GSP
Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Support
Contact Us
Online Help
Handbooks
Domain Status
Man Pages

FAQ
Virtual Servers
Pricing
Billing
Technical

Network
Facilities
Connectivity
Topology Map

Miscellaneous
Server Agreement
Year 2038
Credits
 

USA Flag

 

 

Man Pages
Attribute::Constant(3) User Contributed Perl Documentation Attribute::Constant(3)

Attribute::Constant - Make read-only variables via attribute

$Id: Constant.pm,v 1.1 2013/04/03 14:37:57 dankogai Exp $

 use Attribute::Constant;
 my $sv : Constant( $initial_value );
 my @av : Constant( @values );
 my %hv : Constant( key => value, key => value, ...);

This module uses Data::Lock to make the variable read-only. Check the document and source of Data::Lock for its mechanism.

This module adds only one attribute, "Constant". You give its initial value as shown. Unlike Readonly, parantheses cannot be ommited but it is semantically more elegant and thanks to Data::Lock, it imposes almost no performance penalty.

Multi-line attributes are not allowed in Perl 5.8.x.

  my $o : Constant(Foo->new(one=>1,two=>2,three=>3));    # ok
  my $p : Constant(Bar->new(
                            one   =>1,
                            two   =>2,
                            three =>3
                           )
                 ); # needs Perl 5.10

In which case you can use Data::Lock instead:

  dlock(my $p = Bar->new(
        one   => 1,
        two   => 2,
        three => 3
    )
  );

After all, this module is a wrapper to Data::Lock;

You may be surprised the following code DOES NOT work as you expected:

  #!/usr/bin/perl
  use strict;
  use warnings;
  use Attribute::Constant;
  use Data::Dumper;
  {
    package MyClass;
    sub new {
        my ( $class, %params ) = @_;
        return bless \%params, $class;
    }
  }
  my $o = MyClass->new( a => 1, b => 2 );
  my $x : Constant($o);
  print Dumper( $o, $x );

Which outputs:

  $VAR1 = bless( {
                 'a' => 1,
                 'b' => 2
               }, 'MyClass' );
  $VAR2 = undef;

Why? Because " $x : Constant($o) " happens before "$o = Myclass->new()".

On the other hand, the following works.

  my $y : Constant(MyClass->new(a => 1,b => 2));
  print Dumper( $o, $y );

Rule of the thumb is do not feed variables to constant because varialbes change after the attribute invocation.

Or simply use "Data::Lock::dlock".

  use Data::Lock qw/dlock/;
  dlock my $z = $o;
  print Dumper( $o, $y );

Data::Lock, constant

Dan Kogai, "<dankogai+cpan at gmail.com>"

See Data::Lock.

Readonly

Copyright 2008-2013 Dan Kogai, all rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

2014-03-07 perl v5.32.1

Search for    or go to Top of page |  Section 3 |  Main Index

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with ManDoc.