From: Robert Klemme <shortcutter@...>
Date: 2010-02-13T17:10:45+09:00
Subject: Re: Is there a way to get a method to always run at the end of any   descendent's initialize method?

On 02/12/2010 11:41 PM, Xeno Campanoli wrote:
> Ryan Davis wrote:
>> On Feb 12, 2010, at 13:50 , Xeno Campanoli wrote:
>>
>>> I have an initialize method I want to run at the end of any daughter or granddaughter 'initialize' to make sure the state has been created properly, and I would rather specify the execution from the base class itself than count on those descendents to do it.
>> The most correct way to do it is to have all subclasses properly use super at the end of their initialize bodies:
>>
>> class Subclass < Superclass
>>   def initialize
>>     # ... stuff
>>     super
>>   end
>> end
>>
>> There are "fancier" ways (read: overly clever), but doing it this way is cleaner, faster, and easier to debug / maintain.
>>
>>
>>
> 
> The problem with that is you may want some things from super available before 
> you do other things in initialize.  I figured out another way, which is to 
> define initialize subroutines as pure virtual in the base class, called from 
> initialize, then I put the thing at the end of initialize, and all the daughters 
> only modify the called initializers, and all use the base class initialize 
> method.  There is nothing to force daughters not to make their own initialize, 
> but at least this is nice and formal.

Template method pattern.

Here's another approach, only mildly more sophisticated:

irb(main):001:0> class Base
irb(main):002:1> def self.new(*a, &b)
irb(main):003:2> x = allocate
irb(main):004:2> x.send(:initialize, *a, &b)
irb(main):005:2> x.after_init
irb(main):006:2> x
irb(main):007:2> end
irb(main):008:1> def after_init; puts "hook run for #{self.class}"; end
irb(main):009:1> end
=> nil
irb(main):010:0> class Derived < Base
irb(main):011:1> def initialize; puts "work for #{self.class}"; end
irb(main):012:1> end
=> nil
irb(main):013:0> d = Derived.new
work for Derived
hook run for Derived
=> #<Derived:0x8c6b85c>
irb(main):014:0> class Derived2 < Derived
irb(main):015:1> def initialize;super;puts "in #{self.class}"; end
irb(main):016:1> end
=> nil
irb(main):017:0> e = Derived2.new
work for Derived2
in Derived2
hook run for Derived2
=> #<Derived2:0x8c25ac4>
irb(main):018:0>

The basic trick is to redefine the base class's #new method.

Kind regards

	robert


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