Current File : //usr/share/perl5/HTML/Tree/AboutObjects.pod
#Time-stamp: "2001-02-23 20:07:25 MST" -*-Text-*-
# This document contains text in Perl "POD" format.
# Use a POD viewer like perldoc or perlman to render it.
=head1 NAME
HTML::Tree::AboutObjects -- article: "User's View of Object-Oriented Modules"
=head1 SYNOPSIS
# This an article, not a module.
=head1 DESCRIPTION
The following article by Sean M. Burke first appeared in I<The Perl
Journal> #17 and is copyright 2000 The Perl Journal. It appears
courtesy of Jon Orwant and The Perl Journal. This document may be
distributed under the same terms as Perl itself.
=head1 A User's View of Object-Oriented Modules
-- Sean M. Burke
The first time that most Perl programmers run into object-oriented
programming when they need to use a module whose interface is
object-oriented. This is often a mystifying experience, since talk of
"methods" and "constructors" is unintelligible to programmers who
thought that functions and variables was all there was to worry about.
Articles and books that explain object-oriented programming (OOP), do so
in terms of how to program that way. That's understandable, and if you
learn to write object-oriented code of your own, you'd find it easy to
use object-oriented code that others write. But this approach is the
I<long> way around for people whose immediate goal is just to use
existing object-oriented modules, but who don't yet want to know all the
gory details of having to write such modules for themselves.
This article is for those programmers -- programmers who want to know
about objects from the perspective of using object-oriented modules.
=head2 Modules and Their Functional Interfaces
Modules are the main way that Perl provides for bundling up code for
later use by yourself or others. As I'm sure you can't help noticing
from reading
I<The Perl Journal>, CPAN (the Comprehensive Perl Archive
Network) is the repository for modules (or groups of modules) that
others have written, to do anything from composing music to accessing
Web pages. A good deal of those modules even come with every
installation of Perl.
One module that you may have used before, and which is fairly typical in
its interface, is Text::Wrap. It comes with Perl, so you don't even
need to install it from CPAN. You use it in a program of yours, by
having your program code say early on:
use Text::Wrap;
and after that, you can access a function called C<wrap>, which inserts
line-breaks in text that you feed it, so that the text will be wrapped to
seventy-two (or however many) columns.
The way this C<use Text::Wrap> business works is that the module
Text::Wrap exists as a file "Text/Wrap.pm" somewhere in one of your
library directories. That file contains Perl code...
=over
Footnote: And mixed in with the Perl code, there's documentation, which
is what you read with "perldoc Text::Wrap". The perldoc program simply
ignores the code and formats the documentation text, whereas "use
Text::Wrap" loads and runs the code while ignoring the documentation.
=back
...which, among other things, defines a function called C<Text::Wrap::wrap>,
and then C<exports> that function, which means that when you say C<wrap>
after having said "use Text::Wrap", you'll be actually calling the
C<Text::Wrap::wrap> function. Some modules don't export their
functions, so you have to call them by their full name, like
C<Text::Wrap::wrap(...parameters...)>.
Regardless of whether the typical module exports the functions it
provides, a module is basically just a container for chunks of code that
do useful things. The way the module allows for you to interact with
it, is its I<interface>. And when, like with Text::Wrap, its interface
consists of functions, the module is said to have a B<functional
interface>.
=over
Footnote: the term "function" (and therefore "functionI<al>") has
various senses. I'm using the term here in its broadest sense, to
refer to routines -- bits of code that are called by some name and
which take parameters and return some value.
=back
Using modules with functional interfaces is straightforward -- instead
of defining your own "wrap" function with C<sub wrap { ... }>, you
entrust "use Text::Wrap" to do that for you, along with whatever other
functions its defines and exports, according to the module's
documentation. Without too much bother, you can even write your own
modules to contain your frequently used functions; I suggest having a look at
the C<perlmod> man page for more leads on doing this.
=head2 Modules with Object-Oriented Interfaces
So suppose that one day you want to write a program that will automate
the process of C<ftp>ing a bunch of files from one server down to your
local machine, and then off to another server.
A quick browse through search.cpan.org turns up the module "Net::FTP",
which you can download and install it using normal installation
instructions (unless your sysadmin has already installed it, as many
have).
