Ruby Calling class method from instance

Question

In Ruby  how do you call a class method from one of that class s instances  Say I have  class Truck   def self default make       Class method       mac    end    def initialize       Instance method      Truck default make    gets the default via the class s method        But  I wish to avoid mentioning Truck  Seems I m repeating myself    end end   the line Truck default make retrieves the default  But is there a way of saying this without mentioning Truck  It seems like there should be

User · Answer

Using self class blah is NOT the same as using ClassName blah when it comes to inheritance   class Truck   def self default make      mac    end    def make1     self class default make   end    def make2     Truck default make   end end   class BigTruck  lt  Truck   def self default make      bigmac    end end  ruby-1 9 3-p0  021  gt  b BigTruck new    gt    lt BigTruck 0x0000000307f348 gt   ruby-1 9 3-p0  022  gt  b make1    gt   bigmac   ruby-1 9 3-p0  023  gt  b make2    gt   mac

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Here s an approach on how you might implement a  class method that works as self class for this situation  Note  Do not use this in production code  this is for interest-sake     From  Can you eval code in the context of a caller in Ruby  and also http   rubychallenger blogspot com au 2011 07 caller-binding html    Rabid monkey-patch for Object require  continuation  if RUBY VERSION  gt    1 9 0  class Object   def     eval  self class   caller binding  end   alias   class       def caller binding     cc   nil  count   0     set trace func lambda    event  file  lineno  id  binding  klass        if count    2         set trace func nil         cc call binding       elsif event     return          count    1       end           return callcc    cont  cc   cont     end end    Now we have awesome def Tiger   def roar       self class roar        roar       or  even      class roar   end   def self roar       TODO  tigerness   end end   Maybe the right answer is to submit a patch for Ruby

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One more   class Truck   def self default make      mac    end    attr reader  make    private define method  default make   amp method  default make     def initialize make   default make       make   make   end end  puts Truck new make     gt  mac

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If you have access to the delegate method you can do this    20  pry main  gt  class Foo  20  pry main     def self bar  20  pry main        foo bar   20  pry main     end    20  pry main     delegate  bar  to   self class   20  pry main   end     gt    bar   21  pry main  gt  Foo new bar   gt   foo bar   22  pry main  gt  Foo bar   gt   foo bar    Alternatively  and probably cleaner if you have more then a method or two you want to delegate to class  amp  instance    1  pry main  gt  class Foo  1  pry main     module AvailableToClassAndInstance  1  pry main       def bar  1  pry main          foo bar   1  pry main       end    1  pry main     end    1  pry main     include AvailableToClassAndInstance  1  pry main     extend AvailableToClassAndInstance  1  pry main   end     gt  Foo  2  pry main  gt  Foo new bar   gt   foo bar   3  pry main  gt  Foo bar   gt   foo bar    A word of caution   Don t just randomly delegate everything that doesn t change state to class and instance because you ll start running into strange name clash issues  Do this sparingly and only after you checked nothing else is squashed

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self class default make

User · Answer

You re doing it the right way   Class methods  similar to  static  methods in C   or Java  aren t part of the instance  so they have to be referenced directly   On that note  in your example you d be better served making  default make  a regular method      usr bin ruby  class Truck     def default make           Class method           mac      end      def initialize           Instance method          puts default make    gets the default via the class s method      end end  myTruck   Truck new     Class methods are more useful for utility-type functions that use the class   For example      usr bin ruby  class Truck     attr accessor  make      def default make           Class method           mac      end      def self buildTrucks make  count          truckArray                1  count  each do             truckArray  lt  lt  Truck new make          end          return truckArray     end      def initialize make   nil          if  make    nil                make   default make           else              make   make         end     end end  myTrucks   Truck buildTrucks  Yotota   4   myTrucks each do  truck      puts truck make end

User · Answer

To access a class method inside a instance method  do the following   self class default make   Here is an alternative solution for your problem   class Truck    attr accessor  make   year    def self default make      Toyota    end    def make      make    self class default make   end    def initialize make nil  year nil      self year  self make   year  make   end end   Now let s use our class   t   Truck new  Honda   2000  t make     gt   Honda  t year     gt   2000   t   Truck new t make     gt   Toyota  t year     gt  nil

User · Answer

Similar your question  you could use   class Truck   def default make       Do something   end    def initialize     super     self default make   end end

User · Answer

Rather than referring to the literal name of the class  inside an instance method you can just call self class whatever   class Foo     def self some class method         puts self     end      def some instance method         self class some class method     end end  print  Class method    Foo some class method  print  Instance method    Foo new some instance method   Outputs    Class method  Foo Instance method  Foo

[ruby] Ruby: Calling class method from instance

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