Why use Ruby s attr accessor attr reader and attr writer

Question

Ruby has this handy and convenient way to share instance variables by using keys like  attr accessor  var attr reader  var attr writer  var   Why would I choose attr reader or attr writer if I could simply use attr accessor  Is there something like performance  which I doubt   I guess there is a reason  otherwise they wouldn t have made such keys

User · Answer

It is important to understand that accessors restrict access to variable, but not their content. In ruby, like in some other OO languages, every variable is a pointer to an instance. So if you have an attribute to an Hash, for example, and you set it to be "read only" you always could change its content, but not the content of pointer. Look at this:

irb(main):024:0> class A
irb(main):025:1> attr_reader :a
irb(main):026:1> def initialize
irb(main):027:2> @a = {a:1, b:2}
irb(main):028:2> end
irb(main):029:1> end
=> :initialize
irb(main):030:0> a = A.new
=> #<A:0x007ffc5a10fe88 @a={:a=>1, :b=>2}>
irb(main):031:0> a.a
=> {:a=>1, :b=>2}
irb(main):032:0> a.a.delete(:b)
=> 2
irb(main):033:0> a.a
=> {:a=>1}
irb(main):034:0> a.a = {}
NoMethodError: undefined method `a=' for #<A:0x007ffc5a10fe88 @a={:a=>1}>
        from (irb):34
        from /usr/local/bin/irb:11:in `<main>'

As you can see is possible delete a key/value pair from the Hash @a, as add new keys, change values, eccetera. But you can't point to a new object because is a read only instance variable.

User · Answer

You don t always want your instance variables to be fully accessible from outside of the class  There are plenty of cases where allowing read access to an instance variable makes sense  but writing to it might not  e g  a model that retrieves data from a read-only source   There are cases where you want the opposite  but I can t think of any that aren t contrived off the top of my head

User · Answer

You may use the different accessors to communicate your intent to someone reading your code  and make it easier to write classes which will work correctly no matter how their public API is called   class Person   attr accessor  age       end   Here  I can see that I may both read and write the age   class Person   attr reader  age       end   Here  I can see that I may only read the age   Imagine that it is set by the constructor of this class and after that remains constant   If there were a mutator  writer  for age and the class were written assuming that age  once set  does not change  then a bug could result from code calling that mutator   But what is happening behind the scenes   If you write   attr writer  age   That gets translated into   def age  value     age   value end   If you write   attr reader  age   That gets translated into   def age    age end   If you write   attr accessor  age   That gets translated into   def age  value     age   value end  def age    age end   Knowing that  here s another way to think about it  If you did not have the attr     helpers  and had to write the accessors yourself  would you write any more accessors than your class needed   For example  if age only needed to be read  would you also write a method allowing it to be written

User · Answer

All of the answers above are correct  attr reader and attr writer are more convenient to write than manually typing the methods they are shorthands for  Apart from that they offer much better performance than writing the method definition yourself  For more info see slide 152 onwards from this talk  PDF  by Aaron Patterson

User · Answer

Not all attributes of an object are meant to be directly set from outside the class  Having writers for all your instance variables is generally a sign of weak encapsulation and a warning that you re introducing too much coupling between your classes   As a practical example  I wrote a design program where you put items inside containers  The item had attr reader  container  but it didn t make sense to offer a writer  since the only time the item s container should change is when it s placed in a new one  which also requires positioning information

[ruby] Why use Ruby's attr_accessor, attr_reader and attr_writer?

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