Like Text::Wrap or any other module with a familiarly functional
interface, you start off using Net::FTP in your program by saying:
use Net::FTP;
However, that's where the similarity ends. The first hint of
difference is that the documentation for Net::FTP refers to it as a
B<class>. A class is a kind of module, but one that has an
object-oriented interface.
Whereas modules like Text::Wrap
provide bits of useful code as I<functions>, to be called like
C<function(...parameters...)> or like
C<PackageName::function(...parameters...)>, Net::FTP and other modules
with object-oriented interfaces provide B<methods>. Methods are sort of
like functions in that they have a name and parameters; but methods
look different, and are different, because you have to call them with a
syntax that has a class name or an object as a special argument. I'll
explain the syntax for method calls, and then later explain what they
all mean.
Some methods are meant to be called as B<class methods>, with the class
name (same as the module name) as a special argument. Class methods
look like this:
ClassName->methodname(parameter1, parameter2, ...)
ClassName->methodname() # if no parameters
ClassName->methodname # same as above
which you will sometimes see written:
methodname ClassName (parameter1, parameter2, ...)
methodname ClassName # if no parameters
Basically all class methods are for making new objects, and methods that
make objects are called "B<constructors>" (and the process of making them
is called "constructing" or "instantiating"). Constructor methods
typically have the name "new", or something including "new"
("new_from_file", etc.); but they can conceivably be named
anything -- DBI's constructor method is named "connect", for example.
The object that a constructor method returns is
typically captured in a scalar variable:
$object = ClassName->new(param1, param2...);
Once you have an object (more later on exactly what that is), you can
use the other kind of method call syntax, the syntax for B<object method>
calls. Calling object methods is just like class methods, except
that instead of the ClassName as the special argument,
you use an expression that yields an "object". Usually this is
just a scalar variable that you earlier captured the
output of the constructor in. Object method calls look like this:
$object->methodname(parameter1, parameter2, ...);
$object->methodname() # if no parameters
$object->methodname # same as above
which is occasionally written as:
methodname $object (parameter1, parameter2, ...)
methodname $object # if no parameters
Examples of method calls are:
my $session1 = Net::FTP->new("ftp.myhost.com");
# Calls a class method "new", from class Net::FTP,
# with the single parameter "ftp.myhost.com",
# and saves the return value (which is, as usual,
# an object), in $session1.
# Could also be written:
# new Net::FTP('ftp.myhost.com')
$session1->login("sburke","aoeuaoeu")
|| die "failed to login!\n";
# calling the object method "login"
print "Dir:\n", $session1->dir(), "\n";
$session1->quit;
# same as $session1->quit()
print "Done\n";
exit;
Incidentally, I suggest always using the syntaxes with parentheses and
"->" in them,
=over
Footnote: the character-pair "->" is supposed to look like an
arrow, not "negative greater-than"!
=back
and avoiding the syntaxes that start out "methodname $object" or
"methodname ModuleName". When everything's going right, they all mean
the same thing as the "->" variants, but the syntax with "->" is more
visually distinct from function calls, as well as being immune to some
kinds of rare but puzzling ambiguities that can arise when you're trying
to call methods that have the same name as subroutines you've defined.
But, syntactic alternatives aside, all this talk of constructing objects
and object methods begs the question -- what I<is> an object? There are
several angles to this question that the rest of this article will
answer in turn: what can you do with objects? what's in an object?
what's an object value? and why do some modules use objects at all?
=head2 What Can You Do with Objects?
You've seen that you can make objects, and call object methods with
them. But what are object methods for? The answer depends on the class:
A Net::FTP object represents a session between your computer and an FTP
server. So the methods you call on a Net::FTP object are for doing
whatever you'd need to do across an FTP connection. You make the
session and log in:
my $session = Net::FTP->new('ftp.aol.com');
die "Couldn't connect!" unless defined $session;
# The class method call to "new" will return
# the new object if it goes OK, otherwise it
# will return undef.
$session->login('sburke', 'p@ssw3rD')
|| die "Did I change my password again?";
# The object method "login" will give a true
# return value if actually logs in, otherwise
# it'll return false.
You can use the session object to change directory on that session:
$session->cwd("/home/sburke/public_html")
|| die "Hey, that was REALLY supposed to work!";
# if the cwd fails, it'll return false
...get files from the machine at the other end of the session...
foreach my $f ('log_report_ua.txt', 'log_report_dom.txt',
'log_report_browsers.txt')
{
$session->get($f) || warn "Getting $f failed!"
