Animal.new("cat") 
puts animal.name # => cat

In this way, you can access the instance variable @name by writing attr_reader: name. However, as it is, the setter is not defined, so @name cannot be changed from outside the class.

attr_writer attr_writer is a setter for accessing instance variables.

(Example)

class Animal
  attr_writer :name
  attr_reader :name
  def initialize(name)
    @name = name
  end
end

animal = Animal.new("cat")
puts animal.name
animal.name = "dog"
puts animal.name 
# => cat
#    dog

In this way, by writing attr_writer: name in addition to attr_reader: name, you can change and access the instance variable @name from the outside.

attr_accessor This method combines the functions of attr_reader and attr_writter.

(Example)

class Animal
  attr_accessor :name
  def initialize(name)
    @name = name
  end
end
animal = Animal.new("cat")
puts animal.name
animal.name = "dog"
puts animal.name
# => cat
#    dog

In this way, you can change the instance variable @name from the outside and access it.

Ruby accessor method

What is an accessor method?