How to make instance variables private in Ruby?

Question

Is there any way to make instance variables \"private\"(C++ or Java definition) in ruby? In other words I want following code to result in an error.

class B         


        

Answer 1

It is possible (but inadvisable) to do exactly what you are asking.

There are two different elements of the desired behavior. The first is storing x in a read-only value, and the second is protecting the getter from being altered in subclasses.

---

Read-only value

It is possible in Ruby to store read-only values at initialization time. To do this, we use the closure behavior of Ruby blocks.

class Foo
  def initialize (x)
    define_singleton_method(:x) { x }
  end
end

The initial value of x is now locked up inside the block we used to define the getter #x and can never be accessed except by calling foo.x, and it can never be altered.

foo = Foo.new(2)
foo.x  # => 2
foo.instance_variable_get(:@x)  # => nil

Note that it is not stored as the instance variable @x, yet it is still available via the getter we created using define_singleton_method.

---

Protecting the getter

In Ruby, almost any method of any class can be overwritten at runtime. There is a way to prevent this using the method_added hook.

class Foo
  def self.method_added (name)
    raise(NameError, "cannot change x getter") if name == :x
  end
end

class Bar < Foo
  def x
    20
  end
end

# => NameError: cannot change x getter

This is a very heavy-handed method of protecting the getter.

It requires that we add each protected getter to the method_added hook individually, and even then, you will need to add another level of method_added protection to Foo and its subclasses to prevent a coder from overwriting the method_added method itself.

Better to come to terms with the fact that code replacement at runtime is a fact of life when using Ruby.