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.
Constant
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().
$x : Constant($o)
$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.
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>
<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.
To install Data::Lock, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Data::Lock
CPAN shell
perl -MCPAN -e shell install Data::Lock
For more information on module installation, please visit the detailed CPAN module installation guide.