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rubyinstance-variables

Ruby attr_reader allows one to modify string variable if using <<

Ran into some weird behaviour and wondering if anyone else can confirm what I am seeing.

Suppose you create a class with a member variable, and allow it to be read with attr_reader.

class TestClass
  attr_reader :val

  def initialize(value)
   @val = value
  end
end

Now when I do the following, it seems to modify the value of @val, even though I have only granted it read privileges.

test = TestClass.new('hello')
puts test.val
test.val << ' world'
puts test.val

This returns

hello
hello world

This is just the result from some testing I did in irb so not sure if this is always the case

Solution

  • You are not really writing the val attribute. You are reading it and invoking a method on it (the '<<' method).

    If you want an accessor that prevents the kind of modification you describe then you might want to implement a method that returns a copy of @val instead of using attr_reader.