- Define instance variables
- Distinguish instance variables from local variables
- Describe how instance variables give objects attributes and properties
When we build objects through our own classes we know we can add behavior to the objects through instance methods. But how do we give our objects data? For example, how would we teach a dog what its name is? Or what breed it is? How do we put data inside of an instance of an object so that each dog can have its own name and own breed? In other words, how do we give objects attributes or properties?
Fork and clone this code-along, then run your tests with learn test
to get an
overview of what we are trying to do. Follow along to get the tests to pass.
We've been working with variables for a while now. For example:
bro_greeting = "Sup, bro?"
The code above sets a variable, bro_greeting
, equal to the string "Sup, bro?"
. Now we can use that variable to read and operate on that string.
bro_greeting #=> "Sup, bro?"
bro_greeting.upcase #=> "SUP, BRO?"
The bro_greeting
variable is what's known as a local variable, so named
because it can only be accessed in a specific, local environment.
A local variable that is defined inside one method, for example, cannot be accessed by another method. In order to get around this limitation, we can use instance variables inside our Ruby classes.
An instance variable is a variable that is accessible in any instance method in a particular instance of a class.
Let's say we have a class called Dog
that is responsible for producing
individual dog objects. We want each dog instance to be able to have a name and
to show its name. So we need to write two methods: one to set the dog's name
and one to get the dog's name.
Open dog.rb
inside the lib
folder and add the following two methods to the
Dog
class:
class Dog
def name=(dog_name)
this_dogs_name = dog_name
end
def name
this_dogs_name
end
end
Here we've defined two instance methods, the name=
(or "name equals") method,
and the name
method. The first method takes in an argument of a dog's name and
assigns that value to a variable, this_dogs_name
. The second method is
responsible for reporting the name. Our two methods therefore are responsible
for "setting" and "getting" an individual dog's name.
We want the methods to act as mechanisms to expose data from inside of the object to the outside world; in other words, we want to be able to do this:
lassie = Dog.new
lassie.name = "Lassie"
lassie.name #=> "Lassie"
In the code above, we are creating a new Dog
instance and assigning it a name
then asking it to return its name.
In your dog.rb
file add the above code. Run learn test
again. You should see
output that includes something like this:
Failure/Error: this_dogs_name
NameError:
undefined local variable or method `this_dogs_name' for #<Dog:0x00007fa7909a3078>
Uh-oh. Looks like the #name
method doesn't know about the this_dogs_name
variable from the #name=
method. That is because this_dogs_name
is a local
variable. A local variable has a local scope, which means that it cannot be
accessed outside of the method in which it is defined.
We define an instance variable by prefacing the variable name with an @
symbol.
Instance variables are bound to an instance of a class. That means that when we
set the value of an attribute on an instance of a class, the value we set is
associated with that particular instance. This makes intuitive sense: a dog's
name belongs to that specific dog, not to Dog
s in general. Instance variables
hold information about an instance, i.e., they describe attributes or
properties of the instance, such as its name or breed. They can be called on
throughout the class, without needing to be passed into other methods as
arguments (as would be the case with local variables).
Let's refactor our Dog
class to use an instance variable instead of a local
variable to set and get an individual dog's name.
Inside dog.rb
, change the Dog
class in the following way:
class Dog
def name=(dogs_name)
@this_dogs_name = dogs_name
end
def name
@this_dogs_name
end
end
lassie = Dog.new
lassie.name = "Lassie"
puts lassie.name
Run learn test
again; your tests should now be passing.
To summarize: in Object-Oriented Ruby, we want to be able to set attributes on
the instances of our classes. We also want those objects to "know" the values of
their attributes. To do this, we create a setter method, which takes an
argument and sets the instance variable equal to that value. We also create a
getter method that will return the value of the instance variable. By using
the @
in front of the variable name, the variable that's created inside the
setter method is available inside the getter method (and inside any other
methods in our class).
As we dive deeper into object-oriented Ruby, we'll be using instance variables frequently to pass information around the instance methods of a class. Think of instance variables as the containers for instance-specific information. The ability of instance variables to store information and be accessible within different instance methods is one of the things that makes it possible for us to create similar but unique objects in object-oriented Ruby.