ruby alias vs alias_method

November 29th, 2012

While alias and alias_method looks very similar at first they hide a substantial different behaviour. Look at the following code.

### Basic alias behaviour

class Boss
  def name
     p "Andrea"
  end
  alias :full_name :name
end

Boss.new.name # => Andrea
Boss.new.full_name # => Andrea

This is the the basic behaviour we could expect from the alias method. #full_name is perfectly aliased to #name. But what happens if we subclass Boss?

### Alias in subclass

class Employee < Boss

    def name
       p "Bob"
    end

end

Employee.new.name # => Bob
Employee.new.full_name  # => Andrea

Looks like Employee#full_name points to the superclass #name method and not at Employee#name! And this is the truth.

alias is a ruby keyword and is executed when source code gets parsed. When Boss class is parsed Boss#full_name is aliased to Boss#name and this will be truth for any Boss instance as per any Class instance that subclass Boss.

alias_method is instead, as the word says, a method and is executed in the current self scope.

### Using alias_method

class Boss
  def name
     p "Andrea"
  end
  def self.apply_alias
      alias_method :full_name, :name
  end
  apply_alias
end

class Employee < Boss
    def name
       p "Bob"
    end
    apply_alias
end

Boss.new.name # => Andrea
Boss.new.full_name # => Andrea
Employee.new.name # => Bob
Employee.new.full_name # => Bob

alias_method is executed the first time in the self scope of Boss and then the self scope of Employee and apply the method aliasing in the way we expected. While this looks like a tiny difference using alias_method grant more flexibility and may avoid unpredictable code behaviour.

andreacfm