From: "David A. Black" <dblack@...>
Date: 2005-11-07T00:54:33+09:00
Subject: Re: what does --can't define singleton method-- mean?

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Hi --

On Mon, 7 Nov 2005, Daniel Sch=FCle wrote:

> Sean O'Halpin wrote:
>> On 11/6/05, Daniel Sch=FCle <uval@rz.uni-karlsruhe.de> wrote:
>>=20
>>> irb(main):014:0* class << Complex
>>=20
>>=20
>> You're using the wrong syntax here. To re-open a class, just use
>> normal class definition syntax:
>>=20
>> class Complex
>>   def angle
>>     arg/Math::PI*180.0
>>   end
>> end
>
> I found out that I also can use
>
> Complex.class_eval do
> =09def angle
> =09=09arg / Math::PI * 180
> =09end
> end
>
> this would "update" the method for all instances of Complex class

Or:

   def Complex.angle
     ...

> class << Complex
> def angle; "angle"; end
> end
>
> would create singleton_method for Complex class itself
> Complex.angle =3D> "angle"
>
> what is the advantage or typical use of having singleton method for a cla=
ss?

Singleton methods of classes are generally referred to as "class
methods", and they're actually the most frequent use-case for
singleton methods.  You'll find a lot of them in the core.  Here are
some examples:

   Regexp.escape(string)
   File.open(filename)
   Math.sqrt(num)  # Math is actually a Module, not a Class, but same
                   #  idea
   Class.new

etc.


David

--=20
David A. Black
dblack@wobblini.net
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