From: mrilu <mrilu@...>
Date: 2000-03-30T01:39:38+02:00
Subject: [ruby-talk:02254] Hashing

Hey, 

Is there some reason ostruct.rb starts like this:

class OpenStruct
  def initialize(hash=nil)
    @table = {}
    if hash
      for k,v in hash
        @table[k] = v
      end
    end
  end

I think we're trying to make copy of hash 'hash' to @table. Why don't we
say: 
     @table = hash.dup
or
     @table = hash.clone
or
     @table = Hash.new.replace hash

I guess it has something to do with semantics of the copying. It matters
whether 
  hash["a"] = "b"
OpenStruct.new(hash)'s @table["key"] points to the same "b" or to other
string with contents of "b" (in next example, whether openstruct.@table
has something like c or like d). 

  a = "a"
  b = "b";
  p a.id  # => 123
  p b.id  # => 456
  c = a
  d = a.dup
  p c.id  # => 123
  p d.id  # => 789

Or is it about key too, so that does it point to same "key" or to
"key".dup ?

Well, I think copying like in OpenStruct is so common that Hash should
have builtin method for it.

Now some other points of Hash. First about documentation. This excerpt is
from my html-version of 1.4 documents.

dup
               Returns a newly created hash table which has the save keys
               and values to the receiver. clone returns the complete
               copy of the original hash table, including freeze status
               and instance variables. On the other hand, dup copies the
               hash table containts.

Note: I didn't see any other docs for Hash.clone.
I try to fix few things here, but English is not my mother tongue so this
just can't be right:

dup
	Returns a newly created hash table which has same keys and values
	as the original hash instance. dup differs from clone by copying
	only hash contents (that is key,value-pairs).
	Example: 
	  foo = {"key"=>"value", "baz"=>"zak"}
	  bar = foo.dup
	  print bar[baz]   # => "zak"

clone
	Returns a newly created hash table which is a complete copy of the
	original hash instance. clone differs from dup by copying original
	hash table completely including key,value-pairs, freeze status and
	instance variables.
	Example:
	 ...

I'd like to see good examples for different copying strategies too
(shallow, deep).

One more point (raised in checking experimentation :):

  key = "key"
  value = "value"
  a = { key => value }
  b = a.invert
  c = b.invert

  print key.id       # 537639674
  print value.id     # 537639554

  print a[key].id    # 537639554, it's value
  print b[value].id  # 537639704, it's not key
  print c[key].id    # 537639584, well, it's not value anymore

It seems that invert makes clone (or duplicate) of it's keys and values.
If this is important at all it should make it's way into documentation!

Enough questions for one mail, isn't it?