perlboot
(1)
Name
perlboot - Oriented Tutorial
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLBOOT(1)
NAME
perlboot - Beginner's Object-Oriented Tutorial
DESCRIPTION
If you're not familiar with objects from other languages,
some of the other Perl object documentation may be a little
daunting, such as perlobj, a basic reference in using
objects, and perltoot, which introduces readers to the
peculiarities of Perl's object system in a tutorial way.
So, let's take a different approach, presuming no prior
object experience. It helps if you know about subroutines
(perlsub), references (perlref et. seq.), and packages
(perlmod), so become familiar with those first if you
haven't already.
If we could talk to the animals...
Let's let the 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.
So let's create an entire pasture:
# Cow::speak, Horse::speak, Sheep::speak as before
@pasture = qw(Cow Cow Horse Sheep Sheep);
foreach $animal (@pasture) {
&{$animal."::speak"};
}
This results in:
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a Cow goes moooo!
a Cow goes moooo!
a Horse goes neigh!
a Sheep goes baaaah!
a Sheep goes baaaah!
Wow. That symbolic coderef de-referencing there is pretty
nasty. We're counting on "no strict refs" mode, certainly
not recommended for larger programs. And why was that
necessary? Because the name of the package seems to be
inseparable from the name of the subroutine we want to
invoke within that package.
Or is it?
Introducing the method invocation arrow
For now, let's say that "Class->method" invokes subroutine
"method" in package "Class". (Here, "Class" is used in its
"category" meaning, not its "scholastic" meaning.) That's
not completely accurate, but we'll do this one step at a
time. Now let's use it like so:
# Cow::speak, Horse::speak, Sheep::speak as before
Cow->speak;
Horse->speak;
Sheep->speak;
And once again, this results in:
a Cow goes moooo!
a Horse goes neigh!
a Sheep goes baaaah!
That's not fun yet. Same number of characters, all
constant, no variables. But yet, the parts are separable
now. Watch:
$a = "Cow";
$a->speak; # invokes Cow->speak
Ahh! Now that the package name has been parted from the
subroutine name, we can use a variable package name. And
this time, we've got something that works even when "use
strict refs" is enabled.
Invoking a barnyard
Let's take that new arrow invocation and put it back in the
barnyard example:
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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";
}
@pasture = qw(Cow Cow Horse Sheep Sheep);
foreach $animal (@pasture) {
$animal->speak;
}
There! Now we have the animals all talking, and safely at
that, without the use of symbolic coderefs.
But look at all that common code. Each of the "speak"
routines has a similar structure: a "print" operator and a
string that contains common text, except for two of the
words. It'd be nice if we could factor out the commonality,
in case we decide later to change it all to "says" instead
of "goes".
And we actually have a way of doing that without much fuss,
but we have to hear a bit more about what the method
invocation arrow is actually doing for us.
The extra parameter of method invocation
The invocation of:
Class->method(@args)
attempts to invoke subroutine "Class::method" as:
Class::method("Class", @args);
(If the subroutine can't be found, "inheritance" kicks in,
but we'll get to that later.) This means that we get the
class name as the first parameter (the only parameter, if no
arguments are given). So we can rewrite the "Sheep"
speaking subroutine as:
sub Sheep::speak {
my $class = shift;
print "a $class goes baaaah!\n";
}
And the other two animals come out similarly:
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sub Cow::speak {
my $class = shift;
print "a $class goes moooo!\n";
}
sub Horse::speak {
my $class = shift;
print "a $class goes neigh!\n";
}
In each case, $class will get the value appropriate for that
subroutine. But once again, we have a lot of similar
structure. Can we factor that out even further? Yes, by
calling another method in the same class.
Calling a second method to simplify things
Let's call out from "speak" to a helper method called
"sound". This method provides the constant text for the
sound itself.
{ package Cow;
sub sound { "moooo" }
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n";
}
}
Now, when we call "Cow->speak", we get a $class of "Cow" in
"speak". This in turn selects the "Cow->sound" method,
which returns "moooo". But how different would this be for
the "Horse"?
