Rails - Oct 2006 - Ruby for Rails book question

The book says that it is possible to define a method twice and the last
definition over-rides
previous definitions.

But it does not explain why we might need to do this. Any ideas on scenario
where this might be
helpful? TIA.

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Bala Paranj wrote:
> The book says that it is possible to define a method twice and the last
definition over-rides
> previous definitions.
>
> But it does not explain why we might need to do this. Any ideas on scenario
where this might be
> helpful? TIA.
>
> >
>
>   
Loads of occasions. I''ve just used it to give a method to a parent STI 
class, which is therefore the default method for all children. However, 
one of the child classes redefines that method because it needs to 
behave in a different way just for that child class. Once you get used 
to it, it''s tremendously powerful.

HTH
CT

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I understand about the inheritance. Here is the section that is confusing:

Redefining methods 
Nothing stops you from defining a method twice, or overriding it: 
class C 
  def m 
    puts "First definition of method m" 
  end 
  def m 
    puts "Second definition of method m" 
  end 
end 
What happens when we call m on an instance of C? Let’s find out: 
C.new.m 
The printed result is Second definition of method m. The second definition has 
prevailed: We see the output from that definition, not from the first. When you 
override a method, the new version takes precedence. 

 Is there any real-life scenario for its use?

--- Chris T <ctmailinglists-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>
wrote:
> >   
> Loads of occasions. I''ve just used it to give a method to a parent
STI
> class, which is therefore the default method for all children. However, 
> one of the child classes redefines that method because it needs to 
> behave in a different way just for that child class. Once you get used 
> to it, it''s tremendously powerful.
> 
> HTH
> CT
> 
> > 
> 

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Hi --

On Sun, 1 Oct 2006, Bala Paranj wrote:
> --- Chris T <ctmailinglists-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>
wrote:
>>>
>> Loads of occasions. I''ve just used it to give a method to a
parent STI
>> class, which is therefore the default method for all children. However,
>> one of the child classes redefines that method because it needs to
>> behave in a different way just for that child class. Once you get used
>> to it, it''s tremendously powerful.
>>
>> HTH
>> CT
>
> I understand about the inheritance. Here is the section that is confusing:
>
> Redefining methods
> Nothing stops you from defining a method twice, or overriding it:
> class C
>  def m
>    puts "First definition of method m"
>  end
>  def m
>    puts "Second definition of method m"
>  end
> end
> What happens when we call m on an instance of C? Lets find out:
> C.new.m
> The printed result is Second definition of method m. The second definition
has
> prevailed: We see the output from that definition, not from the first. When
you
> override a method, the new version takes precedence.
>
> Is there any real-life scenario for its use?
Probably not.  The example was mainly just to isolate and explain that
behavior.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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You wouldn''t literally define a method twice in a row like that... but
there are scenarios -- other than inheritance -- where you might
redefine a method.

Like, say you''re using a plugin such as LoginEngine.  Maybe the
controller methods provided don''t do quite what you want.  You can
define your own UserController which has the same methods as
LoginEngine''s UserController.  You''re not subclassing, and
you''re not
replacing the whole UserController class -- just the methods with the
same names will be overwritten.

Make sense?


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Do you mean you would open the class and define the same method with custom
implementation? That
is not the same as defining within the same class. If you could modify that
class, why not just
delete the old method implementation? It''s not being used so we keep
the junk code around?

--- "brian.estlin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org"
<brian.estlin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> Like, say you''re using a plugin such as LoginEngine.  Maybe the
> controller methods provided don''t do quite what you want.  You can
> define your own UserController which has the same methods as
> LoginEngine''s UserController.  You''re not subclassing,
and you''re not
> replacing the whole UserController class -- just the methods with the
> same names will be overwritten.
> 
> Make sense?

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