Programming Perl, 4th Edition

Scalar-Tying Methods

Now that you’ve seen a sample of what’s to come, let’s develop a more elaborate scalar-tying class. Instead of using any canned package for the base class (especially since scalars are so simple), we’ll look at each of the four methods in turn, building an example class named ScalarFile. Scalars tied to this class contain regular strings, and each such variable is implicitly associated with a file where that string is stored. (You might name your variables to remind you to which file you’re referring.) Variables are tied to the class this way:

use ScalarFile;       # load ScalarFile.pm
tie $camel, "ScalarFile", "/tmp/camel.lot";

Once the variable has been tied, its previous contents are clobbered, and the internal connection between the variable and its object overrides the variable’s normal semantics. When you ask for the value of $camel, it now reads the contents of /tmp/camel.lot, and when you assign a value to $camel, it writes the new contents out to /tmp/camel.lot, obliterating any previous occupants.

The tie is on the variable, not the value, so the tied nature of a variable does not propagate across assignment. For example, let’s say you copy a variable that’s been tied:

$dromedary = $camel;

Instead of reading the value in the ordinary fashion from the $camel scalar variable, Perl invokes the FETCH method on the associated underlying object. It’s as though you’d written this:

$dromedary = (tied $camel)–>FETCH():

Or if you remember the object returned by tie, you ...