What is the conventional way to access instance variables within an object in Ruby?

Question

Consider the following code:

class Dog
  attr_accessor :name, :color

  def initialize(name, color)
  end
end

Within a Ruby object, is the convention to access the instance variable directly (ie @name = name) or to use the setter/getter methods (ie name = name)?

The former is more clear to me, but if you implement your own setter/getter methods (eg to increment a class variable at the same time) then you end up having to use both approaches (ie @name = name ; color = color).

What's the convention within the Ruby community? How should I write my code to make it clear to others who will read it?

See: stackoverflow.com/questions/4312801/… and stackoverflow.com/questions/2884715/… — Josh Lee
– Josh Lee, Commented Jan 18, 2011 at 5:00

Josh Lee · Accepted Answer · 2011-01-18 05:29:36Z

6

Using name = value is an error, because that creates a local variable named name. You must use self.name = value.

As for convention, you can only get away with using @name if you can guarantee that the accessors will always be lightweight attr_accessors. In all other cases, using @name over self.name will violate encapsulation and give yourself a headache. You gave the exact reason in your question — if there is extra logic in the getter/setter, you must duplicate it if you access the instance variable directly.

edited Jan 18, 2011 at 5:29

answered Jan 18, 2011 at 5:02

Josh Lee

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5 Comments

nfm Over a year ago

So the local variable color has scope precedence over the method color?

Ed Swangren Over a year ago

Yes, that one will bite you in the ass for sure. Every so often I forget the @ sign and spend five minutes wondering what could have gone wrong.

Ed Swangren Over a year ago

I'm curious; why do you claim that using self.property_name breaks encapsulation? If there is logic in the setter I would much prefer to use it rather than duplicating code all over the place.

Josh Lee Over a year ago

@Ed I meant the opposite. I rephrased it a bit.

Ed Swangren Over a year ago

Ah, now that makes more sense ;). +1

Bob. · Accepted Answer · 2011-01-18 04:55:08Z

1

It would be @name. It's more than just a convention. The @ sign defines the scope of the variable to within an instance of the object of which it's defined.

answered Jan 18, 2011 at 4:55

Bob.

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Comments

intellidiot · Accepted Answer · 2011-01-18 05:23:57Z

1

What you have done is the most conventional way, I believe.

Actually attr_accessor creates two instance methods for read and write. It creates 2 methods like:

def color() #getter
  @color
end

and

def color=(color) #setter
  @color = color
end

And something like color = color wont work, because then color will be treated as a local variable if it's found on the left of an assignment. You can use self.color = color though. Again, if the color is not in the left side of any expression, it'll work fine just like the following:

def show_color
  puts color
end

answered Jan 18, 2011 at 5:23

intellidiot

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