Crystal Notes: Classes

Intro

Crystal is a object orientated language. Classes are the blueprints from which objects can be created.

Class Definition

crystal

# Define a class.
class Stuff
end

# Create an instance of the Stuff class.
stuff = Stuff.new()

# Check the type of "stuff"
puts typeof(stuff) # => Stuff

Class Methods

Class methods allow you to add behaviour to a class.

crystal

# Define a class with a show method.
class StuffAndThings
    # Define instance variables and their types.
    @stuff : String
    @things : String
    @blah : String

    def initialize(@stuff, @things, @blah)
    # Do something with variables on object initialization.
    end

    # Add a show method to the StuffAndThings class
    def show
    puts "Stuff: #{@stuff}"
    puts "Things: #{@things}"
    puts "Blah: #{@blah}"
    end

end

# Create an instance of the StuffAndThings class.
stuff_and_things = StuffAndThings.new(
    stuff: "my stuff",
    things: "my things",
    blah: "blah blah"
    )

# Use the show method.
stuff_and_things.show

Variable Access

By default, class variables are not accessible from outside the class. You cannot get or set them. Crystal has some conveniences to help with this.

crystal

# Define a class.
class StuffAndThings
    # Define instance variables and their types.
    @stuff : String
    @things : String
    @blah : String

    # Set variables read/write properties.
    getter stuff # The stuff variable can be read but not written.
    setter things # The things variable can be written but not read.
    property blah # The blah variable can be read and written.

    def initialize(@stuff, @things, @blah)
    # Do something with variables on object initialization.
    end

end

Class Variables

Class variables are prefixed with a double (@@) symbol. Each object created from a class shares the class variables. Sub-classes get their own copy with the value shared across subclasses.

crystal

# Define a class with class variables.
class Blah
    @@blah = "blah blah blah"

    def blah
        @@blah
    end

end

# Create a new class.
blah = Blah.new()

# Access the class variable.
puts(blah.blah())

Class Inheritance

Class inheritance allows the properties of a superclass to be shared with its subclasses. Class inheritance is defined with the subclass < superclass syntax.

crystal

# Define a parent class.
class Parent
    @greet : String
    property greet

    def initialize(@greet="hello")
    end

end

# Define a child class that inherits from parent.
class Child < Parent
end

# Create an instance of the parent and child classes.
Parent.new().greet # => hello
Child.new().greet # => hello

Abstract Class

An abstract class is like an interface in other languages. It is a blueprint that specifes the methods that must be implemented in its derived classes.

crystal

# Define an abstract class.
abstract class StuffAndThings
    # The stuff and things methods must be implemented 
    # in any derived classes of StuffAndThings.
    abstract def stuff
    abstract def things
end