package Counter;
use strict;
use Carp;

sub new {
    my ($class,$value) = @_;
    $value ||= 0;
    my $self = bless {}, $class;
    croak "Argument to constructor must be positive integer."
    	if ! $self->_good_value( $value );
    $self->{_value} = $value;
    return $self;
}

sub value {
    my $self = shift;
    $self->{_value} = $_[0] if $self->_good_value($_[0]);
    return $self->{_value};
}

sub increment_value {
    my $self = shift;
    $self->{_value}++;
}

sub decrement_value {
    my $self = shift;
    $self->{_value}--;
}

sub _good_value {
    my ( $self, $value ) = @_;
    return 0 if ! defined $value;
    $value =~ /^\d+$/;
}

1;