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Ruby Programming
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Inheritance and Visibility

There's one last wrinkle to class inheritance, and it's fairly obscure.

Within a class definition, you can change the visibility of a method in an ancestor class. For example, you can do something like:

class Base
  def aMethod
    puts "Got here"
  end
  private :aMethod
end

class Derived1 < Base   public :aMethod end

class Derived2 < Base end

In this example, you would be able to invoke aMethod in instances of class Derived1, but not via instances of Base or Derived2.

So how does Ruby pull off this feat of having one method with two different visibilities? Simply put, it cheats.

If a subclass changes the visibility of a method in a parent, Ruby effectively inserts a hidden proxy method in the subclass that invokes the original method using super. It then sets the visibility of that proxy to whatever you requested. This means that the code:

class Derived1 < Base
  public :aMethod
end

is effectively the same as:

class Derived1 < Base
  def aMethod(*args)
    super
  end
  public :aMethod
end

The call to super can access the parent's method regardless of its visibility, so the rewrite allows the subclass to override its parent's visibility rules. Pretty scary, eh?
Ruby Programming
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