Accessor Methods

Accessor methods provide a natural place to perform argument checking

        package Bank_Account;

        sub new { 
          my ($class, $owner, $initial_balance) = @_;
          my $self = { OWNER => $owner, BALANCE => $initial_balance };
          bless $self => Bank_Account;
        }

        sub deposit {
          my ($self, $amount) = @_;
          $self->{BALANCE} += $amount;
        }

        sub withdraw {
          my ($self, $amount) = @_;
          croak "Overdrawn" if $amount > $self->{BALANCE};
          croak "Can't withdraw negative amount" if $amount < 0;
          $self->{BALANCE} -= $amount;
        }

        sub balance {
          my ($self) = @_;
          return $self->{BALANCE};
        }

To use this:

        my $freds_acc = Bank_Account->new('Fred', 100.00);
        $freds_acc->deposit(50.00);
        $freds_acc->withdraw(87.28);
        print "Fred's balance is ", $freds_acc->balance, "\n";

        $freds_acc->withdraw(-1000);   # Fatal error