};
...and plenty else, ending finally with closing the connection:
$session->quit();
In short, object methods are for doing things related to (or with)
whatever the object represents. For FTP sessions, it's about sending
commands to the server at the other end of the connection, and that's
about it -- there, methods are for doing something to the world outside
the object, and the objects is just something that specifies what bit
of the world (well, what FTP session) to act upon.
With most other classes, however, the object itself stores some kind of
information, and it typically makes no sense to do things with such an
object without considering the data that's in the object.
=head2 What's I<in> an Object?
An object is (with rare exceptions) a data structure containing a
bunch of attributes, each of which has a value, as well as a name
that you use when you
read or set the attribute's value. Some of the object's attributes are
private, meaning you'll never see them documented because they're not
for you to read or write; but most of the object's documented attributes
are at least readable, and usually writeable, by you. Net::FTP objects
are a bit thin on attributes, so we'll use objects from the class
Business::US_Amort for this example. Business::US_Amort is a very
simple class (available from CPAN) that I wrote for making calculations
to do with loans (specifically, amortization, using US-style
algorithms).
An object of the class Business::US_Amort represents a loan with
particular parameters, i.e., attributes. The most basic attributes of a
"loan object" are its interest rate, its principal (how much money it's
for), and it's term (how long it'll take to repay). You need to set
these attributes before anything else can be done with the object. The
way to get at those attributes for loan objects is just like the
way to get at attributes for any class's objects: through accessors.
An B<accessor> is simply any method that accesses (whether reading or
writing, AKA getting or putting) some attribute in the given object.
Moreover, accessors are the B<only> way that you can change
an object's attributes. (If a module's documentation wants you to
know about any other way, it'll tell you.)
Usually, for simplicity's sake, an accessor is named after the attribute
it reads or writes. With Business::US_Amort objects, the accessors you
need to use first are C<principal>, C<interest_rate>, and C<term>.
Then, with at least those attributes set, you can call the C<run> method
to figure out several things about the loan. Then you can call various
accessors, like C<total_paid_toward_interest>, to read the results:
use Business::US_Amort;
my $loan = Business::US_Amort->new;
# Set the necessary attributes:
$loan->principal(123654);
$loan->interest_rate(9.25);
$loan->term(20); # twenty years
# NOW we know enough to calculate:
$loan->run;
# And see what came of that:
print
"Total paid toward interest: A WHOPPING ",
$loan->total_paid_interest, "!!\n";
This illustrates a convention that's common with accessors: calling the
accessor with no arguments (as with $loan->total_paid_interest) usually
means to read the value of that attribute, but providing a value (as
with $loan->term(20)) means you want that attribute to be set to that
value. This stands to reason: why would you be providing a value, if
not to set the attribute to that value?
Although a loan's term, principal, and interest rates are all single
numeric values, an objects values can any kind of scalar, or an array,
or even a hash. Moreover, an attribute's value(s) can be objects
themselves. For example, consider MIDI files (as I wrote about in
TPJ#13): a MIDI file usually consists of several tracks. A MIDI file is
complex enough to merit being an object with attributes like its overall
tempo, the file-format variant it's in, and the list of instrument
tracks in the file. But tracks themselves are complex enough to be
objects too, with attributes like their track-type, a list of MIDI
commands if they're a MIDI track, or raw data if they're not. So I
ended up writing the MIDI modules so that the "tracks" attribute of a
MIDI::Opus object is an array of objects from the class MIDI::Track.
This may seem like a runaround -- you ask what's in one object, and get
I<another> object, or several! But in this case, it exactly reflects
what the module is for -- MIDI files contain MIDI tracks, which then
contain data.
=head2 What is an Object Value?
When you call a constructor like Net::FTP->new(I<hostname>), you get
back an object value, a value you can later use, in combination with a
method name, to call object methods.
Now, so far we've been pretending, in the above examples, that the
variables $session or $loan I<are> the objects you're dealing with.
