From: "Eric I." <rubytraining@...>
Date: 2007-12-11T01:15:00+09:00
Subject: Re: Zero-copy String Handling

On Dec 10, 10:52 am, Curt Sampson <c...@cynic.net> wrote:
> It appears to me that this means I don't really have any need to
> copy the data; I ought to just be able to set up a bunch of (likely
> frozen) String objects and then tweak the ptr and len on them and pass
> them around, avoiding any allocations or data copies. From a bit of
> experimentation, I can see that dropping several calls to rb_str_new for
> each row results in an enormous speed increase--about ten-fold--in how
> fast I can scan through the file.
>
> Does anybody have any suggestions on a reasonably safe way to do this?

I've read (although I've never verified this by looking at the source
code) that Ruby's strings are copy-on-write.  So if you have code like
this:

    s1 = "This is some string."
    s2 = s1[5, 7]   # => "is some"
    s3 = s1[5..11]  # => "is some"

no copies are made and data is shared ... until you modify the data,
in which case the copy is made.

If you wanted to enforce the read-only aspect, perhaps you could
create a subclass of String where all the methods and operators that
modify the character data are re-implemented to raise exceptions.

Eric

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