ruby cookbook -- Hash

Hash
   1. Except for strings and other built-in objects, most objects have a hash code equivalent to their internal object ID. As seen above, you can override Object#hash to change this, but the only time you should need to do this is if your class also overrides Object#==. If two objects are considered equal, they should also have the same hash code; otherwise, they will behave strangely when you put them into hashes. Code like the fragment below is a very good idea:

       class StringHolder  
          attr_reader :string  
          def initialize(s)  
            @string = s  
          end  
      
          def ==(other)  
           @string == other.string  
         end  
     
         def hash  
           @string.hash  
         end  
       end  
       a = StringHolder.new("The same string.")  
       b = StringHolder.new("The same string.")  
       a == b                                                    # => true  
       a.hash                                                    # => -1007666862  
       b.hash                                                    # => -1007666862  

  
Except for strings and other built-in objects, most objects have a hash code equivalent to their internal object ID. As seen above, you can override Object#hash to change this, but the only time you should need to do this is if your class also overrides Object#==. If two objects are considered equal, they should also have the same hash code; otherwise, they will behave strangely when you put them into hashes. Code like the fragment below is a very good idea:

	class StringHolder
	  attr_reader :string
	  def initialize(s)
	    @string = s
	  end

	  def ==(other)
	    @string == other.string
	  end

	  def hash
	    @string.hash
	  end
	end
	a = StringHolder.new("The same string.")
	b = StringHolder.new("The same string.")
	a == b                                                    # => true
	a.hash                                                    # => -1007666862
	b.hash                                                    # => -1007666862

Using Symbols as Hash Keys
While 'name' and 'name' appear exactly identical, they're actually different. Each time you create a quoted string in Ruby, you create a unique object. You can see this by looking at the object_id method.

'name'.object_id                                        # => -605973716
'name'.object_id                                        # => -605976356
'name'.object_id                                        # => -605978996

By comparison, each instance of a symbol refers to a single object.

:name.object_id                                         # => 878862
:name.object_id                                         # => 878862
'name'.intern.object_id                                 # => 878862
'name'.intern.object_id                                 # => 878862

Using symbols instead of strings saves memory and time. It saves memory because there's only one symbol instance, instead of many string instances. If you have many hashes that contain the same keys, the memory savings adds up.

Using symbols as hash keys is faster because the hash value of a symbol is simply its object ID. If you use strings in a hash, Ruby must calculate the hash value of a string each time it's used as a hash key.