Rob Richardson wrote:
> Greetings!
>
> I am beginning to seriously consider what I am going to have to do to
> wrestle some reasonable amount of organization out of the unbelievable
> pile of spaghetti I've been trying to update. I managed to add what I
> needed; now I'm going to go back and do it the way it should have been
> done, mainly as an academic exercise.
>
> The new version of my script will be based on objects. So, I was
> reading the Camel Book (2nd ed) about objects. It started talking
> about "tie", and the process of tying variables to classes. It went
> into some detail about how to do it, but it didn't say a durn thing
> about why o do it.
>
> Why would I want to "tie" a variable, and what actually happens when
> a variable is tied?
Tying a variable allows you to write code to handle the normal operations
against that variable. The classic case of this is tying a hash to an
external file or database, so you can write the normal:
$hash{key} = "value";
and have code that automatically stores that value out to a database or
file.
Here's a real simple example with a scalar:
#!/usr/bin/perl
package Timestamp;
use base qw/Tie::Scalar/;
sub TIESCALAR { my $self; bless \$self, shift }
sub FETCH { localtime }
package main;
tie $now, 'Timestamp';
print "The time is $now\n";
sleep 5;
print "Now it's $now\n";
The tied variable $now will evaluate to a timestamp string whenever you use
it. This is because the interpolation of the variable in the print statement
(it could also have been a normal assignment or other expression) causes the
FETCH method of the class Timestamp to be called.
So $now is a "magic" variable that always evaluates to the current
timestamp.
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