NAME
Object::Destroyer - Make objects with circular references DESTROY normally
SYNOPSIS
use Object::Destroyer;
{
# Use a standalone destroyer to destroy something when it falls out of scope
my $Tree = Big::Crustry::Tree->parse('somefile.txt');
my $Cleaner = Object::Destroyer->new( $Tree );
}
{
# Or we can use the destroyer as a near transparent wrapper
# that will pass on method calls normally.
my $Mess = Big::Custy::Mess->new;
print $Mess->hello;
}
package Big::Crusty::Mess;
sub new {
my $self = bless {}, shift;
$self->populate_with_stuff;
return Object::Destroyer->new( $self );
}
sub hello { "Hello World!" }
sub DESTROY {
foreach my $child ( values %$self ) {
$child->DESTROY;
}
%$self = ();
}DESCRIPTION
One of the biggest problem with working with large, nested object trees is implementing a way for a child node to see it's parent. The easiest way to do this is to add a reference to the child back to it's parent.
This results in a "circular" reference, where A refers to B refers to A. Unfortunately, the garbage collector perl uses during runtime is not capable of knowing whether or not something ELSE is refering to these circular references.
In practical terms, this means that object trees in lexically scoped variable ( e.g. my $Object = Tree->new ) will not be cleaned up when they fall out of scope, like normal variables. This results in a memory leak for the life of the process, which is a bad thing when using mod_perl or other processes that live for a long time.
Object::Destroyer allows for the creation of "Destroy" handles. The handle is "attached" to the circular relationship, but is not a part of it. When the destroy handle falls out of scope, it will be cleaned up correctly, and while being cleaned up, it will also force the object it is attached to to be destroyed as well.
Use as a Standalone Handle
The simplest way to use the class is to create a standalone destroyer, preferably in the same lexical content. ( i.e. Immediately after creating the parent object )
sub plagiarise {
# Parse in a large nested document
my $filename = shift;
my $Document = My::XML::Tree->open( $filename );
# Create the Object::Destroyer to clean it up as needed
my $Cleaner = Object::Destroyer->new( $Document );
# Continue with the Document as normal
if ( $Document->author == $me ) {
# Normally this would have leaked the document
return new Error("You already own the Document");
}
$Document->change_author( $me );
$Document->save;
# We don't have to $Document->DESTROY here
return 1;
}When the Cleaner falls out of scope at the end of the sub, it will force the cirularly linked $Document to be cleaned up at the same time, rather than being forced to manually call $Document-DESTROY> at each and every location that the sub could possible return.
Using the Object::Destroy object to force garbage collection to work properly allows you to neatly sidestep the inadequecies of the perl garbage collector and work the way you normally would, even with big objects.
Use as a Transparent Wrapper
For situations where a class is always going to produce circular references, you may wish to build this improved clean up directly into the class itself, and with a few exceptions everything will just work the same.
Take the following example class
package My::Tree;
use strict;
use Object::Destroyer;
sub new {
my $self = bless {}, shift;
###
### Initialise with big nasty circular references
###
# Return the Object::Destroyer, with ourself inside it
my $Wrapper = Object::Destroyer->new( $self );
return $Wrapper;
}
sub param {
my $self = shift;
return $self->{CGI}->param(@_);
}
... code ...
sub DESTROY {
my $self = shift;
foreach ( values %$self ) {
$_->DESTROY if ref $_ eq 'My::Tree::Node';
}
%$self = ();
}We might use the class in something like this
sub process_file {
# Create a new tree
my $Tree = My::Tree->new( source => shift );
# Process the Tree
if ( $Tree->comments ) {
$Tree->remove_comments or return;
} else {
return 1; # Nothing to do
}
my $filename = $Tree->param('target') or return;
$Tree->write( $filename ) or return;
return 1;
}We were able to work with the data, and at no point did we know that we were working with a Object::Destroyer object, rather than the My::Tree object itself.
Encased Objects Must Have a DESTROY Method
At this time, the DESTROY method of the underlying object is the ONLY way that the it can be destroyed. In order to use Object::Destroyer, the object to be destroyed must have a DESTROY method. This is checked each time a destroyer is created and an error will be thrown if it does not have one.
Resource Usage
To implement the transparency, there is a slight CPU penalty when a method is called on the wrapper to allow it to pass the method through to the encased object correctly, and without appearing in the caller() information. Once the method is called on the underlying object, you can make further method calls with no penalty and access the internals of the object normally.
Problems with Wrappers and ref or UNIVERSAL::isa
Although it may ACT exactly like what's inside it, is isn't really it. Calling ref $Wrapper or blessed $Wrapper will return 'Object::Destroyer', and not the class of the object inside it.
Likewise, calling UNIVERSAL::isa( $Wrapper, 'My::Tree' ) or UNIVERSAL::can( $Wrapper, 'param' ) directly as functions will also not work. The two alternatives to this are to either use $Wrapper->isa or $Wrapper->can, which will be caught and treated normally, or simple don't use a wrapper and just use the standalone cleaners.
METHODS
new $Object
The new constructor takes as argument a single blessed object, and returns a new Object::Destroyer object, linked to it. The method will die if passed nothing or anything other than an object. The method will also die if the object passed to it does not have a DESTROY function.
DESTROY
If you wish, you may explicitly DESTROY the Destroyer at any time you wish. This will also DESTROY the encased object at the same. This can allow for legacy cases relating to Wrappers, where a user expects to have to manually DESTROY an object even though it is not needed. The DESTROY call will be accepted and dealt with as it he called it on the encased object.
TO DO
Remove the requirement for a DESTROY by adding the option of a struct crawling manual dereferencer. There's probably one somewhere in CPAN we could just run-time load as needed. Any suggestions?
SUPPORT
Bugs should be reported via the CPAN bug tracker at
http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Object%3A%3ADestroyerFor other issues, contact the author
AUTHOR
Adam Kennedy
cpan@ali.as
http://ali.as/COPYRIGHT
Copyright (c) 2004 Adam Kennedy. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
The full text of the license can be found in the LICENSE file included with this module.