hash reference to itself

ExReg has asked for the wisdom of the Perl Monks concerning the following question:

I am trying to figure out if it is possible for a hash value to reference a key value in the same hash. As a much simplified example, suppose I have the following:

use strict;
my %hash = (
            name => "foo",
            code => sub{
                        print "name: ", "foo", "\n";
                       },
            );
$hash{code}->();
_____
name: foo
[download]

"foo" is the value of the name key and is also in the code subroutine. Is it possible to have a hash refer to itself with something like

use strict;
my %hash = (
            name => "foo",
            code => sub{
                        print "name: ", $hash{name}, "\n";
                       },
            );
$hash{code}->();
[download]

Obviously this syntax does not work, but is there a way to do this?

Comment on hash reference to itself Select or Download Code

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Re: hash reference to itself by dave_the_m (Monsignor) on Aug 02, 2016 at 17:41 UTC
Note that a self-referential hash will leak. This code consumes several Gb after running for a few seconds: `while (1) { my %h; $h{a} = \%h; }` [download] You may need to use `weaken()` from `Scalar::Util` Dave.	[reply] [d/l] [select]
Re^2: hash reference to itself by ExReg (Priest) on Aug 02, 2016 at 18:15 UTC
Good to know. Thanks!	[reply]
Re: hash reference to itself by Anonymous Monk on Aug 02, 2016 at 17:09 UTC
Declare the hash before assignment. `my %hash; %hash = ...` [download]	[reply] [d/l]
Re^2: hash reference to itself by ExReg (Priest) on Aug 02, 2016 at 17:19 UTC
When my forehead hit the desk, I woke up several people sleeping around me. I don't expect this kind of mental flatulence until Fridays. Thanks! Just what I needed.	[reply]
Re^3: hash reference to itself by ikegami (Patriarch) on Aug 02, 2016 at 17:56 UTC
Note that this approach will leak memory unless you do the following when you're done with the hash: `%hash = ();` [download]	[reply] [d/l]
Re^4: hash reference to itself by ExReg (Priest) on Aug 02, 2016 at 18:14 UTC
Re: hash reference to itself by VinsWorldcom (Prior) on Aug 02, 2016 at 17:11 UTC
Perl can do object-oriented code. `#!perl use strict; use warnings; package MyPkg; sub name { return 'foo'; } sub print { print "name: " . name(); } package main; print MyPkg->name() . "\n"; MyPkg->print();` [download] UPDATE: Actually not OO as YourMother points out below.	[reply] [d/l]
Re^2: hash reference to itself by Your Mother (Archbishop) on Aug 02, 2016 at 18:41 UTC
There is no object here, just class calls which might as well be package calls. `print MyPkg::name(), $/; MyPkg::print();` [download]	[reply] [d/l]
Re^3: hash reference to itself by VinsWorldcom (Prior) on Aug 02, 2016 at 20:01 UTC
You're right. Is this better? `#!perl use strict; use warnings; package MyPkg; sub new { my ($self, $name) = @_; my %hash = ( name => $name ); return bless \%hash; } sub print { my $self = shift; print "name: " . $self->{name}; } package main; my $obj = MyPkg->new('foo'); $obj->print();` [download]	[reply] [d/l]
Re^4: hash reference to itself by Your Mother (Archbishop) on Aug 02, 2016 at 21:22 UTC
Re^2: hash reference to itself by ExReg (Priest) on Aug 02, 2016 at 17:30 UTC
I think if I had the time to do it, OO may have been a better way to do it. But for now, I will have to stick with the huge hashes I have. Thanks.	[reply]
Re: hash reference to itself by talexb (Chancellor) on Aug 02, 2016 at 17:05 UTC
What problem are you trying to solve here? Alex / talexb / Toronto Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.	[reply]
Re^2: hash reference to itself by ExReg (Priest) on Aug 02, 2016 at 17:25 UTC
I have a rather large hash that has a somewhat similar structure. There are many keys containing subs that use another key to print messages. I wanted to bring those keys up a level ( the name key was inside the code sub) and another piece of code wanted to see the name value. Hence I wanted to bring the name up where it was visible to others. Anonymous Monk hit the nail on the head.	[reply]


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