Some things have a natural concept of addition. And of multiplication.

You can add numbers - 5 + 7 gives 12. You can add strings of text - "Hello " + "World" = "Hello World". You can add day trippers to a train and get a busier train (and add too many and you'll get an overcrowded unhappy train, add too few and you'll get an unjustified service). But you can't sensibly do certain addion operations. You can't add the colour purple to a DNS enquiry on port 53; it's a meaningless concept.

Perl's "+" operator works by default on numeric values - be they integers or floats. And you can redefine it on any of your own objects ("packages") if you wish, using the overload pragma. There's an example from last week's Perl for larger projects course - source code [here].

In the example (the source code is commented), I have shown an example of how to overload the "+" operator on an object of type box. Adding boxes together stacks them, so you get the greater of the two width and depth values, and the sum of the two heights, in a new box. But you have far more flexibiity than that - if I had wished, I could have returned an object of a different type, or added something other than a box to a box. Indeed, I've shown a multiplication example where I use a stack of identical boxes, and the second parameter in this case was a number.

When are two boxes "equal"? The most obvious answer is when all their attributes are the same, but it could be that you want something else. By overloading ==, I have redefined equality on boxes, considering them to be equal if they have the same volume.

And what do I get when I print out a variable that contains a box object? Without any instruction in the class, I'll get the fact that it's a box, the memory address at which it is held, and a note about the internal variable type that holds all the data about it. Overload the "" operator though, and you can change fromn the default to something which usefully described the object. Again, there's a commented example in the program that illustrates this article.

There's a further article [here], and a shorter source example showing just addition [here].

(written 2011-08-02, updated 2011-08-03)

 P219 - Perl - Libraries and Resources
  [86] Talk review - Idiomatic Perl, David Cross - (2004-10-12)
  [112] Avoid the wheel being re-invented by using Perl modules - (2004-11-08)
  [357] Where do Perl modules load from - (2005-06-24)
  [358] Use standard Perl modules - (2005-06-25)
  [712] Why reinvent the wheel - (2006-05-06)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [760] Self help in Perl - (2006-06-14)
  [1219] Judging the quality of contributed Perl code - (2007-06-06)
  [1235] Outputting numbers as words - MySQL with Perl or PHP - (2007-06-17)
  [1391] Ordnance Survey Grid Reference to Latitude / Longitude - (2007-10-14)
  [1444] Using English can slow you right down! - (2007-11-25)
  [1863] About dieing and exiting in Perl - (2008-11-01)
  [1865] Debugging and Data::Dumper in Perl - (2008-11-02)
  [2229] Do not re-invent the wheel - use a Perl module - (2009-06-11)
  [2234] Loading external code into Perl from a nonstandard directory - (2009-06-12)
  [2427] Operator overloading - redefining addition and other Perl tricks - (2009-09-27)
  [2931] Syncronise - software, trains, and buses. Please! - (2010-08-22)
  [3009] Expect in Perl - a short explanation and a practical example - (2010-10-22)
  [3101] The week before Christmas - (2010-12-23)

P218 - Perl - More Objects
  [227] Bellringing and Programming and Objects and Perl - (2005-02-25)
  [246] When to bless a Perl variable - (2005-03-15)
  [531] Packages in packages in Perl - (2005-12-16)
  [588] Changing @INC - where Perl loads its modules - (2006-02-02)
  [592] NOT Gone phishing - (2006-02-05)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [930] -> , >= and => in Perl - (2006-11-18)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1320] Perl for Larger Projects - Object Oriented Perl - (2007-08-25)
  [1435] Object Oriented Programming in Perl - Course - (2007-11-18)
  [1664] Example of OO in Perl - (2008-06-03)
  [1665] Factory method example - Perl - (2008-06-04)
  [1819] Calling base class constructors - (2008-10-03)
  [1949] Nuclear Physics comes to our web site - (2008-12-17)
  [2651] Calculation within objects - early, last minute, or cached? - (2010-02-26)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2811] Igloos melt in the summer, but houses do not - (2010-06-15)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [3097] Making Perl class definitions more conventional and shorter - (2010-12-20)
  [3098] Learning Object Orientation in Perl through bananas and perhaps Moose - (2010-12-21)
  [3581] Perl - calls to methods that use => - what do they mean? - (2012-01-16)
  [3941] Building an object based on another object in Perl - (2012-12-03)
  [4096] Perl design patterns example - (2013-05-20)
  [4098] Using object orientation for non-physical objects - (2013-05-22)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)

Back to Kennet and Avon - Walk from Bedwyn to Pewsey. TransWilts day out.

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Forward to New product - ensuring that supply matches demand

Report - day trip from Swindon / Chippenham / Melksham to Weymouth
Melksham Jelly - An Occasional Office for Home Workers
Sorting data the way YOU want it sorted
New product - ensuring that supply matches demand
What do I mean when I add things in Perl?
Kennet and Avon - Walk from Bedwyn to Pewsey. TransWilts day out.
How to interact with a Perl program while it is processing data
Speeding up your Perl code
Another busy Week at Well House Manor ... pictures from the midweek
Wearing the new London uniform