In Ruby, should I use ||= or if defined? for memoization?

Question

Should I use if defined?

 return @current_user_session if defined?(@current_user_session)
 @current_user_session = UserSession.find

Answer 1

Additionally, the nicer ||= produces a warning (on 1.8.6 and 1.8.7, at least) about uninitialized instance variables, while the more verbose defined? version does not.

On the other hand, this probably does what you want:

def initialize
  @foo = nil
end

def foo
  @foo ||= some_long_calculation_for_a_foo
end

But this almost certainly does not:

def initialize
  @foo = nil
end

def foo
  return @foo if defined?(@foo)
  @foo = some_long_calculation_for_a_foo
end

since @foo will always be defined at that point.