Perl Module Basics

Tying Scalars

Sometimes a class is so simple that its entire interface can be represented by a tied 

scalar. For example, imagine that the BOA::Thermostat module implements an 

interface to a thermometer and a heater for the BOA spacecraft. If this class pro 

vided a tied scalar interface, then reads could correspond to checking the current 

temperature and writes could correspond to opening or closing the heating vents. 

Here's some example code that keeps compartment 10 of the spacecraft between 

20 and 30 degrees:

use BOA::Thermostat;

# tie $thermo to temperator controls for compartment 10, the captain s quarters

tie $thermo,  BOA::Thermostat , compartment => 10;

# enter infinite loop

while (1) {

   # check temperature

   if ($thermo <= 20) {      # too cool?

      $thermo = 1;           # open the vents

   } elsif ($thermo >= 30) { # too hot?

      $thermo = 0;           # close the vents

   }

   sleep(30);                # pause for 30 seconds

}

The code starts by using the BOA::Thermostat module. Next, a call to the tie function 

is made. This call tells Perl that the $thermo variable's implementation will be pro 

vided by the BOA::Thermostat class. Whenever the program accesses the $thermo

variable, a method in the BOA::Thermostat class is called. The call also passes the 

compartment number to the BOA::Thermostat class using a named parameter 

style. The program then enters an infinite loop, checking the temperature and 

opening or closing the vents as appropriate.

This example highlights a key difference between tying and overloading the 

ability to handle assignment. An overloaded class could not provide this interface 

because after the first assignment the $thermo variable would no longer contain a 

reference to an overloaded object. Tied variables provide magic containers, whereas 

overloaded objects provide magic values that can be assigned to variables. The 

difference is subtle but important to understand.

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