From: Lyle Johnson <lyle@...>
Date: 2002-11-28T00:32:39+09:00
Subject: Re: explicit destroy

Ludo wrote:

> Is there a way to explicitly destroy an object [in] Ruby?

No, you need to get rid of all references to that object and then let 
the garbage collector destroy.

> Am I fool to want to do this ?? ;)

I wouldn't use those words ;) but generally speaking it's dangerous to 
destroy an object that other objects still hold references to. In C++ 
and other languages that use more explicit memory management these are 
referred to as "dangling" pointers, and they're the source of a lot of 
debugging headaches.

> I'm working on agents programation, and I try to use Ruby for this,
> but I'd like to have the possibility to kill an agent,
> so other agents should have to perceive the death ... but if they have a 
> reference on the killed agent, the object is still in ObjectSpace ...

OK. One solution that immediately comes to mind is some application of 
the Observer pattern. When an agent is "killed", part of its dying 
process is to send a message out to any other interested agents:

     class Agent
       def die
         @friends.each do |friend|
           friend.notifyOfDeath(self)
         end
       end
     end

For this to work, we assume that the other agents (the "friends") have 
previously registered themselves with this agent, e.g.

     class Agent
       def befriend(anotherAgent)
         unless @friends.contains? anotherAgent
           @friends << anotherAgent
         end
       end
     end

You'll also need to decide what should happen when an agent gets word of 
another agent's death, e.g.

     class Agent
       def notifyOfDeath(anAgent)
         # The agent "anAgent" is on his deathbed.
         # If I'm still holding any references to this agent
         # I need to get rid of them now!
       end
     end

And so a short example of its use would go like this:

     # Create some agents
     bond = Agent.new
     jinx = Agent.new

     # Jinx wants to know when Bond dies
     bond.befriend(jinx)

     # Kill Bond; this should trigger a call to
     # Jinx's notifyOfDeath() method.
     bond.die

Hope this helps,

Lyle