From: Piotr Szotkowski <chastell@...>
Date: 2011-06-09T19:45:42+09:00
Subject: Re: Where is the #hash method defined?

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I=C3=B1aki Baz Castillo:

> Hi, any object in Ruby has the method #hash which
> returns an integer unique for the object (such value
> doesn't change during the lifetime of the Ruby process).

Note that #hash=E2=80=99s role is to fulfil the implication
a.eql?(b) =E2=86=92 a.hash =3D=3D b.hash, but it=E2=80=99s as weak as possi=
ble
(i.e., it would be perfectly legal, if very inefficient,
for #hash to return the same result for all objects).

In general, the idea is that it=E2=80=99s often possible to have a #hash
method that computes a unique-ish result much faster than performing
a full-blown #eql? check; in these cases it=E2=80=99s worthwhile to compare
#hash results of two objects and call #eql? only if they=E2=80=99re the
same (equal #hash results do not mean the objects are #eql?, but
different #hash results do mean that they are not #eql?).

By default, Kernel#hash uses #object_id to compute the hash,
so it=E2=80=99s quite unique (I=E2=80=99m not sure what happens when #objec=
t_id
rolls over the Fixnum limit; #hash seems to be returning
a Fixnum anyway, so in theory it can be non-unique).

> I cannot find which class or module #hash method belongs to.
> I expected it could be in Object class, but it's not. Where is it?

It=E2=80=99s defined as Kernel#hash and in Ruby 1.9.2 implemented here:
https://github.com/ruby/ruby/blob/ruby_1_9_2/object.c#L2516
=E2=86=92 https://github.com/ruby/ruby/blob/ruby_1_9_2/object.c#L99

=E2=80=94 Piotr Szotkowski
--=20
Log rotation code. It=E2=80=99s right behind cryptography in the
list of software you should never, ever write yourself.
                                           [Jeff Hodges]




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