in reply to Use constant vs. hashes

Another solution might be to protect ordinary scalars using a tie:

use strict;
package ConstantScalar;
use Carp;
require Tie::Scalar;
@ConstantScalar::ISA = 'Tie::StdScalar';

sub STORE 
{
 confess "illegal store into constant";
}

# call with a list of refs to scalars
sub makeConstant
{
 while (my $ref = shift)
 {
  (warn "nonscalar ref", next) if (ref($ref) ne 'SCALAR');
  tie $$ref, 'ConstantScalar', $$ref;
 }
}

package main;

my ($A, $B, $C, $D) = (1, 2, 3, 4);
my $E = 5;
print "$A $B $C $E\n";

ConstantScalar::makeConstant(\($A, $B, $C, $D));
$E = 123;
$A = 4;  # illegal store into constant
$B = 5;

print "$A $B $C $E\n";

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Re: Re: Use constant vs. hashes
by repson (Chaplain) on Jul 03, 2001 at 10:20 UTC
    This is one I developed seperately with more stuff in it, also allowing constant hashes and arrays (much like tuples in python as I understand them). It also uses a different and IMHO cleaner syntax for creating the constants:
    Const my $foo => 42;

    Here's the sample program: 

    #!/usr/bin/perl -w
use strict;

use Const qw/Const ConstHash ConstArray/;

Const my $FOO => 42;
# tie my $FOO, 'Const', 42;

print "$FOO\n";         # prints "42"
$FOO = 43;              # croaks for attempted change
$FOO++;                 # same


ConstHash my %BOO => { foo => 'blah', moo => 41, 2 => 'no' };
# tie my %BOO, 'ConstHash', { foo => 'blah', moo => 41, 2 => 'no' };

print "$BOO{foo} $BOO{2}\n";  # prints "blah no"
$BOO{moo}++;            # croaks for attempted change
%BOO = ();              # same
print "$BOO{Foo}\n";    # croaks for non-existant element


ConstArray my @MOO => [ qw/foo 2 five seven/ ];
# tie my @MOO, 'ConstArray', [ qw/foo 2 five seven ];

print "$MOO[0] $MOO[3]\n";  # prints "foo seven"
$MOO[1]++;              # croaks for attemped change
$MOO[5] = 2;            # same
splice(@MOO,1,1,4);     # same again
print "$MOO[10]\n";     # croaks for non-existant element

    [download]
    And the implementation in Const.pm (click Read More)
    Read more... (3 kB)
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      Thanks to everyone - some great ideas here. And, as I'd hoped, a couple that I hadn't thought of :)
      --
      man with no legs, inc.
[reply]
Re: Re: Use constant vs. hashes
by frag (Hermit) on Jul 03, 2001 at 09:41 UTC
    ++{the idea}, but here's another WTDI. 
    package Fixed;
use Carp;
use strict;

sub TIESCALAR {
        my ($class, $value) = @_;
        croak "No value provided for new Fixed scalar" 
           unless defined $value;
        bless \$value, $class;
}
 
sub FETCH {
        return ${$_[0]};
}
 
sub STORE {
        croak "Illegal assignment to Fixed scalar";
}

package main;

tie my $v, "Fixed" => "32";
tie my $x, "Fixed" => "0";
tie my $str, "Fixed" => "my string";

print ">> $v\n";
$v++;
print ">> $v\n";

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    -- Frag.

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