Learning Perl Objects, References & ModulesLearning Perl Objects, References & ModulesSearch this book

Chapter 8. Introduction to Objects

Contents:

If We Could Talk to the Animals...
Introducing the Method Invocation Arrow
The Extra Parameter of Method Invocation
Calling a Second Method to Simplify Things
A Few Notes About @ISA
Overriding the Methods
Starting the Search from a Different Place
The SUPER Way of Doing Things
What to Do with @_
Where We Are So Far...
Exercises

Object-oriented programming (often called OOP) helps programmers run code sooner and maintain it easier at the cost of making the resulting programs slower. That is to say, a typical program written in an OO language will normally run more slowly than the corresponding one written in a language without objects.

So why would anyone want their programs to run more slowly? Your programs will take less time to read, write, debug, and maintain, and on large projects, these factors are important. How much slowdown are we talking about? Well, that depends. But in general, the more heavily you use objects, the more it will save time for the programmer, and the more it will cost time at runtime. Still, you should remember that computers keep getting faster. Even if using OOP slows down your program, using next year's hardware will probably make up for it.

The benefits of OOP become worthwhile when your program (including all external libraries and modules) exceeds about N lines of code. Unfortunately, nobody can agree on what the value of N is, but for Perl programs, it's arguably around 1,000 lines of code. If your whole program is only be a couple hundred lines of code, using objects is probably a waste.

Like references, Perl's object architecture was grafted on after a substantial amount of existing pre-Perl5 code was already in use, so we had to ensure that it wouldn't break existing syntax. Amazingly, the only additional syntax to achieve object nirvana is the method call, introduced shortly. But the meaning of that syntax requires a bit of study, so let's proceed.

The Perl object architecture relies heavily on packages, subroutines, and references, so if you're skipping around in this book, please go back to the beginning. Ready? Here we go.

8.1. If We Could Talk to the Animals...

Obviously, the castaways can't survive on coconuts and pineapples alone. Luckily for them, a barge carrying random farm animals crashed on the island not long after they arrived, and the castaways began farming and raising animals.

Let's let those animals talk for a moment:

sub Cow::speak {
  print "a Cow goes moooo!\n";
}
sub Horse::speak {
  print "a Horse goes neigh!\n";
}
sub Sheep::speak {
  print "a Sheep goes baaaah!\n";
}

Cow::speak;
Horse::speak;
Sheep::speak;

This results in:

a Cow goes moooo!
a Horse goes neigh!
a Sheep goes baaaah!

Nothing spectacular here: simple subroutines, albeit from separate packages, and called using the full package name. Let's create an entire pasture:

sub Cow::speak {
  print "a Cow goes moooo!\n";
}
sub Horse::speak {
  print "a Horse goes neigh!\n";
}
sub Sheep::speak {
  print "a Sheep goes baaaah!\n";
}

my @pasture = qw(Cow Cow Horse Sheep Sheep);
foreach my $beast (@pasture) {
  &{$beast."::speak"};                # Symbolic coderef
}

This results in:

a Cow goes moooo!
a Cow goes moooo!
a Horse goes neigh!
a Sheep goes baaaah!
a Sheep goes baaaah!

Wow. That symbolic coderef dereferencing there in the body of the loop is pretty nasty. We're counting on no strict 'refs' mode, certainly not recommended for larger programs.[34] And why was that necessary? Because the name of the package seems inseparable from the name of the subroutine you want to invoke within that package.

[34]Although all examples in this book should be valid Perl code, some examples in this chapter will break the rules enforced by use strict to make them easier to understand. By the end of the chapter, though, you'll learn how to make strict-compliant code again.

Or is it?



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