methods_and_instance_variables.md

Methods and instance variables

We can simplify our constructor by using a shorter syntax for assigning a method argument to an instance variable:

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

Right now, we can't do much with a person aside from create it with a name. Its age will always be zero. So lets add a method that makes a person become older:

class Person
  def initialize(@name : String)
    @age = 0
  end
  
  def age
    @age
  end
  
  def become_older
    @age += 1
  end
end

john = Person.new "John"
peter = Person.new "Peter"

john.age #=> 0

john.become_older
john.age #=> 1

peter.age #=> 0

Method names begin with a lowercase letter and, as a convention, only use lowercase letters, underscores and numbers.

Getters and setters

The Crystal Standard Library provides macros which simplify the definition of getter and setter methods:

class Person
  property age
  getter name : String

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

john = Person.new "John"
john.age = 32
john.age #=> 32

For more information on getter and setter macros, see the standard library documentation for Object#getter, Object#setter, and Object#property.

As a side note, we can define become_older inside the original Person definition, or in a separate definition: Crystal combines all definitions into a single class. The following works just fine:

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

class Person
  def become_older
    @age += 1
  end
end

Redefining methods, and previous_def

If you redefine a method, the last definition will take precedence.

class Person
  def become_older
    @age += 1
  end
end

class Person
  def become_older
    @age += 2
  end
end

person = Person.new "John"
person.become_older
person.age #=> 2

You can invoke the previously redefined method with previous_def:

class Person
  def become_older
    @age += 1
  end
end

class Person
  def become_older
    previous_def
    @age += 2
  end
end

person = Person.new "John"
person.become_older
person.age #=> 3

Without arguments or parentheses, previous_def receives the same arguments as the method's arguments. Otherwise, it receives the arguments you pass to it.

Catch-all initialization

Instance variables can also be initialized outside initialize methods:

class Person
  @age = 0

  def initialize(@name : String)
  end
end

This will initialize @age to zero in every constructor. This is useful to avoid duplication, but also to avoid the Nil type when reopening a class and adding instance variables to it.