This idea is innocuous up to a point, but it's really a misconception
that will, at best, limit you in what you know how to do. The reality
is not that the variables $session or $query are objects; it's a little
more indirect -- they I<hold> values that symbolize objects. The kind of
value that $session or $query hold is what I'm calling an object value.
To understand what kind of value this is, first think about the other
kinds of scalar values you know about: The first two scalar values you
probably ever ran into in Perl are B<numbers> and B<strings>, which you
learned (or just assumed) will usually turn into each other on demand;
that is, the three-character string "2.5" can become the quantity two
and a half, and vice versa. Then, especially if you started using
C<perl -w> early on, you learned about the B<undefined value>, which can
turn into 0 if you treat it as a number, or the empty-string if you
treat it as a string.
=over
Footnote: You may I<also> have been learning about references, in which
case you're ready to hear that object values are just a kind of
reference, except that they reflect the class that created thing they point
to, instead of merely being a plain old array reference, hash reference,
etc. I<If> this makes makes sense to you, and you want to know more
about how objects are implemented in Perl, have a look at the
C<perltoot> man page.
=back
And now you're learning about B<object values>. An object value is a
value that points to a data structure somewhere in memory, which is
where all the attributes for this object are stored. That data
structure as a whole belongs to a class (probably the one you named in
the constructor method, like ClassName->new), so that the object value
can be used as part of object method calls.
If you want to actually I<see> what an object value is, you might try
just saying "print $object". That'll get you something like this:
Net::FTP=GLOB(0x20154240)
or
Business::US_Amort=HASH(0x15424020)
That's not very helpful if you wanted to really get at the object's
insides, but that's because the object value is only a symbol for the
object. This may all sound very abstruse and metaphysical, so a
real-world allegory might be very helpful:
=over
You get an advertisement in the mail saying that you have been
(im)personally selected to have the rare privilege of applying for a
credit card. For whatever reason, I<this> offer sounds good to you, so you
fill out the form and mail it back to the credit card company. They
gleefully approve the application and create your account, and send you
a card with a number on it.
Now, you can do things with the number on that card -- clerks at stores
can ring up things you want to buy, and charge your account by keying in
the number on the card. You can pay for things you order online by
punching in the card number as part of your online order. You can pay
off part of the account by sending the credit card people some of your
money (well, a check) with some note (usually the pre-printed slip)
that has the card number for the account you want to pay toward. And you
should be able to call the credit card company's computer and ask it
things about the card, like its balance, its credit limit, its APR, and
maybe an itemization of recent purchases ad payments.
Now, what you're I<really> doing is manipulating a credit card
I<account>, a completely abstract entity with some data attached to it
(balance, APR, etc). But for ease of access, you have a credit card
I<number> that is a symbol for that account. Now, that symbol is just a
bunch of digits, and the number is effectively meaningless and useless
in and of itself -- but in the appropriate context, it's understood to
I<mean> the credit card account you're accessing.
=back
This is exactly the relationship between objects and object values, and
from this analogy, several facts about object values are a bit more
explicable:
* An object value does nothing in and of itself, but it's useful when
you use it in the context of an $object->method call, the same way that
a card number is useful in the context of some operation dealing with a
card account.
Moreover, several copies of the same object value all refer to the same
object, the same way that making several copies of your card number
won't change the fact that they all still refer to the same single
account (this is true whether you're "copying" the number by just
writing it down on different slips of paper, or whether you go to the
trouble of forging exact replicas of your own plastic credit card). That's
why this:
$x = Net::FTP->new("ftp.aol.com");
$x->login("sburke", "aoeuaoeu");
does the same thing as this:
$x = Net::FTP->new("ftp.aol.com");
$y = $x;
$z = $y;
$z->login("sburke", "aoeuaoeu");
That is, $z and $y and $x are three different I<slots> for values,
but what's in those slots are all object values pointing to the same
object -- you don't have three different FTP connections, just three
variables with values pointing to the some single FTP connection.
* You can't tell much of anything about the object just by looking at
the object value, any more than you can see your credit account balance
by holding the plastic card up to the light, or by adding up the digits
in your credit card number.
* You can't just make up your own object values and have them work --
they can come only from constructor methods of the appropriate class.
Similarly, you get a credit card number I<only> by having a bank approve
your application for a credit card account -- at which point I<they>
let I<you> know what the number of your new card is.
