CODEWITHSUNDEEP
ruby
Victor
Victor

Reputation: 1879

Ruby: Extended instance variables

How can I set some instance variables when extending a instance in the same way it can be done when creating it, with initialize.

In this example the variable set when extending is "lost":

module Mod
  def self.extended(base)
    @from_module = "Hello from Mod"
    puts "Setting variable to: #{@from_module}"
  end

  def hello_from_module
    return @from_module
  end
end

class Klass
  def initialize
    @from_class = "Hello from Klass"
  end

  def hello_from_class
    return @from_class
  end
end

klass = Klass.new       #=> #<Klass:0x00000000ed8618 @from_class="Hello from Klass">
klass.extend(Mod)       #=> #<Klass:0x00000000ed8618 @from_class="Hello from Klass">
"Setting variable to: Hello from Mod"

klass.hello_from_class  #=> "Hello from Klass"
klass.hello_from_module #=> nil (warning: instance variable @from_module not initialized)

Upvotes: 1

Views: 456

Answers (1)

tompave
tompave

Reputation: 12427

There are a number of ways to do what you describe.

The most common one would be to use instance_variable_get and instance_variable_set:

module ModA
  def self.extended(base)
    base.instance_variable_set(:@from_module, "Hello from Mod A")
    puts "Setting variable to: #{base.instance_variable_get(:@from_module)}"
  end

  def hello_from_module
    return @from_module
  end
end

Another common technique is to use any of the eval or exec methods. In this case instance_exec:

module ModB
  def self.extended(base)
    base.instance_exec { @from_module = "Hello from Mod B" }
    puts "Setting variable to: #{base.instance_exec { @from_module }}"
  end

  def hello_from_module
    return @from_module
  end
end

Upvotes: 2

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