Alias and inheritance

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What is the return value of the following Ruby code?

class Device
  def description
    'I\'m a device'
  end

  def self.alias_description
    alias describe description
  end
end

class Microwave < Device
  def description
    'I\' a microwave'
  end

  alias_description
end

m = Microwave.new
p m.description # => "I'm a microwave"

p m.describe # => ???

The correct answer is

"I'm a device"

"I'm a microwave"

Explanation

The alias keyword

Now let’s see what happens with the alias keyword in Device

class Device
  def description
    'I\'m a device'
  end

  def self.alias_description
    alias describe description
  end
end

class Microwave < Device
  def description
    'I\' a microwave'
  end

  alias_description
end

m = Microwave.new

p m.description # => "I'm a microwave"
p m.describe    # => "I'm a device"

Here we can see that the call to alias within Device.alias_description sets the describe alias on the Device#description method and not on the Microwave#description one.

Module#alias_method

The Module#alias_method acts differently than the alias keyword on one specific point: the scope.

Let’s have a look at this example

class Device
  def description
    'I\'m a device'
  end

  def self.alias_description
    alias_method :describe, :description
  end
end

class Microwave < Device
  def description
    'I\' a microwave'
  end

  alias_description
end

m = Microwave.new

p m.description  # => "I'm a microwave"
p m.describe     # => "I'm a microwave"

Here we can see that the call to alias_method within the Device.alias_description method defines the describe alias on the Microwave#description method and not on Device#description.

Alias and inheritance

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Explanation

The alias keyword

Module#alias_method

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