{ package Horse;
sub sound { "neigh" }
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n";
}
}
Only the name of the package and the specific sound change.
So can we somehow share the definition for "speak" between
the Cow and the Horse? Yes, with inheritance!
Inheriting the windpipes
We'll define a common subroutine package called "Animal",
with the definition for "speak":
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{ package Animal;
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n";
}
}
Then, for each animal, we say it "inherits" from "Animal",
along with the animal-specific sound:
{ package Cow;
@ISA = qw(Animal);
sub sound { "moooo" }
}
Note the added @ISA array (pronounced "is a"). We'll get to
that in a minute.
But what happens when we invoke "Cow->speak" now?
First, Perl constructs the argument list. In this case,
it's just "Cow". Then Perl looks for "Cow::speak". But
that's not there, so Perl checks for the inheritance array
@Cow::ISA. It's there, and contains the single name
"Animal".
Perl next checks for "speak" inside "Animal" instead, as in
"Animal::speak". And that's found, so Perl invokes that
subroutine with the already frozen argument list.
Inside the "Animal::speak" subroutine, $class becomes "Cow"
(the first argument). So when we get to the step of
invoking "$class->sound", it'll be looking for "Cow->sound",
which gets it on the first try without looking at @ISA.
Success!
A few notes about @ISA
This magical @ISA variable has declared that "Cow" "is a"
"Animal". Note that it's an array, not a simple single
value, because on rare occasions, it makes sense to have
more than one parent class searched for the missing methods.
If "Animal" also had an @ISA, then we'd check there too.
The search is recursive, depth-first, left-to-right in each
@ISA by default (see mro for alternatives). Typically, each
@ISA has only one element (multiple elements means multiple
inheritance and multiple headaches), so we get a nice tree
of inheritance.
When we turn on "use strict", we'll get complaints on @ISA,
since it's not a variable containing an explicit package
name, nor is it a lexical ("my") variable. We can't make it
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a lexical variable though (it has to belong to the package
to be found by the inheritance mechanism), so there's a
couple of straightforward ways to handle that.
The easiest is to just spell the package name out:
@Cow::ISA = qw(Animal);
Or declare it as package global variable:
package Cow;
our @ISA = qw(Animal);
Or allow it as an implicitly named package variable:
package Cow;
use vars qw(@ISA);
@ISA = qw(Animal);
If the "Animal" class comes from another (object-oriented)
module, then just employ "use base" to specify that "Animal"
should serve as the basis for the "Cow" class:
package Cow;
use base qw(Animal);
Now that's pretty darn simple!
Overriding the methods
Let's add a mouse, which can barely be heard:
# Animal package from before
{ package Mouse;
@ISA = qw(Animal);
sub sound { "squeak" }
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n";
print "[but you can barely hear it!]\n";
}
}
Mouse->speak;
which results in:
a Mouse goes squeak!
[but you can barely hear it!]
Here, "Mouse" has its own speaking routine, so
"Mouse->speak" doesn't immediately invoke "Animal->speak".
This is known as "overriding". In fact, we don't even need
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to say that a "Mouse" is an "Animal" at all, because all of
the methods needed for "speak" are completely defined for
"Mouse"; this is known as "duck typing": "If it walks like a
duck and quacks like a duck, I would call it a duck" (James
Whitcomb). However, it would probably be beneficial to allow
a closer examination to conclude that a "Mouse" is indeed an
"Animal", so it is actually better to define "Mouse" with
"Animal" as its base (that is, it is better to "derive
"Mouse" from "Animal"").
Moreover, this duplication of code could become a
maintenance headache (though code-reuse is not actually a
good reason for inheritance; good design practices dictate
that a derived class should be usable wherever its base
class is usable, which might not be the outcome if code-
reuse is the sole criterion for inheritance. Just remember
that a "Mouse" should always act like an "Animal").
So, let's make "Mouse" an "Animal"!
