Why can I access private/protected methods using Object#send in Ruby?

Question

The class

class A

  private
  def foo
    puts :foo
  end

  public
  def bar
    puts :bar
  end

  private
  def zim
    puts :zim
  end

  protected
  def dib
    puts :dib
  end
end

instance of A

a = A.new

test

a.foo rescue puts :fail
a.bar rescue puts :fail
a.zim rescue puts :fail
a.dib rescue puts :fail
a.gaz rescue puts :fail

test output

fail
bar
fail
fail
fail

.send test

[:foo, :bar, :zim, :dib, :gaz].each { |m| a.send(m) rescue puts :fail }

.send output

foo
bar
zim
dib
fail

The question

The section labeled "Test Output" is the expected result. So why can I access private/protected method by simply Object#send?

Perhaps more important:

What is the difference between public/private/protected in Ruby? When to use each? Can someone provide real world examples for private and protected usage?

Answer 1

Technically: Because send doesn't do anything to check method visibility. (It would be more work to do so.)

Philosophically: Ruby is a very permissive language. You can already just open up a class and make any method you want public. The language designers implemented send in a way that allows it to override the restrictions normally imposed by private. Ruby 1.9 was originally going to have two variants, a private-respecting send and an unsafe variant called send!, but this was apparently dropped for backwards compatibility.

As for what private, protected and public mean:

• public methods can be called by any sender
• protected methods cannot be called outside of an instance of the method's class or an instance of a subclass
• private methods cannot be called with an explicit receiver (with a couple of exceptions, such as setter methods, which always have to have an explicit receiver, and so can be called within the class that way)