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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Building an object based on another object in Perl

When you create an object, you call a constructor method. In many languages, there's a special name for this method and the word new or init is often involved ... so in Perl it's a good convention to use new as well.

Example:
  $first = new roundtable(1800,"Pine");

Where does Perl find the new method?

Firstly, it will look for a sub called new in the roundtable package. If there isn't one, it will look in any packages which roundtable inherits from by looking in the list @ISA, and it will recursively explore all parent packages until it finds a new sub.

If no new sub is available, the same traversal will be applied looking for a sub called AUTOLOAD. And if that fails too, then an error / excpetion is thrown.

How does Perl run two new methods?

There are occasions when you'll want to construct an object and within its constrcutor call the constructor of its base class. This is the super call in Python, and you may be familiar with other ways of doing this in other languages - it's pretty nonstandard.

In Perl, you can call the constructor of the parent class, and the
re-bless the resulting reference into the subclass. Here's an example, where a roundtable is based on a table but with a single extra parameter.

table constructor:

  sub new {
    my ($type,$mat) = @_;
    my %self;
    $self{m} = $mat;
    bless \%self,$type;
    }


roundtable constructor which calls in table constructor:

  sub new {
    my ($type, $diam, $material) = @_;
    my $this = new table($material);
    $this->{h} = $diam;
    bless $this,$type;
    }


Complete, working example (also shows AUTOLOAD in use) [here] from last week's Perl Course.
(written 2012-12-03, updated 2012-12-08)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
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)
  [2427] Operator overloading - redefining addition and other Perl tricks - (2009-09-27)
  [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)
  [3377] What do I mean when I add things in Perl? - (2011-08-02)
  [3581] Perl - calls to methods that use => - what do they mean? - (2012-01-16)
  [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)


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Run other processes from within your Perl program
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Some other Articles
vargs in Python - how to call a method with unknown number of parameters
Melksham Christmas Lights, 2012
With us for a year already!
Santa Trip - Melksham Railway Development Group, 2012
Building an object based on another object in Perl
Run other processes from within your Perl program
Lots of ways of doing the same thing in Perl - list iteration
A long overdue meeting - a steeping stone towards coordinate transport user inputs
Agenda for Wiltshire Link Travel groups meeting, Saturday 1st December 2012
Lisa is home today ...
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