The obvious solution is to invoke "Animal::speak" directly:
# Animal package from before
{ package Mouse;
@ISA = qw(Animal);
sub sound { "squeak" }
sub speak {
my $class = shift;
Animal::speak($class);
print "[but you can barely hear it!]\n";
}
}
Note that we're using "Animal::speak". If we were to invoke
"Animal->speak" instead, the first parameter to
"Animal::speak" would automatically be "Animal" rather than
"Mouse", so that the call to "$class->sound" in
"Animal::speak" would become "Animal->sound" rather than
"Mouse->sound".
Also, without the method arrow "->", it becomes necessary to
specify the first parameter to "Animal::speak" ourselves,
which is why $class is explicitly passed:
"Animal::speak($class)".
However, invoking "Animal::speak" directly is a mess:
Firstly, it assumes that the "speak" method is a member of
the "Animal" class; what if "Animal" actually inherits
"speak" from its own base? Because we are no longer using
"->" to access "speak", the special method look up mechanism
wouldn't be used, so "speak" wouldn't even be found!
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The second problem is more subtle: "Animal" is now hardwired
into the subroutine selection. Let's assume that
"Animal::speak" does exist. What happens when, at a later
time, someone expands the class hierarchy by having "Mouse"
inherit from "Mus" instead of "Animal". Unless the
invocation of "Animal::speak" is also changed to an
invocation of "Mus::speak", centuries worth of taxonomical
classification could be obliterated!
What we have here is a fragile or leaky abstraction; it is
the beginning of a maintenance nightmare. What we need is
the ability to search for the right method wih as few
assumptions as possible.
Starting the search from a different place
A better solution is to tell Perl where in the inheritance
chain to begin searching for "speak". This can be achieved
with a modified version of the method arrow "->":
ClassName->FirstPlaceToLook::method
So, the improved "Mouse" class is:
# same Animal as before
{ package Mouse;
# same @ISA, &sound as before
sub speak {
my $class = shift;
$class->Animal::speak;
print "[but you can barely hear it!]\n";
}
}
Using this syntax, we start with "Animal" to find "speak",
and then use all of "Animal"'s inheritance chain if it is
not found immediately. As usual, the first parameter to
"speak" would be $class, so we no longer need to pass $class
explicitly to "speak".
But what about the second problem? We're still hardwiring
"Animal" into the method lookup.
The SUPER way of doing things
If "Animal" is replaced with the special placeholder "SUPER"
in that invocation, then the contents of "Mouse"'s @ISA are
used for the search, beginning with $ISA[0]. So, all of the
problems can be fixed as follows:
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# same Animal as before
{ package Mouse;
# same @ISA, &sound as before
sub speak {
my $class = shift;
$class->SUPER::speak;
print "[but you can barely hear it!]\n";
}
}
In general, "SUPER::speak" means look in the current
package's @ISA for a class that implements "speak", and
invoke the first one found. The placeholder is called
"SUPER", because many other languages refer to base classes
as "superclasses", and Perl likes to be eclectic.
Note that a call such as
$class->SUPER::method;
does not look in the @ISA of $class unless $class happens to
be the current package.
Let's review...
So far, we've seen the method arrow syntax:
Class->method(@args);
or the equivalent:
$a = "Class";
$a->method(@args);
which constructs an argument list of:
("Class", @args)
and attempts to invoke:
Class::method("Class", @args);
However, if "Class::method" is not found, then @Class::ISA
is examined (recursively) to locate a class (a package) that
does indeed contain "method", and that subroutine is invoked
instead.
Using this simple syntax, we have class methods, (multiple)
inheritance, overriding, and extending. Using just what
we've seen so far, we've been able to factor out common code
(though that's never a good reason for inheritance!), and
provide a nice way to reuse implementations with variations.
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Now, what about data?
A horse is a horse, of course of course, or is it?
