Table of contents
What is a getter method?
A getter method is a method that gets a value of an instance variable.
Without a getter method, you can not retrieve a value of an instance variable outside the class the instance variable is instantiated from.
Here is an example.
class Movie
def initialize(name)
@name = name
end
end
obj1 = Movie.new('Forrest Gump')
p obj1.name #=> undefined method `name' for #<Movie:0x007fecd08cb288 @name="Forrest Gump"> (NoMethodError)
As you can see, the value of obj1
(name
) can not be retrieved outside Movie
class. if you try to retrive a value of an instance variable outside its class without a getter method, Ruby raises No Mothod Error.
Now, Let's see how to retrieve the value of obj1
outside Movie
class with a getter method.
All you have to do here is to define a getter method
named name
. Though the name of a getter method can be anything, it is common practice to name a getter method the instance variable’s name.
class Movie
def initialize(name)
@name = name
end
def name
@name
end
end
obj1 = Movie.new('Forrest Gump')
p obj1.name #=> "Forrest Gump"
What is a setter method?
A setter is a method that sets a value of an instance variable.
Without a setter method, you can not assign a value to an instance variable outside its class.
if you try to set a value of an instance variable outside its class, Ruby raises No Method Error just like it does when you try to retrieve a value of an instance variable outside its class without a getter method.
class Movie
def initialize(name)
@name = name
end
def name #getter method
@name
end
end
obj1 = Movie.new('Forrest Gump')
p obj1.name #=> "Forrest Gump"
obj1.name = 'Fight Club' #=> undefined method `name=' for #<Movie:0x007f9937053368 @name="Forrest Gump"> (NoMethodError)
Defining a setter method inside a class makes it possible to set a value of an instance variable outside the class.
You can define a setter method like the code below.
class Movie
def initialize(name)
@name = name
end
def name #getter method
@name
end
def name=(name) #setter method
@name = name
end
end
obj1 = Movie.new('Forrest Gump')
p obj1.name #=> "Forrest Gump"
obj1.name = 'Fight Club'
p obj1.name #=> "Fight Club"
By using name=
, you can now assign a new value Fight Club
to obj1
.
What are accessors?
Accessors
are a way to create getter and setter methods without explicitly defining them in a class.
There are three types fo accessors in Ruby.
-
attr_reader
automatically generates a getter method for each given attribute. -
attr_writer
automatically generates a setter method for each given attribute. -
attr_accessor
automatically generates a getter and setter method for each given attribute.
First, let's take a look at attr_reader
!
As you can see in the code below, name
and year
are retrieved outside Movie
class even though there is no getter method for either of them. This is because attr_reader
generates a getter method for each given attribute.
class Movie
attr_reader :name, :year
def initialize(name, year)
@name = name
@year = year
end
end
obj1 = Movie.new('Forrest Gump', 1994)
p obj1.name #=> Forrest Gump
p obj1.year #=> 1994
Second, let's see how attr_writer
works!
As I mentioned above, attr_witer
generates a setter method for each given attribute. Therefore you can assign new values to ob1
without explicitly writing setter methods for name
and year
!
class Movie
attr_reader :name, :year
attr_writer :name, :year
def initialize(name, year)
@name = name
@year = year
end
end
obj1 = Movie.new('Forrest Gump', 1994)
obj1.name = 'Fight Club'
obj1.year = 1999
p obj1.name #=> "Fight Club"
p obj1.year #=> 1999
Last but certainly not least, attr_accessor
does what attr_reader
and attr_writer
do with just one line of code! It will automatically generate a getter and setter mehod for each given attribute.
class Movie
attr_accessor :name, :year
def initialize(name, year)
@name = name
@year = year
end
end
obj1 = Movie.new('Forrest Gump', 1994)
obj1.name = 'Fight Club'
obj1.year = 1999
p obj1.name #=> "Fight Club"
p obj1.year #=> 1999
References
Ruby Getters and Setters
How getter/setter methods work in Ruby
What is attr_accessor in Ruby?
rubylearning.com
Top comments (11)
Another neat trick is you can mark these as private so you can use them internally without worrying about them leaking out by throwing a
private
into the class.This can be useful if you don't like remembering when to use
@year
vsyear
or perhaps you want to be prepared to abstract it later.It seems odd to declare these as
private
since you can always refer to them by their@
name internally.It can be odd, but it can also be useful as it allows you to more easily override it later without having to go find/replace all instances of
@year
in your class.Or, if a class inherits from it and needs to change how
year
is generated it's easier it doesn't have to worry about parent class using@year
.If that makes sense.
It just seems inconsistent to use
year
andself.year = ...
in the code where one's bare and the other's necessarily prefixed, instead of@year
consistently. This plus the way that@year
is by default "private".One of the major goals of object-oriented design is to properly contain any changes like that, so replacing
@year
with something else is at least contained to the one file or scope.Subclass concerns are valid, though in Ruby it's generally understood you need to play nice with your parent's properties and avoid really getting in there and wrecking around.
Thanks for sharing !!
These are also called "accessors" (read) and "mutators" (write) in other languages, but the principle is the same. Useful terminology for those coming to Ruby from places where those terms are used instead.
Ruby's way of declaring them as
x=
type methods is fairly unique and makes for some extremely concise code since there's no need forgetX
/setX
pairs, it's justx
andx=
.Another thing worth mentioning is if you have a "setter" or
attr_writer
you can't use that without prefixing it with some kind of object, evenself
.For example:
That's because in the code
test = :assigned
creates a variable namedtest
, it doesn't call thetest=
method. To use those you must doself.test = :assigned
inside the context of that method orexample.test = :assigned
by using some kind of variable for reference.This leads to a lot of confusion in places like ActiveRecord where assigning to the auto-generated attributes "doesn't work".
I have a question how to generate this getter dynamically, for example you get a string argument and you have to generate a setter method for it which sets an instance variable which gets triggered when someone assign a value to it
Thank you for the detailed explanation of getter Death By AI and setter methods! It’s clear how crucial these methods are for accessing and modifying instance variables in Ruby.
Thanks for the detailed explanation of the getter, setter, and accessor methods in Ruby. This information is very helpful to understand Mapquest Directions how to manage and access instance variables in a class efficiently.
Thank you👍!
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