Programming Perl, 4th Edition

Magical Counter Variables

Here’s a simple Tie::Counter class, inspired by the CPAN module of the same name. Variables tied to this class increment themselves by 1 every time they’re used. For example:

tie my $counter, "Tie::Counter", 100;
@array = qw /Red Green Blue/;
for my $color (@array) {               # Prints:
    print "  $counter  $color\n";      #   101  Red
}                                      #   102  Green
                                       #   103  Blue

The constructor takes as an optional extra argument the first value of the counter, which defaults to 0. Assigning to the counter will set a new value. Here’s the class:

package Tie::Counter;
sub FETCH     { ++ ${ $_[0] } }
sub STORE     { ${ $_[0] } = $_[1] }
sub TIESCALAR {
    my ($class, $value) = @_;
    $value = 0 unless defined $value;
    bless \$value => $class;
}
1;  # if in module

See how small that is? It doesn’t take much code to put together a class like this.