From: ts <decoux@...>
Date: 2001-09-16T01:18:38+09:00
Subject: [ruby-talk:21214] Re: using my own objects as hash keys - a question

>>>>> "Y" == Yuri Leikind <YuriLeikind@scnsoft.com> writes:

Y> Have I missed something ?

 Yes, when you want to re-define #hash you must also redefine #eql?

 You have at http://www.ruby-lang.org/en/man-1.4/Object.html#hash


    Returns the integer hash value of the object. It is used in the Hash
    class to calculate the holding place of an object. The hash value of
    two objects must be equal, where these two objects are equal by using
    eql? operator. So, whenever you redefine the definition of the eql?
    operator in any class, don't forget to redefine hash method according
    to the definition.  

 For example :


pigeon% cat b.rb
#!/usr/bin/ruby
class MyObject
   attr_reader :someAttr
   def initialize (someAttr)
      @someAttr=someAttr
   end

   def eql? other
      other.kind_of?(self.class) && someAttr == other.someAttr
   end

   def hash
      @someAttr.hash
   end
end

key1=MyObject.new('qwerty')
key2=MyObject.new('qwerty')

puts key1.hash == key2.hash #=> true  

hash=Hash.new

hash[key1]=6

puts hash[key1]  #=> 6

puts hash[key2]  #=> nil  

puts hash.has_key?(key1) #=> true
puts hash.has_key?(key2) #=> false
pigeon% 

pigeon% b.rb
true
6
6
true
true
pigeon% 


-- 

Guy Decoux