Now, there's even more to the fact that you can't just make up your own
object value: even though you can print an object value and get a string
like "Net::FTP=GLOB(0x20154240)", that string is just a
I<representation> of an object value.
Internally, an object value has a basically different type from a
string, or a number, or the undefined value -- if $x holds a real
string, then that value's slot in memory says "this is a value of type
I<string>, and its characters are...", whereas if it's an object value,
the value's slot in memory says, "this is a value of type I<reference>,
and the location in memory that it points to is..." (and by looking at
what's at that location, Perl can tell the class of what's there).
Perl programmers typically don't have to think about all these details
of Perl's internals. Many other languages force you to be more
conscious of the differences between all of these (and also between
types of numbers, which are stored differently depending on their size
and whether they have fractional parts). But Perl does its best to
hide the different types of scalars from you -- it turns numbers into
strings and back as needed, and takes the string or number
representation of undef or of object values as needed. However, you
can't go from a string representation of an object value, back to an
object value. And that's why this doesn't work:
$x = Net::FTP->new('ftp.aol.com');
$y = Net::FTP->new('ftp.netcom.com');
$z = Net::FTP->new('ftp.qualcomm.com');
$all = join(' ', $x,$y,$z); # !!!
...later...
($aol, $netcom, $qualcomm) = split(' ', $all); # !!!
$aol->login("sburke", "aoeuaoeu");
$netcom->login("sburke", "qjkxqjkx");
$qualcomm->login("smb", "dhtndhtn");
This fails because $aol ends up holding merely the B<string representation>
of the object value from $x, not the object value itself -- when
C<join> tried to join the characters of the "strings" $x, $y, and $z,
Perl saw that they weren't strings at all, so it gave C<join> their
string representations.
Unfortunately, this distinction between object values and their string
representations doesn't really fit into the analogy of credit card
numbers, because credit card numbers really I<are> numbers -- even
thought they don't express any meaningful quantity, if you stored them
in a database as a quantity (as opposed to just an ASCII string),
that wouldn't stop them from being valid as credit card numbers.
This may seem rather academic, but there's there's two common mistakes
programmers new to objects often make, which make sense only in terms of
the distinction between object values and their string representations:
The first common error involves forgetting (or never having known in the
first place) that when you go to use a value as a hash key, Perl uses
the string representation of that value. When you want to use the
numeric value two and a half as a key, Perl turns it into the
three-character string "2.5". But if you then want to use that string
as a number, Perl will treat it as meaning two and a half, so you're
usually none the wiser that Perl converted the number to a string and
back. But recall that Perl can't turn strings back into objects -- so
if you tried to use a Net::FTP object value as a hash key, Perl actually
used its string representation, like "Net::FTP=GLOB(0x20154240)", but
that string is unusable as an object value. (Incidentally, there's
a module Tie::RefHash that implements hashes that I<do> let you use
real object-values as keys.)
The second common error with object values is in
trying to save an object value to disk (whether printing it to a
file, or storing it in a conventional database file). All you'll get is the
string, which will be useless.
When you want to save an object and restore it later, you may find that
the object's class already provides a method specifically for this. For
example, MIDI::Opus provides methods for writing an object to disk as a
standard MIDI file. The file can later be read back into memory by
a MIDI::Opus constructor method, which will return a new MIDI::Opus
object representing whatever file you tell it to read into memory.
Similar methods are available with, for example, classes that
manipulate graphic images and can save them to files, which can be read
back later.
But some classes, like Business::US_Amort, provide no such methods for
storing an object in a file. When this is the case, you can try
using any of the Data::Dumper, Storable, or FreezeThaw modules. Using
these will be unproblematic for objects of most classes, but it may run
into limitations with others. For example, a Business::US_Amort
object can be turned into a string with Data::Dumper, and that string
written to a file. When it's restored later, its attributes will be
accessible as normal. But in the unlikely case that the loan object was
saved in mid-calculation, the calculation may not be resumable. This is
because of the way that that I<particular> class does its calculations,
but similar limitations may occur with objects from other classes.
But often, even I<wanting> to save an object is basically wrong -- what would
saving an ftp I<session> even mean? Saving the hostname, username, and
password? current directory on both machines? the local TCP/IP port
number? In the case of "saving" a Net::FTP object, you're better off
just saving whatever details you actually need for your own purposes,
so that you can make a new object later and just set those values for it.
