Classes and methods

A class is a blueprint from which objects are created. Class names, as with all type names, starts with a capital letter.

class Animal
end

By calling .new we can create an instance of a class.

animal = Animal.new

In this case, animal is an instance of Animal. To try this out, create a file named animal.cr, and add the following code:

animal = Animal.new

puts animal

class Animal
end

And run it by entering crystal animal.cr in a terminal within the directory that you saved the file. The output should be something like this: #<Animal:0x1d88fe0>.

Now, to spice things up a bit, we’re going to add some methods to our Animal class. The initialize method is being called when using the .new and creating an instance of the class. What it does is setting the instance variable (more on those further on) @name to the value of the parameter name, and setting the instance variable @age to 0.

def initialize(name : String)
    @name = name
    @age = 0
end

If we run this code now, we will still end up with the same output as before. So we need to add two more methods and update how we instanciate our Animal class. These methods make the value within our instance variables available to use on our instance of the Animal class and can be used as such: animal.name and animal.age.

def name
  @name
end

def age
  @age
end

To create a new animal we can now update our previous instanciation of our Animal class.

cat = Animal.new "Floofkins"
dog = Animal.new "Pluto"

And print them

puts cat.name #=> Floofkins
puts cat.age #=> 0

puts dog.name #=> Pluto
puts dog.age #=> 0

Note: In Crystal parameters are required to have a type specified, more on that topic here.