When we have
an instance of any kind of object in Ruby, it will have certain variables that
are unique to it. These variables are therefore referred to as Instance
variables and they are denoted by the use of the @ sigil. You’ve probably read that in Ruby everything is an object,
therefore we’re practically always going to have a collection of instance
variables available to call on.
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Class
variables are set-up when we define a Class in our script, they are denoted by
the use of the @@ sigil and are
common across all instances of the Class.
It’s
important to note that we define our Class variables when write up the Class,
with the structure we want, at the time we create the script. These will be
re-used by all future instances of the Class and will carry the same data into
all instances.
The value of
individual Instance variables is set when we create an instance of the Class
when the script is running. The data / value it holds is unique to the instance
and gives it some variation on other Instances.
Let’s look
at an example which shows @instance and @@class variables being used.
Copy the
following code into a new .rb file, then run it via a cmd window.
# instanceAndClassVariables_001.rb
# Define a Class called Creature and
set a Class variable with @@
class Creature
@@mobType = "NPC"
def initialize creatureName
@name = creatureName
end #def initialize
creatureName
def to_s
"Creature name is #{@name}. It is an
#{@@mobType}."
end # def to_s
end #Creature class
# Create a new instance of the Creature
class, then assign it a name via the @name Instance variable
mob_001 = Creature.new "Silver
Kobold"
# View the details of the Class,
calling the Class and Instance variables
print mob_001
In this
example of a simple Class, we see both Instance and Class variables being used.
Our Class variable is @@mobType and this example it’s an “NPC” (a Non-Player
Character). We can tell that all instances of this Creature class will be the
NPC mob type. However, the particular name of this instance of the creature
isn’t set until we create an instance of it.
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