Programming Perl, 4th Edition

A Subtle Untying Trap

If you intend to make use of the object returned from tie or tied, and the class defines a destructor, there is a subtle trap you must guard against. Consider this (admittedly contrived) example of a class that uses a file to log all values assigned to a scalar:

package Remember;

sub TIESCALAR {
    my $class = shift;
    my $filename = shift;
    open(my $handle, ">", $filename)
         || die "Cannot open $filename: $!\n";
    print $handle "The Start\n";
    bless {FH => $handle, VALUE => 0}, $class;
}

sub FETCH {
    my $self = shift;
    return $self–>{VALUE};
}

sub STORE {
    my $self = shift;
    my $value = shift;
    my $handle = $self–>{FH};
    print $handle "$value\n";
    $self–>{VALUE} = $value;
}

sub DESTROY {
    my $self = shift;
    my $handle = $self–>{FH};
    print $handle "The End\n";
    close $handle;
}

1;

Here is an example that makes use of our Remember class:

use strict;
use Remember;

my $fred;
$x = tie $fred, "Remember", "camel.log";
$fred = 1;
$fred = 4;
$fred = 5;
untie $fred;
system "cat camel.log";

This is the output when it is executed:

The Start
1
4
5
The End

So far, so good. Let’s add an extra method to the Remember class that allows comments in the file—say, something like this:

sub comment {
    my $self = shift;
    my $message = shift;
    print { $self–>{FH} } $handle $message, "\n";
}

And here is the previous example, modified to use the comment method:

use strict; use Remember; my ($fred, $x); $x = tie $fred, "Remember", "camel.log"; $fred = 1; $fred = 4; comment $x "changing..."; $fred = 5; untie $fred; system ...