Let’s say you have a class Person.
class Person end person = Person.new person.name # => no method error
Obviously we never defined method name. Let’s do that.
class Person
def name
@name # simply returning an instance variable @name
end
end
person = Person.new
person.name # => nil
person.name = "Dennis" # => no method error
Aha, we can read the name, but that doesn’t mean we can assign the name. Those are two different methods. The former is called reader and latter is called writer. We didn’t create the writer yet so let’s do that.
class Person
def name
@name
end
def name=(str)
@name = str
end
end
person = Person.new
person.name = 'Dennis'
person.name # => "Dennis"
Awesome. Now we can write and read instance variable @name using reader and writer methods. Except, this is done so frequently, why waste time writing these methods every time? We can do it easier.
class Person attr_reader :name attr_writer :name end
Even this can get repetitive. When you want both reader and writer just use accessor!
class Person attr_accessor :name end person = Person.new person.name = "Dennis" person.name # => "Dennis"
Works the same way! And guess what: the instance variable @name in our person object will be set just like when we did it manually, so you can use it in other methods.
class Person
attr_accessor :name
def greeting
"Hello #{@name}"
end
end
person = Person.new
person.name = "Dennis"
person.greeting # => "Hello Dennis"
That’s it. In order to understand how attr_reader, attr_writer, and attr_accessor methods actually generate methods for you, read other answers, books, ruby docs.