`finalize' method?

[Daniel Schierbeck]

Yeah, it's me again, your friendly neighbourhood power-suggester!

I'm wondering why there isn't a `finalize' method classes can define,
that will be called immediately before instances of that class are
garbage collected? I'm aware of ObjectSpace.define_finalizer, but it
seems un-OO, and it doesn't seem to work when being called from within
the instance methods. It also seems as if the Proc sent to
`define_finalizer' isn't called until *after* the object has been destroyed.

class Klass
def finalize
# finalize something
end
end

A use case (the best I can come up with at the moment):

class Contact
def initialize(filename)
@filename = filename
parse_xml_file(@filename)
end

def finalize
save_as_xml(@filename)
end
end


Cheers,
Daniel

 

[Jim Weirich]

Yeah, it's me again, your friendly neighbourhood power-suggester!

I'm wondering why there isn't a `finalize' method classes can define,
that will be called immediately before instances of that class are
garbage collected? I'm aware of ObjectSpace.define_finalizer, but it
seems un-OO, and it doesn't seem to work when being called from within
the instance methods. It also seems as if the Proc sent to
`define_finalizer' isn't called until *after* the object has been
destroyed.

That's correct, it prevents problems like the following code:

class Klass
def finalize
$global_reference = self
end
end

 

[Daniel Schierbeck]

That's correct, it prevents problems like the following code:

class Klass
def finalize
$global_reference = self
end
end

In that case I'd expect $global_reference to be nil after the `finalize'
method is done executing, since the Klass instance has already been
deemed ready for destruction.


Cheers,
Daniel

 

[Jim Weirich]

In that case I'd expect $global_reference to be nil after the `finalize'
method is done executing, since the Klass instance has already been
deemed ready for destruction.

"after the 'finalize' method is done" => Tricky.

How would this be implemented? Seems to me Ruby would have to scan all
of the current image to determine if any new references were created
during the execution of the finalize method. Especially since the
references may be created deep inside other methods invoked by finalize.

And it might be more subtle than just checking for variable bindings.
Consider:

class NamedObject
attr_reader :name
def initialize(name)
@name = name
end
def finalize
proc { name }
end
end

k = NamedObject.new("Billy")
p = k.finalize # Here p is a proc that has a binding that includes
# a "finalized" object.

The point is that the current mechanism avoids all these questions by
only calling the finalizer /after/ the object has been destroyed.

 

[Daniel Schierbeck]

"after the 'finalize' method is done" => Tricky.

How would this be implemented? Seems to me Ruby would have to scan all
of the current image to determine if any new references were created
during the execution of the finalize method. Especially since the
references may be created deep inside other methods invoked by finalize.

And it might be more subtle than just checking for variable bindings.
Consider:

class NamedObject
attr_reader :name
def initialize(name)
@name = name
end
def finalize
proc { name }
end
end

k = NamedObject.new("Billy")
p = k.finalize # Here p is a proc that has a binding that includes
# a "finalized" object.

The point is that the current mechanism avoids all these questions by
only calling the finalizer /after/ the object has been destroyed.

I just think it's weird that PHP has destructors and Ruby doesn't

http://www.php.net/manual/en/language.oop5.decon.php

I'm not really into C, neither am I familiar with the Ruby
implementation, but doesn't all variables just reference an object? So
somewhere, an object is stored, and each variable referencing it really
only holds that object's id. I imagine the `finalize' method could be
run, and afterwards, the object could be replaced with nil, maybe
retaining the old object id. That way any variables that point to the
object will be nil.

Now, I don't know if that's how it works, but it sounds logical to me,
speaking as a guy who's never taken any programming lessons or
programmed/scripted using other languages than PHP, JavaScript and Ruby...


Cheers,
Daniel

 

[Hugh Sasse]

I just think it's weird that PHP has destructors and Ruby doesn't

http://www.php.net/manual/en/language.oop5.decon.php

I find this tricky too: the only model I can think of is that where
you need destructors, your constructor desstructs, and you get at
the object with a block, in the style of open("file"){|f| ...}; if
you see what I mean.