Let's start with the code for the "Animal" class and the
"Horse" class:
{ package Animal;
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n";
}
}
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
}
This lets us invoke "Horse->speak" to ripple upward to
"Animal::speak", calling back to "Horse::sound" to get the
specific sound, and the output of:
a Horse goes neigh!
But all of our Horse objects would have to be absolutely
identical. If we add a subroutine, all horses automatically
share it. That's great for making horses the same, but how
do we capture the distinctions of an individual horse? For
example, suppose we want to give our first horse a name.
There's got to be a way to keep its name separate from the
other horses.
That is to say, we want particular instances of "Horse" to
have different names.
In Perl, any reference can be an "instance", so let's start
with the simplest reference that can hold a horse's name: a
scalar reference.
my $name = "Mr. Ed";
my $horse = \$name;
So, now $horse is a reference to what will be the instance-
specific data (the name). The final step is to turn this
reference into a real instance of a "Horse" by using the
special operator "bless":
bless $horse, Horse;
This operator stores information about the package named
"Horse" into the thing pointed at by the reference. At this
point, we say $horse is an instance of "Horse". That is,
it's a specific horse. The reference is otherwise
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unchanged, and can still be used with traditional
dereferencing operators.
Invoking an instance method
The method arrow can be used on instances, as well as
classes (the names of packages). So, let's get the sound
that $horse makes:
my $noise = $horse->sound("some", "unnecessary", "args");
To invoke "sound", Perl first notes that $horse is a blessed
reference (and thus an instance). It then constructs an
argument list, as per usual.
Now for the fun part: Perl takes the class in which the
instance was blessed, in this case "Horse", and uses that
class to locate the subroutine. In this case, "Horse::sound"
is found directly (without using inheritance). In the end,
it is as though our initial line were written as follows:
my $noise = Horse::sound($horse, "some", "unnecessary", "args");
Note that the first parameter here is still the instance,
not the name of the class as before. We'll get "neigh" as
the return value, and that'll end up as the $noise variable
above.
If Horse::sound had not been found, we'd be wandering up the
@Horse::ISA array, trying to find the method in one of the
superclasses. The only difference between a class method and
an instance method is whether the first parameter is an
instance (a blessed reference) or a class name (a string).
Accessing the instance data
Because we get the instance as the first parameter, we can
now access the instance-specific data. In this case, let's
add a way to get at the name:
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
sub name {
my $self = shift;
$$self;
}
}
Inside "Horse::name", the @_ array contains:
($horse, "some", "unnecessary", "args")
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so the "shift" stores $horse into $self. Then, $self gets
de-referenced with $$self as normal, yielding "Mr. Ed".
It's traditional to "shift" the first parameter into a
variable named $self for instance methods and into a
variable named $class for class methods.
Then, the following line:
print $horse->name, " says ", $horse->sound, "\n";
outputs:
Mr. Ed says neigh.
How to build a horse
Of course, if we constructed all of our horses by hand, we'd
most likely make mistakes from time to time. We're also
violating one of the properties of object-oriented
programming, in that the "inside guts" of a Horse are
visible. That's good if you're a veterinarian, but not if
you just like to own horses. So, let's have the Horse class
handle the details inside a class method:
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
sub name {
my $self = shift; # instance method, so use $self
$$self;
}
sub named {
my $class = shift; # class method, so use $class
my $name = shift;
bless \$name, $class;
}
}
Now with the new "named" method, we can build a horse as
follows:
my $horse = Horse->named("Mr. Ed");
Notice we're back to a class method, so the two arguments to
"Horse::named" are "Horse" and "Mr. Ed". The "bless"
operator not only blesses "\$name", it also returns that
reference.
This "Horse::named" method is called a "constructor".
We've called the constructor "named" here, so that it
quickly denotes the constructor's argument as the name for
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this particular "Horse". You can use different constructors
with different names for different ways of "giving birth" to
the object (like maybe recording its pedigree or date of
birth). However, you'll find that most people coming to
Perl from more limited languages use a single constructor
named "new", with various ways of interpreting the arguments
to "new". Either style is fine, as long as you document
your particular way of giving birth to an object. (And you
were going to do that, right?)
