Problem

You want to add special processing to a variable or handle.

Solution

Use the tie function to give your ordinary variables object hooks.

Discussion

Anyone who's ever used a DBM file under Perl has already used tied objects. Perhaps the most excellent way of using objects is such that the user never notices them. With tie, you can bind a variable or handle to a class, after which all access to the tied variable or handle is transparently intercepted by specially named object methods.

The most important tie methods are FETCH to intercept read access, STORE to intercept write access, and the constructor, which is one of TIESCALAR, TIEARRAY, TIEHASH, or TIEHANDLE.

tie $s, "SomeClass"

SomeClass->TIESCALAR()

$p = $s

$p = $obj->FETCH()

$s = 10

$obj->STORE(10)

Where did that $obj come from? The tie triggers a call to the class's TIESCALAR constructor method. Perl squirrels away the object returned and surreptitiously uses it for later access.

Here's a simple example of a tie class that implements a value ring. Every time the variable is read from, the next value on the ring is displayed. When it's written to, a new value is pushed on the ring. Here's an example:

#!/usr/bin/perl
# demo_valuering - show tie class
use ValueRing;
tie $color, 'ValueRing', qw(red blue);
print "$color $color $color $color $color $color\n";
red blue red blue red blue

$color = 'green';
print "$color $color $color $color $color $color\n";
green red blue green red blue

The simple implementation is shown in Example 13.3.

package ValueRing;

# this is the constructor for scalar ties
sub TIESCALAR {
    my ($class, @values) = @_;
    bless  \@values, $class;
    return \@values;
} 

# this intercepts read accesses
sub FETCH {
    my $self = shift;
    push(@$self, shift(@$self));
    return $self->[-1];
} 

# this intercepts write accesses
sub STORE {
    my ($self, $value) = @_;
    unshift @$self, $value;
    return $value;
} 

1;

This example might not be compelling, but it illustrates how easy it is to write ties of arbitrary complexity. To the user, $color is just a plain old variable, not an object. All the magic is hidden beneath the tie. You don't have to use a scalar reference just because you're tying a scalar. Here we've used an array reference, but you can use anything you'd like. Usually a hash reference will be used irrespective of what's being tied to because it's the most flexible object representation.

For arrays and hashes, more elaborate operations are possible. Tied handles didn't appear until the 5.004 release, and prior to 5.005 use of tied arrays was somewhat limited, but tied hashes have always been richly supported. Because so many object methods are needed to fully support tied hashes, most users choose to inherit from the standard Tie::Hash module, which provides default methods for these.

Following are numerous examples of interesting uses of ties.

Tie Example: Outlaw $_

This curious tie class is used to outlaw unlocalized uses of the implicit variable, $_. Instead of pulling it in with use, which implicitly invokes the class's import ( ) method, this one should be loaded with no to call the seldom-used unimport ( ) method. The user says:

no UnderScore;

Then, all uses of the unlocalized global $_ will raise an exception.

Here's a little test suite for the module.

#!/usr/bin/perl
# nounder_demo - show how to ban $_ from your program
no UnderScore;
@tests = (
    "Assignment"  => sub { $_ = "Bad" },
    "Reading"     => sub { print }, 
    "Matching"    => sub { $x = /badness/ },
    "Chop"        => sub { chop },
    "Filetest"    => sub { -x }, 
    "Nesting"     => sub { for (1..3) { print } },
);

while ( ($name, $code) = splice(@tests, 0, 2) ) {
    print "Testing $name: ";
    eval { &$code };
    print $@ ? "detected" : "missed!";
    print "\n";
} 

The result is the following:

Testing Assignment: detected
Testing Reading: detected
Testing Matching: detected
Testing Chop: detected
Testing Filetest: detected
Testing Nesting: 123missed!

The reason the last one was missed is that it was properly localized by the for loop, so it was considered safe.

The UnderScore module itself is shown in Example 13.4. Notice how small it is. The module itself does the tie in its initialization code.

package UnderScore;
use Carp;
sub TIESCALAR {
    my $class = shift;
    my $dummy;
    return bless \$dummy => $class;
} 
sub FETCH { croak "Read access to \$_ forbidden"  } 
sub STORE { croak "Write access to \$_ forbidden" } 
sub unimport { tie($_, __PACKAGE__) }
sub import { untie $_ } 
tie($_, __PACKAGE__) unless tied $_;
1;

You can't usefully mix calls to use and no for this class in your program, because they all happen at compile time, not run time. To renege and let yourself use $_ again, localize it.

Tie Example: Make a Hash That Always Appends

The class shown below produces a hash whose keys accumulate in an array.

#!/usr/bin/perl 
# appendhash_demo - show magic hash that autoappends
use Tie::AppendHash;
tie %tab, 'Tie::AppendHash';

$tab{beer} = "guinness";
$tab{food} = "potatoes";
$tab{food} = "peas";

while (my($k, $v) = each %tab) {
    print "$k => [@$v]\n";
}

Here is the result:

food => [potatoes peas]
beer => [guinness]

To make this class easy, we will use the boilerplate hash tying module from the standard distribution, shown in Example 13.5. To do this, we load the Tie::Hash module and then inherit from the Tie::StdHash class. (Yes, those are different names. The file Tie/Hash.pm provides both the Tie::Hash and Tie::StdHash classes, which are slightly different.)

package Tie::AppendHash;
use strict;
use Tie::Hash;
use Carp;
use vars qw(@ISA);
@ISA = qw(Tie::StdHash);
sub STORE {
    my ($self, $key, $value) = @_;
    push @{$self->{$key}}, $value;
} 
1;

Tie Example: Case-Insensitive Hash

Here's a fancier hash tie called Tie::Folded. It provides a hash with case-insensitive keys.

