Chapter 2

To implement a tied scalar class, you need to provide three methods 

TIESCALAR(), FETCH(), and STORE(). The first, TIESCALAR(), is the constructor for the 

tied scalar class. It works just like the new() methods you've seen previously it 

takes some parameters and returns a bless()'d reference. Here's a possible imple 

mentation for BOA::Thermostat::TIESCALAR():

package BOA::Thermostat;

sub TIESCALAR {

   my $pkg = shift;

   my $self = { @_ }; # retrieve named options into $self hash ref

   # check for required  compartment  option

   croak("Missing compartment number!") unless exists $self >{compartment};

   # the vent is initially closed

   $self >{vent_state} = 0;

   # bless $self and return

   return bless($self, $pkg);

}

This should look very familiar by now it's just a simple variation on a normal con 

structor. The only difference is the name and the way it will be called by tie()

instead of directly. Notice that even though this code is emulating a scalar, there's 

no need to use a scalar underneath the object itself is a hash in this example.

The remaining two methods are FETCH() and STORE(), which get called when 

the tied variable is read and written, respectively. Here's the FETCH() implemen 

tation for the BOA::Thermostat class:

# method called when scalar is read

sub FETCH {

   my $self = shift;

   return get_temp($self >{compartment});

}

FETCH() receives only the object as an argument and returns the results of calling 

the class method get_temp() with the compartment number as a parameter. This 

method will check the temperature in the given compartment and return it. I'll 

leave implementing this method as an exercise for budding rocket scientists in the 

audience.

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