Inheriting the constructor
But was there anything specific to "Horse" in that method?
No. Therefore, it's also the same recipe for building
anything else that inherited from "Animal", so let's put
"name" and "named" there:
{ package Animal;
sub speak {
my $class = shift;
print "a $class goes ", $class->sound, "!\n";
}
sub name {
my $self = shift;
$$self;
}
sub named {
my $class = shift;
my $name = shift;
bless \$name, $class;
}
}
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
}
Ahh, but what happens if we invoke "speak" on an instance?
my $horse = Horse->named("Mr. Ed");
$horse->speak;
We get a debugging value:
a Horse=SCALAR(0xaca42ac) goes neigh!
Why? Because the "Animal::speak" routine is expecting a
classname as its first parameter, not an instance. When the
instance is passed in, we'll end up using a blessed scalar
reference as a string, and that shows up as we saw it just
now.
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Making a method work with either classes or instances
All we need is for a method to detect if it is being called
on a class or called on an instance. The most
straightforward way is with the "ref" operator. This
returns a string (the classname) when used on a blessed
reference, and an empty string when used on a string (like a
classname). Let's modify the "name" method first to notice
the change:
sub name {
my $either = shift;
ref $either ? $$either : "Any $either";
}
Here, the "?:" operator comes in handy to select either the
dereference or a derived string. Now we can use this with
either an instance or a class. Note that I've changed the
first parameter holder to $either to show that this is
intended:
my $horse = Horse->named("Mr. Ed");
print Horse->name, "\n"; # prints "Any Horse\n"
print $horse->name, "\n"; # prints "Mr Ed.\n"
and now we'll fix "speak" to use this:
sub speak {
my $either = shift;
print $either->name, " goes ", $either->sound, "\n";
}
And since "sound" already worked with either a class or an
instance, we're done!
Adding parameters to a method
Let's train our animals to eat:
{ package Animal;
sub named {
my $class = shift;
my $name = shift;
bless \$name, $class;
}
sub name {
my $either = shift;
ref $either ? $$either : "Any $either";
}
sub speak {
my $either = shift;
print $either->name, " goes ", $either->sound, "\n";
}
sub eat {
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my $either = shift;
my $food = shift;
print $either->name, " eats $food.\n";
}
}
{ package Horse;
@ISA = qw(Animal);
sub sound { "neigh" }
}
{ package Sheep;
@ISA = qw(Animal);
sub sound { "baaaah" }
}
And now try it out:
my $horse = Horse->named("Mr. Ed");
$horse->eat("hay");
Sheep->eat("grass");
which prints:
Mr. Ed eats hay.
Any Sheep eats grass.
An instance method with parameters gets invoked with the
instance, and then the list of parameters. So that first
invocation is like:
Animal::eat($horse, "hay");
More interesting instances
What if an instance needs more data? Most interesting
instances are made of many items, each of which can in turn
be a reference or even another object. The easiest way to
store these is often in a hash. The keys of the hash serve
as the names of parts of the object (often called "instance
variables" or "member variables"), and the corresponding
values are, well, the values.
But how do we turn the horse into a hash? Recall that an
object was any blessed reference. We can just as easily
make it a blessed hash reference as a blessed scalar
reference, as long as everything that looks at the reference
is changed accordingly.
Let's make a sheep that has a name and a color:
my $bad = bless { Name => "Evil", Color => "black" }, Sheep;
so "$bad->{Name}" has "Evil", and "$bad->{Color}" has
"black". But we want to make "$bad->name" access the name,
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and that's now messed up because it's expecting a scalar
reference. Not to worry, because that's pretty easy to fix
up.
