attr_reader, attr_writer and attr_accessor

[Khoa]

Could some body please give me some examples about attr_reader, attr_writer and attr_accessor? I don't know what do they do and how to use them (><).

 

[hanmac]

from: http://www.ruby-doc.org/core/classes/Mo ... ml#M001701
  attr(symbol, writable=false) => nil

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. If the optional writable argument is true, also creates a method called name= to set the attribute.

  module Mod
    attr  :size, true
  end

is equivalent to:

  module Mod
    def size
      @size
    end
    def size=(val)
      @size = val
    end
  end

attr_accessor(symbol, ...) => nil

Equivalent to calling ``attrsymbol, true’’ on each symbol in turn.

  module Mod
    attr_accessorone, :two)
  end
  Mod.instance_methods.sort  #=> ["one", "one=", "two", "two="]

attr_reader(symbol, ...) => nil

Creates instance variables and corresponding methods that return the value of each instance variable. Equivalent to calling ``attr:name’’ on each name in turn.
attr_writer(symbol, ...) => nil

Creates an accessor method to allow assignment to the attribute aSymbol.id2name.

 

[Khoa]

thank hanmac for your help, but your example is too difficult to understand (><).
Could you please make it more simple? (^^)

 

[hanmac]

attr_reader :name
is the same as
def name
return @name
end


attr_writer :name
is the same as
def name=(value)
@name = value
end


attr_accessor :name

is both

 

[Khoa]

Does that mean create a method and a variable with the same name?

 

[hanmac]

yes and no.

it make that you can read/write the variable from out of the object.

Sample:

class ClassA
attr_accessor :alpha
end

x=ClassA.new
x.alpha = 45

 

[Khoa]

Thank you, hanmac, I understand alot (^^).
But I don't know when should I use it and when shouldn't use (><).

 

[hanmac]

use it when you need a access.

use it not when you need a spezific access ala:

def test=(vaule)
  @test=[[0,value].max,100].min
end

 

[khmp]

I find it helpful when coding to ask myself a ton of questions about what I'm trying to do. This applies to attributes as well. To decide which one you will need. You just need to ask yourself; Will I ever need to change the value from outside the class? Will I only ever just need to see the value from outside of this class? Will I need to do both from outside the class' definition?

Answering yes to the first one means that the value should have either an attr_writer written for it or a method like the one hanmac has just above.

Answering yes to the second one means that the value should have either an attr_reader written for it or a method like this:

attr_reader :value
...
def value
  return @value
end

And lastly answering yes to both questions means that you use an attr_accessor for the value or write out both methods of accessing and manipulating:

attr_accessor :value
...
def value
  return @value
end

def value=(value)
  @value = value
end

Hope that helps.

Good luck with it Khoa! :thumb:

 

[Khoa]

Thank you, khmp & hanmac. I understand it now (^^)