What does addition actually mean? It means different things depending on what you're adding together, and in Perl you can redefine additions - and other operators - on objects of types which you define. This is known as operator overloading, and there's a complete code example here.

So ... how do you do it? Within your class, call up the overload program to redirect the operator that you're overloading to a sub of your choice. And provide that sub - with two parameters passed in through @_ for the two objects being added. Within the sub you should check that you really do have objects (my example is kept simple and does not have this check) and if you do, construct a new object that's a combination of the inputs.

I've chosen - for my example - to stack boxes which have an "x" and a "y" dimension; the new combined box when you add two of them together has an "x" which is the sum of the two "x"es, and a "y" which is the greater of the two "y"s.

Operators can be overloaded in Python too (example) and in C++ (example), but not in PHP, which is a loosely typed language.


Illustation ... I might look for a clever title like "adding pieces to a jigsaw puzzle" ... but actually these are delegates on a private Perl course at our training centre and hotel, relaxing during the evening in the old fashioned way by doing a jigsaw. Our courses provide not only the lectures and practical sessions, but also a chance to get to know your colleagues better, and to exchange ideas with other delegates learning the same subject.
(written 2009-09-27, updated 2010-06-20)

 P219 - Perl - Libraries and Resources
  [3377] What do I mean when I add things in Perl? - (2011-08-02)
  [3101] The week before Christmas - (2010-12-23)
  [3009] Expect in Perl - a short explanation and a practical example - (2010-10-22)
  [2931] Syncronise - software, trains, and buses. Please! - (2010-08-22)
  [2234] Loading external code into Perl from a nonstandard directory - (2009-06-12)
  [2229] Do not re-invent the wheel - use a Perl module - (2009-06-11)
  [1865] Debugging and Data::Dumper in Perl - (2008-11-02)
  [1863] About dieing and exiting in Perl - (2008-11-01)
  [1444] Using English can slow you right down! - (2007-11-25)
  [1391] Ordnance Survey Grid Reference to Latitude / Longitude - (2007-10-14)
  [1235] Outputting numbers as words - MySQL with Perl or PHP - (2007-06-17)
  [1219] Judging the quality of contributed Perl code - (2007-06-06)
  [760] Self help in Perl - (2006-06-14)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [712] Why reinvent the wheel - (2006-05-06)
  [358] Use standard Perl modules - (2005-06-25)
  [357] Where do Perl modules load from - (2005-06-24)
  [112] Avoid the wheel being re-invented by using Perl modules - (2004-11-08)
  [86] Talk review - Idiomatic Perl, David Cross - (2004-10-12)

P303 - Perl - Miscellany
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [3012] Exception handling in Perl - using eval - (2010-10-23)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [2219] Configuring httpd, or Tomcat, to run CGI scripts in Perl - (2009-06-05)

P218 - Perl - More Objects
  [3581] Perl - calls to methods that use => - what do they mean? - (2012-01-16)
  [3098] Learning Object Orientation in Perl through bananas and perhaps Moose - (2010-12-21)
  [3097] Making Perl class definitions more conventional and shorter - (2010-12-20)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [2811] Igloos melt in the summer, but houses do not - (2010-06-15)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2651] Calculation within objects - early, last minute, or cached? - (2010-02-26)
  [1949] Nuclear Physics comes to our web site - (2008-12-17)
  [1819] Calling base class constructors - (2008-10-03)
  [1665] Factory method example - Perl - (2008-06-04)
  [1664] Example of OO in Perl - (2008-06-03)
  [1435] Object Oriented Programming in Perl - Course - (2007-11-18)
  [1320] Perl for Larger Projects - Object Oriented Perl - (2007-08-25)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [930] -> , >= and => in Perl - (2006-11-18)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [592] NOT Gone phishing - (2006-02-05)
  [588] Changing @INC - where Perl loads its modules - (2006-02-02)
  [531] Packages in packages in Perl - (2005-12-16)
  [246] When to bless a Perl variable - (2005-03-15)
  [227] Bellringing and Programming and Objects and Perl - (2005-02-25)

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Forward to Diluting History

Moving busstop!
Not just a PHP program - a good web application
Tcl scripts / processes on a web server via CGI
Diluting History
Operator overloading - redefining addition and other Perl tricks
Which version of MySQL am I running?
Weekend and Christmas Promotion - Well House Manor Hotel, Melksham
A Winter Weekend Special at Well House Manor
What is a JVM, a JRE, a JDK - components of the core Java Environment
Looking inside Java classes - javap and javadoc