I'm not really into C, neither am I familiar with the Ruby implementation, but
doesn't all variables just reference an object? So somewhere, an object is
stored, and each variable referencing it really only holds that object's id. I
imagine the `finalize' method could be run, and afterwards, the object could
be replaced with nil, maybe retaining the old object id. That way any
variables that point to the object will be nil.

But there is only one nil in the system, so it can't have many ids.

 

[Daniel Schierbeck]

I find this tricky too: the only model I can think of is that where
you need destructors, your constructor desstructs, and you get at
the object with a block, in the style of open("file"){|f| ...}; if
you see what I mean.

No, not really. I really can't stretch enough that I'm a noob...

But there is only one nil in the system, so it can't have many ids.

Could the object then by replaced by a reference to the nil object?
Again, I'm talking theoretically...


Cheers,
Daniel

 

[Logan Capaldo]

------=_Part_23819_162370.1132935286481
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline

No, not really. I really can't stretch enough that I'm a noob...

t

Could the object then by replaced by a reference to the nil object?
Again, I'm talking theoretically...


Cheers,
Daniel

Matz has also said that define_finalizer was in ObjectSpace on purpose to
discourage the use of finalizers. Finalizers are kind of weird, I would
recommend adding a clean-up method to any object you think "needs" a
finalizer. At least then the object still exists as in in a known state. Tr=
y
to not to think of them as destructors (in C++ sense) because they aren't.

------=_Part_23819_162370.1132935286481--

 

[Hugh Sasse]

No, not really. I really can't stretch enough that I'm a noob...

The braces, curly brackets, are a block. The CS people will tell
you it is a closure. What it means is: In the current scope (local
variables defined, etc) execute open("file") to open the file for
reading, and then [when inside open it effectively calls yield] you
will have a parameter which we'll call f, and we'll do something
with it in the local scope until the closing brace of the block. On
the closing brace, open will close the file for us. The oft' used
each method behaves in a similar way.

For an intro:
http://www.rubycentral.com/book/tut_containers.html#S2

For info about using them with constructors:
http://www.rubycentral.com/articles/insteval.html

[...]

Could the object then by replaced by a reference to the nil object? Again, I'm
talking theoretically...

Ruby doesn't have references, otherwise it could.

Cheers,
Daniel

Hugh

 

[Daniel Schierbeck]

No, not really. I really can't stretch enough that I'm a noob...

The braces, curly brackets, are a block. The CS people will tell
you it is a closure. What it means is: In the current scope (local
variables defined, etc) execute open("file") to open the file for
reading, and then [when inside open it effectively calls yield] you
will have a parameter which we'll call f, and we'll do something
with it in the local scope until the closing brace of the block. On
the closing brace, open will close the file for us. The oft' used
each method behaves in a similar way.

Oh, sorry, I misunderstood you: You mean something like this:

class Klass
def initialize
yield self
finalize
end

def finalize
# do something
end
end

Sorry, I'm kinda groggy


Cheers,
Daniel

 

[Hugh Sasse]

Oh, sorry, I misunderstood you: You mean something like this:

class Klass
def initialize # setup usual stuff, then:
yield self
finalize
end

def finalize
# do something

# like releasing resources, yes.

end
end

Sorry, I'm kinda groggy

Not that groggy, that's exactly what I meant.

 

[Jim Weirich]

What's the problem if this method ran on the first GC once and then
the object just stayed around? I.e. have a three-pass GC, mark, run,
sweep?

There are two problems:

(1) Detecting that the finalizer has created new reference to a object
scheduled for collection is problematic. I'm thinking this would
require a new mark pass to detect.

(2) What is the state of an object that has been finalized, but sticks
around afterwards? Is it valid to call methods on it? After all,
supposedly all of its resources have been finalized at this point. And
when it becomes eligible for collection again, does the finalizer need
to run again or not?

