From: Lars Christensen <larsch@...>
Date: 2008-05-06T00:40:04+09:00
Subject: String copy-on-write question

Hello group,

Ruby implements copy-on-write for strings, so you can do stuff like
this very cheaply:

   str = 0.chr * (2**24)  # 16MiB allocated
   str[100..-1]  # this costs only a small amount of memory

How come this optimization does not apply in this case?:

  str[100..-2]   # this costs around 16MiB bytes of memory

As a side effect, if using regexps on a large string, the pre-match
and post-match variables behave differently:

  s = 0.chr * (2**23) + "Hello" + 0.chr * (2**23)   # About 16MiB
allocated (after GC)
  s.scan(/Hello/) { |m| p m }   # This is free
  p $'.size  # This is free
  p $`.size  # This costs another 8MiB.

Any insights?

Lars