How to create a private class method

Question

How come this approach of creating a private class method works   class Person    def self get name     persons name   end    class  lt  lt  self      private      def persons name        Sam      end   end end  puts  Hey      Person get name puts  Hey      Person persons name     gt  raises  private method  persons name  called for Person Class  NoMethodError     But this does not   class Person    def self get name     persons name   end    private    def self persons name      Sam    end end  puts  Hey      Person get name puts  Hey      Person persons name

User · Answer

I too  find Ruby  or at least my knowledge of it  short of the mark in this area   For instance the following does what I want but is clumsy   class Frob     attr reader  val1   val2      Tolerance   2   Float  EPSILON      def initialize val1  val2           val2   val1          val2   val2                 end        Stuff that s likely to change and I don t want part       of a public API   Furthermore  the method is operating       solely upon  reference  and  under test  and will be flagged as having       low cohesion by quality metrics unless made a class method      def self compare reference  under test            special floating point comparison          reference - under test  abs  lt   Tolerance     end     private class method  compare      def    arg          self class send  compare  val1  arg val1   amp  amp          self class send  compare  val2  arg val2   amp  amp                  end end   My problems with the code above is that the Ruby syntax requirements and my code quality metrics conspire to made for cumbersome code  To have the code both work as I want and to quiet the metrics  I must make compare   a class method   Since I don t want it to be part of the class  public API  I need it to be private  yet  private  by itself does not work   Instead I am force to use  private class method  or some such work-around   This  in turn  forces the use of  self class send  compare     for each variable I test in         Now that s a bit unwieldy

User · Answer

ExiRe wrote      Such behavior of ruby is really frustrating  I mean if you move   to private section self method then it is NOT private  But if    you move it to class  lt  lt  self then it suddenly works    It is just disgusting    Confusing it probably is  frustrating it may well be  but disgusting it is definitely not    It makes perfect sense once you understand Ruby s object model and the corresponding method lookup flow  especially when taking into consideration that private is NOT an access visibility modifier  but actually a method call  with the class as its recipient  as discussed here    there s no such thing as  a private section  in Ruby   To define private instance methods  you call private on the instance s class to set the default visibility for subsequently defined methods to private    and hence it makes perfect sense to define private class methods by calling private on the class s class  ie  its metaclass   Other mainstream  self-proclaimed OO languages may give you a less confusing syntax  but you definitely trade that off against a confusing and less consistent  inconsistent   object model without the power of Ruby s metaprogramming facilities

User · Answer

private doesn t seem to work if you are defining a method on an explicit object  in your case self    You can use private class method to define class methods as private  or like you described    class Person   def self get name     persons name   end    def self persons name      Sam    end    private class method  persons name end  puts  Hey      Person get name puts  Hey      Person persons name   Alternatively  in ruby 2 1    since a method definition returns a symbol of the method name  you can also use this as follows   class Person   def self get name     persons name   end    private class method def self persons name      Sam    end end  puts  Hey      Person get name puts  Hey      Person persons name

User · Answer

Instance methods are defined inside a class definition block  Class methods are defined as singleton methods on the singleton class of a class  also informally known as the  metaclass  or  eigenclass   private is not a keyword  but a method  Module private    This is a call to method self private A private which  toggles  private access on for all forthcoming instance method definitions until toggled otherwise   class A   private     def instance method 1  end     def instance method 2  end          and so forth end   As noted earlier  class methods are really singleton methods defined on the singleton class   def A class method  end   Or using a special syntax to open the definition body of the anonymous  singleton class of A   class  lt  lt  A   def class method  end end   The receiver of the  message private  - self - inside class A is the class object A  self inside the class  lt  lt  A block is another object  the singleton class   The following example is in reality calling two different methods called private  using two different recipients or targets for the call  In the first part  we define a private instance method   on class A    in the latter we define a private class method  is in fact a singleton method on the singleton class object of A    class A     self is A and private call  A private      private def instance method  end    class  lt  lt  self       self is A s singleton class and private call  A singleton class private        private def class method  end   end end   Now  rewrite this example a bit   class A   private     def self class method  end end   Can you see the mistake  that Ruby language designers  made  You toggle on private access for all forthcoming instance methods of A  but proceed to declare a singleton method on a different class  the singleton class

User · Answer

Just for the completeness  we can also avoid declaring private class method in a separate line  I personally don t like this usage but good to know that it exists   private class method  def self method name       end

User · Answer

As of ruby 2 3 0  class Check   def self first method     second method   end    private   def self second method     puts  well I executed    end end  Check first method    gt  well I executed

User · Answer

Ruby seems to provide a poor solution   To explain  start with a simple C   example that shows access to private class methods    include  lt iostream gt   class C       public          void instance method void                        std  cout  lt  lt   instance method n               class method            LOOK     no  send  required  We can access it                                 because  private  allows access within the class               private          void static class method void    std  cout  lt  lt   class method n         int main         C c       c instance method       works        C  class method   does not compile - it s properly private     return 0      Running the above         a out    instance method    class method   Now Ruby does not seem to provide the equivalent   Ruby s rules  I think  are that private methods must not be accessed with a receiver   That is   inst pvt method    FAILS pvt method   WORKS only within the class  good    That s OK for private instance methods  but causes problems with private class methods   I would like Ruby to function this way   class C     def instance method         STDOUT  lt  lt   instance method n             Simple access to the private class method would be nice          class method     DOES NOT WORK  RUBY WON T FIND THE METHOD         C class method   DOES NOT WORK  RUBY WON T ALLOW IT            ONLY THIS WORKS  While I am happy such capability exists I think           the way  send  should be used is when the coder knows he she is           doing a no-no   The semantic load on the coder for this is also           remarkably clumsy for an elegant language like ruby          self class send  class method      end      private class method def self class method   STDOUT  lt  lt   class method n   end end   But  alas  the above does not work   Does someone know a better way   When I see  send  prior to a method  it s a clear sign the code is violating the intent of the API s designer  but in this case the design is specifically to have an instance method of the class call the private class method

User · Answer

By default all class methods are public  To make them private you can use Module private class method like  tjwallace wrote or define them differently  as you did   class  lt  lt  self    private    def method name           end end   class  lt  lt  self opens up self s singleton class  so that methods can be redefined for the current self object  This is used to define class module   static   method  Only there  defining private methods really gives you private class methods

[ruby] How to create a private class method?

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