Ruby: How should I access instance variables inside a class?

Question

In ruby you can internally access variables directly via @var_name or via private getters attr_reader :var_name.

Which solution is more (semantically?) correct? Any advantages/disadvantages of using either solution 1 or solution 2?

Solution 1:

class Point
 def initialize(x, y)
   @x = x
   @y = y
 end

 def distance
   Math.sqrt(@x ** 2 + @y ** 2)
 end
end

Solution 2:

class Point
  def initialize(x, y)
   @x = x
   @y = y
  end

  def distance
    Math.sqrt(x ** 2 + y ** 2)
  end

private 
  attr_reader :x, :y

end

There are times when you want to use "Solution 3": Object#instance_variable_get. Suppose, for example, you wanted to compute the sum of squares of the values of all instance variables. You could write def sum_of_squares; instance_variables.reduce(0) { |t,v| t + instance_variable_get(v) ** 2 }; end, then Point.new(3,5).sum_of_squares #=> 34. Object#instance_variables returns an array of the instance variables. — Cary Swoveland
– Cary Swoveland, Commented Oct 31, 2015 at 19:25
@CarySwoveland That's tricky, I like it! I believe this approach is perfect for codegolf.stackexchange.com :D — Filip Bartuzi
– Filip Bartuzi, Commented Nov 1, 2015 at 9:48
attr_reader is just a shortcut to avoid writing the same, simple and generic getter methods for every attribute. It's part of DRY, imo. — Charles
– Charles, Commented Nov 1, 2015 at 15:33
@c650 it's not "just" a shortcut. It's C extension which is few times faster than writing methods by hand. omniref.com/ruby/2.2.0/files/… — Filip Bartuzi
– Filip Bartuzi, Commented Nov 1, 2015 at 15:49
@FilipBartuzi so I've heard. To me, however, if you're using Ruby there's a chance that you don't care about the few extra seconds. Wouldn't you use a different language if you actually cared about speed? — Charles
– Charles, Commented Nov 1, 2015 at 18:44

spickermann · Accepted Answer · 2023-03-14 08:03:52Z

4

I would use the second option:

class Point
  def initialize(x, y)
   @x = x
   @y = y
  end

  def distance
    Math.sqrt(x ** 2 + y ** 2)
  end

private 

  attr_reader :x, :y    
end

For two reasons:

attr_reader might be faster (as Filip Bartuzi already pointed out)
Using attr_reader might make it easier to refactor that class later on by replacing the attr_reader with a custom getter method.

edited Mar 14, 2023 at 8:03

answered Oct 31, 2015 at 13:23

spickermann

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1 Comment

Cary Swoveland Over a year ago

Good solution. I can think of a third reason. Have a look at the comment I left on the question. Another way of doing that is def sum_of_squares; instance_variables.reduce(0) { |t,sym| t + send(sym.to_s[1..-1]) ** 2 }; end, then Point.new(3,5).sum_of_squares #=> 34. For each instance variable, this sends the string representation of its getter to the instance to obtain the value of the instance variable.

Community · Accepted Answer · 2017-05-23 11:44:32Z

0

attr_reader :x, :y defines the following functions:

def x
  @x
end

def y
  @y
end

So both methods are equivalent and in fact identical. Performance is also similar in both cases, although attr_reader can be faster than some other variable accessing methods..

edited May 23, 2017 at 11:44

CommunityBot

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answered Oct 31, 2015 at 13:03

Leo Brito

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2 Comments

Filip Bartuzi Over a year ago

attr_reader is 3x faster than define_method: omniref.com/ruby/2.2.0/files/…

Leo Brito Over a year ago

@FilipBartuzi the question is asking about direct access, though, which is not mentioned in the article.

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