From: email@...
Date: 2016-01-25T19:49:50+00:00
Subject: [ruby-core:73440] [CommonRuby - Feature #12021] [Open] Final	instance variables

Issue #12021 has been reported by Petr Chalupa.

----------------------------------------
Feature #12021: Final instance variables
https://bugs.ruby-lang.org/issues/12021

* Author: Petr Chalupa
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Having a final instance variables in Ruby will allow: to construct thread-save immutable objects without additional synchronisation on instance variable reading

~~~ ruby
# Immutable and thread-safe 
class TreeNode
  attr :left, :right, :value, final: true
  attr_reader :left, :right, :value

  def initialize(left, right, value)
    @left, @right, @value = left, right, value
  end
end
~~~

And to fix the an issue shown in the following example:

~~~ ruby
attr :lock, final: true
def initialize
  @lock = Mutex.new
  # ...
end  
   
def a_protected_method
  @lock.synchronize do
    # ...
  end
end
~~~

The issue lies in initialization of instance variable `@lock` which is not ensured to be visible in subsequent call to `a_protected_method` method on a different thread.

Summary can be found in this document https://docs.google.com/document/d/1c07qfDArx0bhK9sMr24elaIUdOGudiqBhTIRALEbrYY/edit#.

The aggregating issue of this effort can be found [here](https://bugs.ruby-lang.org/issues/12019).



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