From: Robert Klemme <bob.news@...>
Date: 2004-08-22T19:40:45+09:00
Subject: Re: Callback for instance var creation?


"T. Onoma" <transami@runbox.com> schrieb im Newsbeitrag
news:200408212304.28814.transami@runbox.com...
> Is there a callback for when an instance variable is first used? I know
they
> are "instantiated" upon use and set to nil.

No, they are instantiated on first assignment:

>> class Foo;end
=> nil
>> f=Foo.new
=> #<Foo:0x101858b8>
>> f.instance_eval { puts @bar }
nil
=> nil
>> f
=> #<Foo:0x101858b8>
>> f.instance_variables
=> []
>> f.instance_eval { @bar = "yes" }
=> "yes"
>> f
=> #<Foo:0x101858b8 @bar="yes">
>> f.instance_variables
=> ["@bar"]

> I would like to set this to
> something else dependent on the name of the variable.

Which problem are you trying to solve?  Usually attribute accessors can do
what you want.

class Foo
  attr_accessor :bar
  def bar=(b)
    @bar = "foo #{b}"
  end
end

>> f=Foo.new
=> #<Foo:0x1019f940>
>> f.bar = "bar"
=> "bar"
>> f.bar
=> "foo bar"
>> f
=> #<Foo:0x1019f940 @bar="foo bar">
>>

Other than that if you need to intercept all variable settings a modified
Hash might be better for managing your instance vars:

class Foo
  class FooHash < Hash
    def []=(k,v)
      case k
        when :foo
          super(k, "bar #{v}")
        else
          super
      end
    end
  end

  def initialize
    @vars = FooHash.new
  end

  def method_missing(sym, *args, &b)
    case sym.to_s
      when /^(\w+)=$/
        @vars[$1.intern] = args[0]
      when /^(\w+)$/
        @vars[sym]
      else
        super
    end
  end
end

>> f=Foo.new
=> #<Foo:0x1018e690 @vars={}>
>> f.foo = 10
=> 10
>> f.bar = 20
=> 20
>> f
=> #<Foo:0x1018e690 @vars={:bar=>20, :foo=>"bar 10"}>
>>

This of course is just an example.  It depends on the real problem at hand
that you are trying to solve.

Regards

    robert