One solution is to override "Animal::name" and
"Animal::named" by defining them anew in "Sheep", but then
any methods added later to "Animal" might still mess up, and
we'd have to override all of those too. Therefore, it's
never a good idea to define the data layout in a way that's
different from the data layout of the base classes. In fact,
it's a good idea to use blessed hash references in all
cases. Also, this is why it's important to have constructors
do the low-level work. So, let's redefine "Animal":
## in Animal
sub name {
my $either = shift;
ref $either ? $either->{Name} : "Any $either";
}
sub named {
my $class = shift;
my $name = shift;
my $self = { Name => $name };
bless $self, $class;
}
Of course, we still need to override "named" in order to
handle constructing a "Sheep" with a certain color:
## in Sheep
sub named {
my ($class, $name) = @_;
my $self = $class->SUPER::named(@_);
$$self{Color} = $class->default_color;
$self
}
(Note that @_ contains the parameters to "named".)
What's this "default_color"? Well, if "named" has only the
name, we still need to set a color, so we'll have a class-
specific default color. For a sheep, we might define it as
white:
## in Sheep
sub default_color { "white" }
Now:
my $sheep = Sheep->named("Bad");
print $sheep->{Color}, "\n";
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outputs:
white
Now, there's nothing particularly specific to "Sheep" when
it comes to color, so let's remove "Sheep::named" and
implement "Animal::named" to handle color instead:
## in Animal
sub named {
my ($class, $name) = @_;
my $self = { Name => $name, Color => $class->default_color };
bless $self, $class;
}
And then to keep from having to define "default_color" for
each additional class, we'll define a method that serves as
the "default default" directly in "Animal":
## in Animal
sub default_color { "brown" }
Of course, because "name" and "named" were the only methods
that referenced the "structure" of the object, the rest of
the methods can remain the same, so "speak" still works as
before.
A horse of a different color
But having all our horses be brown would be boring. So
let's add a method or two to get and set the color.
## in Animal
sub color {
$_[0]->{Color}
}
sub set_color {
$_[0]->{Color} = $_[1];
}
Note the alternate way of accessing the arguments: $_[0] is
used in-place, rather than with a "shift". (This saves us a
bit of time for something that may be invoked frequently.)
And now we can fix that color for Mr. Ed:
my $horse = Horse->named("Mr. Ed");
$horse->set_color("black-and-white");
print $horse->name, " is colored ", $horse->color, "\n";
which results in:
Mr. Ed is colored black-and-white
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Perl Programmers Reference Guide PERLBOOT(1)
Summary
So, now we have class methods, constructors, instance
methods, instance data, and even accessors. But that's still
just the beginning of what Perl has to offer. We haven't
even begun to talk about accessors that double as getters
and setters, destructors, indirect object notation,
overloading, "isa" and "can" tests, the "UNIVERSAL" class,
and so on. That's for the rest of the Perl documentation to
cover. Hopefully, this gets you started, though.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
For more information, see perlobj (for all the gritty
details about Perl objects, now that you've seen the
basics), perltoot (the tutorial for those who already know
objects), perltooc (dealing with class data), perlbot (for
some more tricks), and books such as Damian Conway's
excellent Object Oriented Perl.
Some modules which might prove interesting are
Class::Accessor, Class::Class, Class::Contract,
Class::Data::Inheritable, Class::MethodMaker and
Tie::SecureHash
COPYRIGHT
Copyright (c) 1999, 2000 by Randal L. Schwartz and
Stonehenge Consulting Services, Inc.
Copyright (c) 2009 by Michael F. Witten.
Permission is hereby granted to distribute this document
intact with the Perl distribution, and in accordance with
the licenses of the Perl distribution; derived documents
must include this copyright notice intact.
Portions of this text have been derived from Perl Training
materials originally appearing in the Packages, References,
Objects, and Modules course taught by instructors for
Stonehenge Consulting Services, Inc. and used with
permission.
perl v5.12.5 Last change: 2012-11-03 18
Perl Programmers Reference Guide PERLBOOT(1)
Portions of this text have been derived from materials
originally appearing in Linux Magazine and used with
permission.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
perl v5.12.5 Last change: 2012-11-03 19