From: sheepman <sheepman@...>
Date: 2002-01-31T20:41:03+09:00
Subject: Re: Policy Classes - Challenge

 Hello.

On Thu, 31 Jan 2002 11:38:41 +0900
John Carter <john.carter@tait.co.nz> wrote:

> Alexandrescu's book Modern C++ Design
>   http://cseng.aw.com/book/0,3828,0201704315,00.html
> describes (See the "Sample Chapter" link) the concept of Policy Classes.
> 
> I see this, like the laser was, a "Solution looking for a problem".
> 
> Basically it is the idea that the client of a library should...
>   * Not be forced to inherit from library objects.
>   * Should be able to plug in any suitable class provided by the
>     client or the library as the base class of a library class.
>   * Should be able to plug in several orthoganal policy classes as
>     multiply inherited base classes.
> 
> He does all this magic by doing arcane things with C++ templates. (Michael
> Feathers mostly wondrously called the book "Porn for Programmers".)
>

I'm reading the book in translation. 
His C++ template magic is great and a little complicated.

> Question. Can you think of a neat and compelling use for this concept, I'm
> prepare to write the code as a proof of concept.

How about this?
It uses [] like template in C++

class PolicyPattern  
  def self.[](*mod)
    @klass_with_policy ||= {}
    unless @klass_with_policy[mod]
      klass = self.dup
      make_klass_include_modules(klass, mod)
      @klass_with_policy[mod] = klass
    end
    return @klass_with_policy[mod]
  end

  def self.make_klass_include_modules(klass, mods)
    class << klass
      def include__(m)
	include(m)
      end
    end
    mods.each{|mod|
	klass.include__(mod)
    }
    class << klass
      remove_method :include__
    end
  end
  private_class_method :make_klass_include_modules
end

class HostClass < PolicyPattern
end

module PolicyHoge
  def some_method
    puts "hoge"
  end
end

module PolicyBar
  def some_method
    puts "bar"
  end
end

HostClass[PolicyHoge].new.some_method #=> hoge
HostClass[PolicyBar].new.some_method  #=> bar

-- 
sheepman / TAMURA Takashi
sheepman@tcn.zaq.ne.jp http://www.tcn.zaq.ne.jp/sheepman/
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