Is it safe to override #hash and #eql? to enable hash key equality?

ruby-talk
1

Hi,

I would like to use object instances as hash keys where two objects
containing the same datum both have the same key value. This does not
work by default (as expected given that these are two different
instances):

class Foo
  def initialize(datum)
    @datum = datum
  end
end

# Two hash keys for the same datum
a = {Foo.new(:baz) => 1, Foo.new(:baz) => 2}
p a

# => {#<Foo:0x2a95e68888 @datum=:baz>=>1, #<Foo:0x2a95e68860
@datum=:baz>=>2}

However, this functionality can be obtained by overriding Object#hash
and Object#eql?:

class Bar

  attr_reader :datum

  def initialize(datum)
    @datum = datum
  end

  def hash
    @datum.hash
  end

  def eql?(other)
    @datum.eql? other.datum
  end

end

# One hash key for the same datum
b = {Bar.new(:baz) => 1, Bar.new(:baz) => 2}
p b

#=> {#<Bar:0x2a95e684f0 @datum=:baz>=>2}

My simple question is if this is safe/normal/OK for my intentions.
Based on perusal of the online Ruby documentation it looks like #eql?
has been overridden in a number of sub-classes for the same purpose.
But I wanted to get feedback from the community - has anyone here
tried this? Are there any known side-effects?

Thanks,
-James

2

Hi,

I would like to use object instances as hash keys where two objects
containing the same datum both have the same key value. This does not
work by default (as expected given that these are two different
instances):

class Foo
  def initialize(datum)
    @datum = datum
  end
end

# Two hash keys for the same datum
a = {Foo.new(:baz) => 1, Foo.new(:baz) => 2}
p a

# => {#<Foo:0x2a95e68888 @datum=:baz>=>1, #<Foo:0x2a95e68860
@datum=:baz>=>2}

However, this functionality can be obtained by overriding Object#hash
and Object#eql?:

class Bar

  attr_reader :datum

  def initialize(datum)
    @datum = datum
  end

  def hash
    @datum.hash
  end

  def eql?(other)
    @datum.eql? other.datum
  end

end

# One hash key for the same datum
b = {Bar.new(:baz) => 1, Bar.new(:baz) => 2}
p b

#=> {#<Bar:0x2a95e684f0 @datum=:baz>=>2}

My simple question is if this is safe/normal/OK for my intentions.

Not only that: this is exactly how one should do it.

Based on perusal of the online Ruby documentation it looks like #eql?
has been overridden in a number of sub-classes for the same purpose.
But I wanted to get feedback from the community - has anyone here
tried this? Are there any known side-effects?

No, your use is absolutely correct. This is what this methods are meant for. The only thing to remark is this: you can save yourself a lot typing by doing:

Bar = Struct.new :datum

If you need additional methods in that class you can even do

Bar = Struct.new :datum do
   def another_method
      123
   end
end

Kind regards

  robert

···

On 18.03.2009 18:59, James wrote:

3

Excellent - thanks for confirming!

···

On Mar 18, 11:04 am, Robert Klemme <shortcut...@googlemail.com> wrote:

Not only that: this is exactly how one should do it.