instance_eval and yield

Great example from http://stackoverflow.com/questions/1425055/is-yield-self-the-same-as-instance-eval

class Foo
  def initialize(&block)
    instance_eval(&block) if block_given?
  end
end
x = Foo.new { def foo; 'foo'; end }            #=> #<Foo:0xb800f6a0>                                            
x.foo  #=> "foo"                                                        
z = Foo.new  #=> #<Foo:0xb800806c>                                            
z.foo #=>NoMethodError: undefined method `foo' for #<Foo:0xb800806c>

class Foo2
  def initialize
    yield if block_given?
  end
end
x = Foo2.new { def foo; 'foo'; end } #=> #<Foo:0xb7ff1bb4>
x.foo #=> private method `foo' called for #<Foo2:0xb8004930> (NoMethodError)
x.send :foo => "foo"
z = Foo.new  #=> #<Foo:0xb800806c> 
z.send :foo => "foo"