What is attr accessor in Ruby

Question

I am having a hard time understanding attr accessor in Ruby  Can someone explain this to me

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I am new to ruby and had to just deal with understanding the following weirdness  Might help out someone else in the future  In the end it is as was mentioned above  where 2 functions  def myvar  def myvar   both get implicitly for accessing  myvar  but these methods can be overridden by local declarations   class Foo   attr accessor  myvar    def initialize      myvar    A      myvar    B      puts  myvar   A     puts myvar   B - myvar declared above overrides myvar method   end    def test     puts  myvar   A     puts myvar   A - coming from myvar accessor      myvar    C    local myvar overrides accessor     puts  myvar   A     puts myvar   C      send  myvar     E    not running  myvar     but instead calls setter for  myvar     puts  myvar   E     puts myvar   C   end end

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Basically they fake publicly accessible data attributes  which Ruby doesn t have

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attr accessor is just a method   The link should provide more insight with how it works - look at the pairs of methods generated  and a tutorial should show you how to use it    The trick is that class is not a definition in Ruby  it is  just a definition  in languages like C   and Java   but it is an expression that evaluates  It is during this evaluation when the attr accessor method is invoked which in turn modifies the current class - remember the implicit receiver  self attr accessor  where self is the  open  class object at this point   The need for attr accessor and friends  is  well    Ruby  like Smalltalk  does not allow instance variables to be accessed outside of methods1 for that object  That is  instance variables cannot be accessed in the x y form as is common in say  Java or even Python  In Ruby y is always taken as a message to send  or  method to call    Thus the attr   methods create wrappers which proxy the instance  variable access through dynamically created methods  Boilerplate sucks   Hope this clarifies some of the little details  Happy coding     1 This isn t strictly true and there are some  techniques  around this  but there is no syntax support for  public instance variable  access

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Simple Explanation Without Any Code  Most of the above answers use code  This explanation attempts to answer it without using any  via an analogy story   Outside parties cannot access internal CIA secrets   Let s imagine a really secret place  the CIA  Nobody knows what s happening in the CIA apart from the people inside the CIA  In other words  external people cannot access any information in the CIA  But because it s no good having an organisation that is completely secret  certain information is made available to the outside world - only things that the CIA wants everyone to know about of course  e g  the Director of the CIA  how environmentally friendly this department is compared to all other government departments etc  Other information  e g  who are its covert operatives in Iraq or Afghanistan - these types of things will probably remain a secret for the next 150 years  If you re outside the CIA you can only access the information that it has made available to the public  Or to use CIA parlance you can only access information that is  cleared   The information that the CIA wants to make available to the general public outside the CIA are called  attributes     The meaning of read and write attributes    In the case of the CIA  most attributes are  read only   This means if you are a party external to the CIA  you can ask   who is the director of the CIA   and you will get a straight answer  But what you cannot do with  read only  attributes is to make changes changes in the CIA  e g  you cannot make a phone call and suddenly decide that you want Kim Kardashian to be the Director  or that you want Paris Hilton to be the Commander in Chief   If the attributes gave you  write  access  then you could make changes if you want to  even if you were outside  Otherwise  the only thing you can do is read   In other words accessors allow you to make inquiries  or to make changes  to organisations that otherwise do not let external people in  depending on whether the accessors are read or write accessors    Objects inside a class can easily access each other   On the other hand  if you were already inside the CIA  then you could easily call up your CIA operative in Kabul because this information is easily accessible given you are already inside  But if you re outside the CIA  you simply will not be given access   you will not be able to know who they are  read access   and you will not be able to change their mission  write access     Exact same thing with classes and your ability to access variables  properties and methods within them  HTH  Any questions  please ask and I hope i can clarify