Most languages that allow finalizers explicitly state that creating new
references duing finalization is not allowed and depend upon the
programmer to follow that rule. Ruby handles it by making it impossible
for the programmer to break that rule (i.e. the finalizer doesn't get a
reference to the object, making it difficult to create a reference to
it).

-- Jim Weirich

 

[Matthias Georgi]

That's correct, it prevents problems like the following code:

In that case I'd expect $global_reference to be nil after the `finalize'
method is done executing, since the Klass instance has already been
deemed ready for destruction.

This would force the interpreter to keep a back reference for all
objects to variables which keep references to them, not very efficient
in regard of cpu and memory. Java solves this problem by reviving those
objects, which seems at least for me very odd.

So generally you should avoid finalizers, because most of the time you
are only interested in running some code before the application exits.

This should help in most cases:

BEGIN { $exit_listeners = [] }

def add_exit_listener(object)
$exit_listeners << object
end

END { $exit_listeners.each {|listener| listener.on_exit } }

Alternatively you may declare an END block inside your initialize
method.

 

[Daniel Schierbeck]

# like releasing resources, yes.

Not that groggy, that's exactly what I meant.
Hugh

What would you call such an approach? Encapsulating? I use this often
enough that I think it would fit lovely in a module:

module Encapsulatable
def self.included(klass)
super
klass.module_eval do
def self.new(*args)
yield obj = __new__(*args)
obj.finalize if obj.respond_to? :finalize
end
end
end
end


Cheers,
Daniel

 

[ts]

D> What would you call such an approach? Encapsulating? I use this often
D> enough that I think it would fit lovely in a module:

A bad approach, at least for me.

D> def self.new(*args)
D> yield obj = __new__(*args)
D> obj.finalize if obj.respond_to? :finalize
D> end

There is a convention in ruby : the method ::new must return an object of
the class. You must change the name of the method if its return nil, or
another object.


Guy Decoux

 

[Daniel Schierbeck]

D> What would you call such an approach? Encapsulating? I use this often
D> enough that I think it would fit lovely in a module:

A bad approach, at least for me.

D> def self.new(*args)
D> yield obj = __new__(*args)
D> obj.finalize if obj.respond_to? :finalize
D> end

There is a convention in ruby : the method ::new must return an object of
the class. You must change the name of the method if its return nil, or
another object.

Roger. That's not that hard to do:

module Encapsulatable
def self.included(klass)
super
klass.metaclass_eval do
private :new

def encapsulate(*args)
yield obj = new(*args)
obj.finalize if obj.respond_to? :finalize
end
end
end
end

Note that this doesn't prevent the user from calling methods on the
object after its `finalize' method has been called.

class Klass
include Encapsulatable

def initialize
puts "initializing..."
end

def foo
puts "called method `foo'"
end

def finalize
puts "finalizing..."
end
end

ref = nil
Klass.encapsulate { |obj| obj.foo; ref = obj }
ref.foo

But then again, this ain't Java. If you want to, you could wreak havoc
in seconds. But we're all friends, right?


Cheers,
Daniel

 

[Peter C. Verhage]

Hi,

How about making self a WeakRef in the finalize method?
http://www.ruby-doc.org/stdlib/libdoc/weakref/rdoc/

Regards,

Peter

 

[Eric Mahurin]

Hi,

How about making self a WeakRef in the finalize method?
http://www.ruby-doc.org/stdlib/libdoc/weakref/rdoc/

Or just say in the docs that any references to self will become
invalid after finalize is called. I don't think doing this is any
less safe than using #object_id and ObjectSapce::_id2ref (same type of
error if you use this on an object_id already GCed). Right now if you
are using ObjectSapce::define_finalizer, you are likely using
ObjectSpace::_id2ref somewhere (you probably have some data-structure
holding object_id's).

Maybe this method could be called #_finalize to say that it isn't safe
(like ObjectSpace::_id2ref).