From: Robert Klemme <bob.news@...>
Date: 2004-12-09T21:27:28+09:00
Subject: Re: Inheritance, mixins and initializers


"Johan Nilsson" <johan.nilsson@---.esrange.ssc.se> schrieb im Newsbeitrag
news:1102594375.344eddcc1c357ac539046c764cd8c6b8@teranews...
> Hi,
>
> I've got a question regarding inheritance, mixins, order of
initialization
> and calling module/super class "initializers". I'd like to do something
> along the lines of (pseudo-ruby code):
>
> ---
> class Base
>     def initialize(field_count)
>         @fields = Array.new(field_count)
>     end
> end
>
> module FooMixin
>     def initialize(foo_field)
>         @foo_field = foo_field
>         self.fields[@foo_field] = Foo.new
>     end
>
>     def foo
>         self.fields[@foo_field]
>     end
> end
>
> module BarMixin
>     def initialize(bar_field)
>         @bar_field = bar_field
>         self.fields[@bar_field] = Bar.new
>     end
>
>     def bar
>         self.fields[@bar_field]
>     end
> end
>
> class Derived < Base
>     include FooMixin
>     include BarMixin
>     def initialize
>         super(2)
>         FooMixin(0)
>         BarMixin(1)
>     end
> end
>
> ---
>
> Obviously this doesn't work - is there any way of achieving something
> similar? The Foo/Bar mixins depend on Base being initialized before them
> (ok, maybe not in this particular example).

I don't know which real problem you are trying to solve.  I can think of
several other approaches.  You might want to store your mixin created
instances directly in instance variables and record only the names of
these.

class Foo;end
class Bar;end

class Base
    attr_reader :fields
    def initialize()
        @fields = []
    end

    def add_field(name) @fields << name end
    def all_fields() @fields.map {|f| instance_variable_get f} end
end

module FooMixin
    attr_reader :foo

    def initialize(*a,&b)
        super
        @foo = Foo.new
        add_field "@foo"
    end
end

module BarMixin
    attr_reader :bar

    def initialize(*a,&b)
        super
        @bar = Bar.new
        add_field "@bar"
    end
end

class Derived < Base
    include FooMixin
    include BarMixin
    def initialize
      super
    end
end


Or you can determine the position automatically.

class Foo;end
class Bar;end

class Base
    attr_reader :fields
    def initialize()
        @fields = []
    end
end

module FooMixin
    def initialize(*a,&b)
        super
        @foo_field = self.fields.size
        self.fields << Foo.new
    end

    def foo
        self.fields[@foo_field]
    end
end

module BarMixin
    def initialize(*a,&b)
        super
        @bar_field = self.fields.size
        self.fields << Bar.new
    end

    def bar
        self.fields[@bar_field]
    end
end

class Derived < Base
    include FooMixin
    include BarMixin
    def initialize
      super
    end
end

Kind regards

    robert