That can be done easily with an object that takes charge of the reblessing and which reverts what it did at destruction time. Your illustration code would look like:
package boo;
sub to_s { "a boo" };
package bar;
sub to_s { "a bar" };
package main;
sub doit {
my $boo = shift;
my $reblesser = Reblesser->new;
$reblesser->rebless($boo, "bar");
print "within: ", $boo->to_s, "\n"; # "a bar"
# here the scope of the reblesser ends and so the reblessing it did
}
my $obj = bless {}, 'boo';
print "before: ", $obj->to_s, "\n"; # "a boo"
doit($obj);
print "after: ", $obj->to_s, "\n"; # "a boo" again
The output should be:
$ perl reblesser.pl
before: a boo
[DEBUG] reblessing boo=HASH(0x102efcf8) to bar
within: a bar
[DEBUG] reverting blessing on bar=HASH(0x102efcf8): back to boo
after: a boo
This way you just have to be sure that the scope in which you want to limit the reblessing is the same as the scope of the reblesser object.
A trivial implementation of such reblesser would be:
package Reblesser;
use strict;
use warnings;
use base qw(Class::Accessor);
__PACKAGE__->mk_accessors(qw(reblessed));
use Scalar::Util qw(blessed);
# reblessed holds pairs [ (object => original_package) ]
sub new {
my $self = shift;
return $self->SUPER::new({ reblessed => [] });
}
sub rebless {
my $self = shift;
my $obj = shift;
my $package = shift;
die "not blessed" unless blessed $obj;
push @{$self->reblessed}, [ $obj, blessed $obj ];
warn "[DEBUG] reblessing $obj to $package\n";
return bless $obj, $package
}
sub DESTROY {
my $self = shift;
for (@{$self->reblessed}) {
my ($obj, $pkg) = @$_;
warn "[DEBUG] reverting blessing on $obj: back to $pkg\n";
bless $obj, $pkg;
}
}
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