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Scalar Quickie
This is a fast example of a class that will tie() a scalar variable to a text file. This is different from the earlier example in the "Constructors/Destructors" section of a string value living in a text file being assigned to the tying scalar. In this case, the text file keeps a log of all the times the scalar variable is changed or its value is FETCHED. Take what you've learned about tying with hashes and apply it to the module below:
# Usage: tie($VARIABLE,'TrackScalar', FILE, "\$VARIABLE name/description");
# use TrackScalar;
# my $var;
# tie($var, 'TrackScalar', 'track.txt', "\$var (keeps count)");
package TrackScalar;
use strict;
use vars qw($VERSION @ISA);
# Get Revision number from RCS
($VERSION = substr(q$Revision: 0.2 $, 10)) =~ s/\s+$//;
sub Version {return $VERSION;}
use IO::File;
# Create tied scalar
sub TIESCALAR {
my $class = shift;
my $log = shift;
my $var = shift || "(undefined)";
my $fh = new IO::File ">> $log" or die "Cannot open $log: $!\n";
# Notice that the variable being blessed in the object is an
# anonymous hash, and this is tied to the scalar
return bless {FH => $fh, VAL => 0, VAR => $var}, $class;
}
sub FETCH {
my $self = shift;
my ($package, $filename, $line) = caller();
my $fh = $self->{FH};
print $fh "package $package, $filename line $line FETCHED $self->{VAR}\n";
return $self->{VAL};
}
sub STORE {
my $self = shift;
my $var = shift;
my $fh = $self->{FH};
my ($package, $filename, $line) = caller();
print $fh "package $package, $filename line $line changed $self->{VAR} to $var\n";
$self->{VAL} = $var;
}
sub DESTROY {
undef ${$_[0]};
}
1;