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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Changing a variable behaviour in Perl - tieing

What can you do with a scalar variable? When you reduce it to lowest level programming principles, not a lot ... you can create it, destroy it, save a value into it, and read a value back from it. That's about it, when you think of it!

In Perl, the low level memory accessors within the language go through just a handful of routines (it's really an object interface), which you can override for individual variables if you like. What does this mean? It means you can change the characteristics of a variable if you like. Look at this program:

use propercase;
tie $yourname,"propercase";
print "Enter your name ";
$yourname = <STDIN>;
print "Welcome, $yourname";

Good - but because of my propercase class it will force the name to be properly cased:

earth-wind-and-fire:~/jun09 grahamellis$ perl ppc
Enter your name graHam
Welcome, Graham
earth-wind-and-fire:~/jun09 grahamellis$

Clever, isn't it? ... Here is the module that defines "propercase"

package propercase;
  my ($class,$value) = @_;
  bless \$value,$class; }
sub STORE {
  my ($inst,$value) = @_;
  $$inst = ucfirst(lc($value)); }
sub FETCH {
  my ($inst) = @_;
  return $$inst; }

Other examples of a tied scalar are for a variable which automatically has spaces trimmed off the start and end, a stack (where each time to save a value it is actually pushed onto a list, and each time you call it back, the next value is returned) and a variable that persists between runs of your program, as tieing links tha variable to a file on the disc.

Tieing is also possible to lists and hashes - in the case of a hash it can provide a very useful interface indeed to a database table - you can insert / replace / select / update database records with simple assignment statements. The methods you need to implement to use a tied hash are TIEHASH (the constructor), STORE, FETCH, EXISTS, DELETE, CLEAR, FIRSTKEY and NEXTKEY. There are examples in our Tieing in Perl resource module, which is covered on our Perl for Larger Projects course.

Illustration - delegates on a Perl course at Well House Manor
(written 2009-06-16, updated 2010-06-23)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P304 - Perl - Tieing
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3409] When variables behave differently - Tie in Perl - (2011-08-28)

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What should a web site cost you?
Changing a variable behaviour in Perl - tieing
So what is this thing called Perl that I keep harping on about?
Perl references - $$var and \$var notations
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Handling nasty characters - Perl, PHP, Python, Tcl, Lua
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