#!/usr/bin/perl 
# folded_demo - demo hash that magically folds case
use Tie::Folded;
tie %tab, 'Tie::Folded';

$tab{VILLAIN}  = "big "; 
$tab{herOine}  = "red riding hood";
$tab{villain} .= "bad wolf";   

while ( my($k, $v) = each %tab ) {
    print "$k is $v\n";
}

The following is the output of this demo program:

heroine is red riding hood
villain is big bad wolf

Because we have to trap more accesses, the class in Example 13.6 is slightly more complicated than the one in Example 13.5.

package Tie::Folded;
use strict;
use Tie::Hash;
use vars qw(@ISA);
@ISA = qw(Tie::StdHash);
sub STORE {
    my ($self, $key, $value) = @_;
    return $self->{lc $key} = $value;
    } 
sub FETCH {
    my ($self, $key) = @_;
    return $self->{lc $key};
} 
sub EXISTS {
    my ($self, $key) = @_;
    return exists $self->{lc $key};
} 
sub DEFINED {
    my ($self, $key) = @_;
    return defined $self->{lc $key};
} 
1;

Tie Example: Hash That Allows Look-Ups by Key or Value

Here is a hash that lets you look up members by key or by value. It does this by having a store method that not only uses the key to store the value, it also uses the value to store the key.

Normally there could be a problem if the value being stored were a reference, since you can't normally use a reference as a key. The standard distribution comes with the Tie::RefHash class that avoids this problem. We'll inherit from it so that we can also avoid this difficulty.

#!/usr/bin/perl -w
# revhash_demo - show hash that permits key *or* value lookups
use strict;
use Tie::RevHash;
my %tab;
tie %tab, 'Tie::RevHash';
%tab = qw{
    Red         Rojo
    Blue        Azul
    Green       Verde
};
$tab{EVIL} = [ "No way!", "Way!!" ];

while ( my($k, $v) = each %tab ) {
    print ref($k) ? "[@$k]" : $k, " => ",
        ref($v) ? "[@$v]" : $v, "\n";
} 

When run, revhash_demo produces this:

[No way! Way!!] => EVIL
EVIL => [No way! Way!!]
Blue => Azul
Green => Verde
Rojo => Red
Red => Rojo
Azul => Blue
Verde => Green

The module is shown in Example 13.7. Notice how small it is!

package Tie::RevHash;
use Tie::RefHash;
use vars qw(@ISA);
@ISA = qw(Tie::RefHash);
sub STORE {
    my ($self, $key, $value) = @_;
    $self->SUPER::STORE($key, $value);
    $self->SUPER::STORE($value, $key);
}

sub DELETE {
    my ($self, $key) = @_;
    my $value = $self->SUPER::FETCH($key);
    $self->SUPER::DELETE($key);
    $self->SUPER::DELETE($value);
}

1;

Tie Example: Handle That Counts Access

Here's an example of tying a filehandle:

use Counter;
tie *CH, 'Counter';
while (<CH>) {
    print "Got $_\n";
} 

When run, that program keeps printing Got 1, Got 2, and so on until the universe collapses, you hit an interrupt, or your computer reboots, whichever comes first. Its simple implementation is shown in Example 13.8.

package Counter;
sub TIEHANDLE {
    my $class = shift;
    my $start = shift;
    return bless \$start => $class;
} 
sub READLINE {
    my $self = shift;
    return ++$$self;
} 
1;

Tie Example: Multiple Sink Filehandles

Finally, here's an example of a tied handle that implements a tee-like functionality by twinning standard out and standard error:

use Tie::Tee;
tie *TEE, 'Tie::Tee', *STDOUT, *STDERR;
print TEE "This line goes both places.\n";

Or, more elaborately:

#!/usr/bin/perl
# demo_tietee
use Tie::Tee;
use Symbol;

@handles = (*STDOUT);
for $i ( 1 .. 10 ) {
    push(@handles, $handle = gensym());
    open($handle, ">/tmp/teetest.$i");
} 

tie *TEE, 'Tie::Tee', @handles;
print TEE "This lines goes many places.\n";

The Tie/Tee.pm file is shown in Example 13.9.

package Tie::Tee;

sub TIEHANDLE {
    my $class   = shift;
    my $handles = [@_];

    bless $handles, $class;
    return $handles;
}

sub PRINT {
    my $href = shift;
    my $handle;
    my $success = 0;

    foreach $handle (@$href) {
        $success += print $handle @_;
    }

    return $success == @$href;
}                                     

1;

See Also

The tie function in perlfunc (1); perltie (1); the section on "Using Tied Variables" in Chapter 5 of Programming Perl