=head2 So Why Do Some Modules Use Objects?
All these details of using objects are definitely enough to make you
wonder -- is it worth the bother? If you're a module author, writing
your module with an object-oriented interface restricts the audience of
potential users to those who understand the basic concepts of objects
and object values, as well as Perl's syntax for calling methods. Why
complicate things by having an object-oriented interface?
A somewhat esoteric answer is that a module has an object-oriented
interface because the module's insides are written in an
object-oriented style. This article is about the basics of
object-oriented I<interfaces>, and it'd be going far afield to explain
what object-oriented I<design> is. But the short story is that
object-oriented design is just one way of attacking messy problems.
It's a way that many programmers find very helpful (and which others
happen to find to be far more of a hassle than it's worth,
incidentally), and it just happens to show up for you, the module user,
as merely the style of interface.
But a simpler answer is that a functional interface is sometimes a
hindrance, because it limits the number of things you can do at once --
limiting it, in fact, to one. For many problems that some modules are
meant to solve, doing without an object-oriented interface would be like
wishing that Perl didn't use filehandles. The ideas are rather simpler
-- just imagine that Perl let you access files, but I<only> one at a
time, with code like:
open("foo.txt") || die "Can't open foo.txt: $!";
while(readline) {
print $_ if /bar/;
}
close;
That hypothetical kind of Perl would be simpler, by doing without
filehandles. But you'd be out of luck if you wanted to read from
one file while reading from another, or read from two and print to a
third.
In the same way, a functional FTP module would be fine for just
uploading files to one server at a time, but it wouldn't allow you to
easily write programs that make need to use I<several> simultaneous
sessions (like "look at server A and server B, and if A has a file
called X.dat, then download it locally and then upload it to server B --
except if B has a file called Y.dat, in which case do it the other way
around").
Some kinds of problems that modules solve just lend themselves to an
object-oriented interface. For those kinds of tasks, a functional
interface would be more familiar, but less powerful. Learning to use
object-oriented modules' interfaces does require becoming comfortable
with the concepts from this article. But in the end it will allow you
to use a broader range of modules and, with them, to write programs
that can do more.
B<[end body of article]>
=head2 [Author Credit]
Sean M. Burke has contributed several modules to CPAN, about half of
them object-oriented.
[The next section should be in a greybox:]
=head2 The Gory Details
For sake of clarity of explanation, I had to oversimplify some of the
facts about objects. Here's a few of the gorier details:
* Every example I gave of a constructor was a class method. But object
methods can be constructors, too, if the class was written to work that
way: $new = $old->copy, $node_y = $node_x->new_subnode, or the like.
* I've given the impression that there's two kinds of methods: object
methods and class methods. In fact, the same method can be both,
because it's not the kind of method it is, but the kind of calls it's
written to accept -- calls that pass an object, or calls that pass a
class-name.
* The term "object value" isn't something you'll find used much anywhere
else. It's just my shorthand for what would properly be called an
"object reference" or "reference to a blessed item". In fact, people
usually say "object" when they properly mean a reference to that object.
* I mentioned creating objects with I<con>structors, but I didn't
mention destroying them with I<de>structor -- a destructor is a kind of
method that you call to tidy up the object once you're done with it, and
want it to neatly go away (close connections, delete temporary files,
free up memory, etc). But because of the way Perl handles memory,
most modules won't require the user to know about destructors.
* I said that class method syntax has to have the class name, as in
$session = B<Net::FTP>->new($host). Actually, you can instead use any
expression that returns a class name: $ftp_class = 'Net::FTP'; $session
= B<$ftp_class>->new($host). Moreover, instead of the method name for
object- or class-method calls, you can use a scalar holding the method
name: $foo->B<$method>($host). But, in practice, these syntaxes are
rarely useful.
And finally, to learn about objects from the perspective of writing
your own classes, see the C<perltoot> documentation,
or Damian Conway's exhaustive and clear book I<Object Oriented Perl>
(Manning Publications 1999, ISBN 1-884777-79-1).
=head1 BACK
Return to the L<HTML::Tree|HTML::Tree> docs.
=cut
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