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I faced this problem as well and wrote a somewhat lengthy answer to this question  There are some great answers on this already  but anyone looking for more clarification  I hope my answer can help  Initialize Method  Initialize allows you to set data to an instance of an object upon creation of the instance rather than having to set them on a separate line in your code each time you create a new instance of the class    class Person    def initialize name       name   name   end     def greeting      Hello    name     end end  person   Person new  Denis   puts person greeting   In the code above we are setting the name    Denis    using the initialize method by passing Dennis through the parameter in Initialize  If we wanted to set the name without the initialize method we could do so like this   class Person   attr accessor  name      def initialize name           name   name     end    def greeting      Hello   name     end end  person   Person new person name    Dennis  puts person greeting   In the code above  we set the name by calling on the attr accessor setter method using person name  rather than setting the values upon initialization of the object    Both    methods    of doing this work  but initialize saves us time and lines of code   This is the only job of initialize  You cannot call on initialize as a method  To actually get the values of an instance object you need to use getters and setters  attr reader  get   attr writer set   and attr accessor both     See below for more detail on those   Getters  Setters  attr reader  attr writer  attr accessor   Getters  attr reader  The entire purpose of a getter is to return the value of a particular instance variable   Visit the sample code below for a breakdown on this     class Item    def initialize item name  quantity       item name   item name      quantity   quantity   end    def item name      item name   end    def quantity       quantity   end end  example   Item new  TV  2  puts example item name puts example quantity   In the code above you are calling the methods    item name    and    quantity    on the instance of Item    example     The    puts example item name    and    example quantity    will return  or    get     the value for the parameters that were passed into the    example    and display them to the screen   Luckily in Ruby there is an inherent method that allows us to write this code more succinctly  the attr reader method   See the code below   class Item  attr reader  item name   quantity    def initialize item name  quantity       item name   item name      quantity   quantity   end  end  item   Item new  TV  2  puts item item name puts item quantity   This syntax works exactly the same way  only it saves us six lines of code  Imagine if you had 5 more state attributable to the Item class  The code would get long quickly   Setters  attr writer  What crossed me up at first with setter methods is that in my eyes it seemed to perform an identical function to the initialize method  Below I explain the difference based on my understanding    As stated before  the initialize method allows you to set the values for an instance of an object upon object creation    But what if you wanted to set the values later  after the instance was created  or change them after they have been initialized   This would be a scenario where you would use a setter method   THAT IS THE DIFFERENCE  You don   t have to    set    a particular state when you are using the attr writer method initially    The code below is an example of using a setter method to declare the value item name for this instance of the Item class  Notice that we continue to use the getter method attr reader so that we can get the values and print them to the screen  just in case you want to test the code on your own    class Item  attr reader  item name    def item name  str       item name    str    end  end   The code below is an example of using attr writer to once again shorten our code and save us time   class Item  attr reader  item name attr writer  item name  end  item   Item new puts item item name    TV    The code below is a reiteration of the initialize example above of where we are using initialize to set the objects value of item name upon creation   class Item  attr reader  item name    def initialize item name       item name   item name   end  end  item   Item new  TV   puts item item name   attr accessor  Performs the functions of both attr reader and attr writer  saving you one more line of code

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Let s say you have a class Person   class Person end  person   Person new person name     gt  no method error   Obviously we never defined method name  Let s do that   class Person   def name      name   simply returning an instance variable  name   end end  person   Person new person name     gt  nil person name    Dennis      gt  no method error   Aha  we can read the name  but that doesn t mean we can assign the name  Those are two different methods  The former is called reader and latter is called writer  We didn t create the writer yet so let s do that   class Person   def name      name   end    def name  str       name   str   end end  person   Person new person name    Dennis  person name     gt   Dennis    Awesome  Now we can write and read instance variable  name using reader and writer methods  Except  this is done so frequently  why waste time writing these methods every time  We can do it easier   class Person   attr reader  name   attr writer  name end   Even this can get repetitive  When you want both reader and writer just use accessor   class Person   attr accessor  name end  person   Person new person name    Dennis  person name     gt   Dennis    Works the same way  And guess what  the instance variable  name in our person object will be set just like when we did it manually  so you can use it in other methods   class Person   attr accessor  name    def greeting      Hello    name     end end  person   Person new person name    Dennis  person greeting     gt   Hello Dennis    That s it  In order to understand how attr reader  attr writer  and attr accessor methods actually generate methods for you  read other answers  books  ruby docs

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If you are familiar with OOP concept  You must familiar with getter and setter method  attr accessor does the same in Ruby   Getter and Setter in General Way  class Person   def name      name   end    def name  str       name   str   end end  person   Person new person name    Eshaan  person name     gt   Eshaan    Setter Method  def name  val     name   val end   Getter method   def name    name end   Getter and Setter method in Ruby  class Person   attr accessor  name end  person   Person new person name    Eshaan  person name     gt   Eshaan

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Simply put it will define a setter and getter for the class   Note that   attr reader  v is equivalant to  def v    v end  attr writer  v is equivalant to def v  value     v value end   So   attr accessor  v which means  attr reader  v  attr writer  v    are equivalant to define a setter and getter for the class

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I think part of what confuses new Rubyists programmers  like myself  is    Why can t I just tell the instance it has any given attribute  e g   name  and give that attribute a value all in one swoop    A little more generalized  but this is how it clicked for me   Given   class Person end   We haven t defined Person as something that can have a name or any other attributes for that matter    So if we then   baby   Person new      and try to give them a name     baby name    Ruth    We get an error because  in Rubyland  a Person class of object is not something that is associated with or capable of having a  name      yet   BUT we can use any of the given methods  see previous answers  as a way to say   An instance of a Person class  baby  can now have an attribute called  name   therefore we not only have a syntactical way of getting and setting that name  but it makes sense for us to do so    Again  hitting this question from a slightly different and more general angle  but I hope this helps the next instance of class Person who finds their way to this thread

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Another way to understand it is to figure out what error code it eliminates by having attr accessor    Example   class BankAccount       def initialize  account owner        owner   account owner      balance   0   end    def deposit  amount        balance    balance   amount   end    def withdraw  amount        balance    balance - amount   end end   The following methods are available     bankie   BankAccout new  Iggy     bankie    bankie deposit 100    bankie withdraw 5    The following methods throws error     bankie owner      undefined method  owner        bankie balance    undefined method  balance       owner and balance are not  technically  a method  but an attribute  BankAccount class does not have def owner and def balance  If it does  then you can use the two commands below  But those two methods aren t there  However  you can access attributes as if you d access a method via attr accessor   Hence the word attr accessor  Attribute  Accessor  It accesses attributes like you would access a method    Adding attr accessor  balance   owner allows you to read and write balance and owner  method   Now you can use the last 2 methods     bankie balance   bankie owner

