Ruby mark instance variable as unassignable

Question

Is it possible to mark an instance variable such as @var as unassingnable, such that the following test would pass? The aim is simulate a readonly variable:

class Test
  def initialize(var)
    @var = var
    @var.freeze
    make_unassignable(:@var)
  end

  def set_var(new_var)
    @var = new_var
  end

  def make_unassignable(symbol)
    #insert algorithm here
  end
end

test = Test.new('some var')

expect { test.set_var = 'some other var' }.to raise_error
expect { test.instance_variable_set(:@var, 'some other var') }.to raise_error

@Andrey: Please remove the duplicate flag. The question you have given is similar but I am asking about disabling the ability to assign a variable altogether (not just build protections via exceptions). — abstractx1
– abstractx1, Commented Feb 13, 2017 at 11:16
please unaccept my answer since it does not answer the question about making variable unassignable (which is not possible, as another answer correctly states) — Andrey Deineko
– Andrey Deineko, Commented Feb 13, 2017 at 12:42
There are read-only variables in Ruby (for example $<), but Ruby doesn't expose this feature through its API. — Stefan
– Stefan, Commented Feb 13, 2017 at 13:03

Jörg W Mittag · Accepted Answer · 2017-02-13 12:31:49Z

3

No, this is not possible. Variables are variable in Ruby, always. In fact, even constants are variable in Ruby, they only generate a warning when re-assigned.

answered Feb 13, 2017 at 12:31

Jörg W Mittag

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1 Comment

Aleksei Matiushkin Over a year ago

Overwritten instance_variable_set on Test’s eigenclass might drastically hamper it’s resetting (not to make it impossible, though.)

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