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The main functionality of attr accessor over the other ones is the capability of accessing data from other files  So you usually would have attr reader or attr writer but the good news is that Ruby lets you combine these two together with attr accessor  I think of it as my to go method because it is more well rounded or versatile  Also  peep in mind that in Rails  this is eliminated because it does it for you in the back end  So in other words  you are better off using attr acessor over the other two because you don t have to worry about being to specific  the accessor covers it all  I know this is more of a general explanation but it helped me as a beginner    Hope this helped

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attr accessor is very simple   attr accessor  foo   is a shortcut for   def foo  val     foo   val end  def foo    foo end   it is nothing more than a getter setter for an object

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Defines a named attribute for this module  where the name is symbol id2name  creating an instance variable   name  and a corresponding access method to read it  Also creates a method called name  to set the attribute   module Mod   attr accessor  one   two  end Mod instance methods sort      gt    one   one    two   two

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To summarize an attribute accessor aka attr accessor gives you two free methods   Like in Java they get called getters and setters   Many answers have shown good examples so I m just going to be brief    the attribute  and   the attribute   In the old ruby docs a hash tag   means a method  It could also include a class name prefix    MyClass my method

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Simply attr-accessor creates the getter and setter methods for the specified attributes

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attr accessor is  as  pst stated  just a method  What it does is create more methods for you   So this code here   class Foo   attr accessor  bar end   is equivalent to this code   class Foo   def bar      bar   end   def bar   new value        bar   new value   end end   You can write this sort of method yourself in Ruby   class Module   def var  method name       inst variable name       method name   to sym     define method method name do       instance variable get inst variable name     end     define method    method name    do  new value        instance variable set inst variable name  new value     end   end end  class Foo   var  bar end  f   Foo new p f bar        gt  nil f bar   42 p f bar        gt  42

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It is just a method that defines getter and setter methods for instance variables  An example implementation would be   def self attr accessor  names    names each do  name      define method name   instance variable get     name       This is the getter     define method    name       arg  instance variable set     name    arg     This is the setter   end end

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Hmmm  Lots of good answers  Here is my few cents on it     attr accessor is a simple method that helps us in cleaning DRY-ing  up the repeating getter and setter methods   So that we can focus more on writing business logic and not worry about the setters and getters

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Attributes and accessor methods  Attributes are class components that can be accessed from outside the object  They are known as properties in many other programming languages  Their values are accessible by using the  dot notation   as in object name attribute name  Unlike Python and a few other languages  Ruby does not allow instance variables to be accessed directly from outside the object   class Car   def initialize      wheels   4    This is an instance variable   end end  c   Car new c wheels       Output  NoMethodError  undefined method  wheels  for   lt Car 0x00000000d43500 gt    In the above example  c is an instance  object  of the Car class  We tried unsuccessfully to read the value of the wheels instance variable from outside the object  What happened is that Ruby attempted to call a method named wheels within the c object  but no such method was defined  In short  object name attribute name tries to call a method named attribute name within the object  To access the value of the wheels variable from the outside  we need to implement an instance method by that name  which will return the value of that variable when called  That s called an accessor method  In the general programming context  the usual way to access an instance variable from outside the object is to implement accessor methods  also known as getter and setter methods  A getter allows the value of a variable defined within a class to be read from the outside and a setter allows it to be written from the outside   In the following example  we have added getter and setter methods to the Car class to access the wheels variable from outside the object  This is not the  Ruby way  of defining getters and setters  it serves only to illustrate what getter and setter methods do   class Car   def wheels    getter method      wheels   end    def wheels  val     setter method      wheels   val   end end  f   Car new f wheels   4    The setter method was invoked f wheels    The getter method was invoked   Output    gt  4   The above example works and similar code is commonly used to create getter and setter methods in other languages  However  Ruby provides a simpler way to do this  three built-in methods called attr reader  attr writer and attr acessor  The attr reader method makes an instance variable readable from the outside  attr writer makes it writeable  and attr acessor makes it readable and writeable    The above example can be rewritten like this   class Car   attr accessor  wheels end  f   Car new f wheels   4 f wheels    Output    gt  4   In the above example  the wheels attribute will be readable and writable from outside the object  If instead of attr accessor  we used attr reader  it would be read-only  If we used attr writer  it would be write-only  Those three methods are not getters and setters in themselves but  when called  they create getter and setter methods for us  They are methods that dynamically  programmatically  generate other methods  that s called metaprogramming   The first  longer  example  which does not employ Ruby s built-in methods  should only be used when additional code is required in the getter and setter methods  For instance  a setter method may need to validate data or do some calculation before assigning a value to an instance variable    It is possible to access  read and write  instance variables from outside the object  by using the instance variable get and instance variable set built-in methods  However  this is rarely justifiable and usually a bad idea  as bypassing encapsulation tends to wreak all sorts of havoc

[ruby] What is attr_accessor in Ruby?

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