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HTML::Tree::AboutObjecUserpContributed Perl DocumHTML::Tree::AboutObjects(3pm)



NAME
       HTML::Tree::AboutObjects -- article: "User's View of Object-Oriented
       Modules"

SYNOPSIS
         # This an article, not a module.

DESCRIPTION
       The following article by Sean M. Burke first appeared in The Perl Jour-
       nal #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.

A User's View of Object-Oriented Modules
       -- Sean M. Burke

       The first time that most Perl programmers run into object-oriented pro-
       gramming when they need to use a module whose interface is object-ori-
       ented.  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 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.

       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 The Perl Journal, CPAN (the Comprehensive Perl Archive
       Network) is the repository for modules (or groups of modules) that oth-
       ers 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 "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 "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...

           Footnote: And mixed in with the Perl code, there's documentation,
           which is what you read with "perldoc Text::Wrap".  The perldoc pro-
           gram simply ignores the code and formats the documentation text,
           whereas "use Text::Wrap" loads and runs the code while ignoring the
           documentation.

       ...which, among other things, defines a function called
       "Text::Wrap::wrap", and then "exports" that function, which means that
       when you say "wrap" after having said "use Text::Wrap", you'll be actu-
       ally calling the "Text::Wrap::wrap" function.  Some modules don't
       export their functions, so you have to call them by their full name,
       like "Text::Wrap::wrap(...parameters...)".

       Regardless of whether the typical module exports the functions it pro-
       vides, 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 interface.  And when, like with Text::Wrap, its interface
       consists of functions, the module is said to have a functional inter-
       face.

           Footnote: the term "function" (and therefore "functional") has var-
           ious 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.

       Using modules with functional interfaces is straightforward -- instead
       of defining your own "wrap" function with "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 documenta-
       tion.  Without too much bother, you can even write your own modules to
       contain your frequently used functions; I suggest having a look at the
       "perlmod" man page for more leads on doing this.

       Modules with Object-Oriented Interfaces

       So suppose that one day you want to write a program that will automate
       the process of "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 inter-
       face, 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 differ-
       ence is that the documentation for Net::FTP refers to it as a class.  A
       class is a kind of module, but one that has an object-oriented inter-
       face.

       Whereas modules like Text::Wrap provide bits of useful code as func-
       tions, to be called like "function(...parameters...)" or like "Package-
       Name::function(...parameters...)", Net::FTP and other modules with
       object-oriented interfaces provide 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 syn-
       tax 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 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 "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 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 yeilds 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,

           Footnote: the character-pair "->" is supposed to look like an
           arrow, not "negative greater-than"!

       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 try-
       ing 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 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?

       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 ses-
       sion 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.

       What's 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.  Busi-
       ness::US_Amort is a very simple class (available from CPAN) that I
       wrote for making calculations to do with loans (specifically, amortiza-
       tion, using US-style algorithms).

       An object of the class Business::US_Amort represents a loan with par-
       ticular 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
       accessor is simply any method that accesses (whether reading or writ-
       ing, AKA getting or putting) some attribute in the given object.  More-
       over, accessors are the 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 "principal", "interest_rate", and
       "term".  Then, with at least those attributes set, you can call the
       "run" method to figure out several things about the loan.  Then you can
       call various accessors, like "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 them-
       selves.  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 writ-
       ing 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 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.

       What is an Object Value?

       When you call a constructor like Net::FTP->new(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 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 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 numbers and 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
       "perl -w" early on, you learned about the 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.

           Footnote: You may 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.  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 "perltoot" man page.

       And now you're learning about 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 struc-
       ture 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 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:

           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, this offer sounds good to you,
           so you fill out the form and mail it back to the credit card com-
           pany.  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 (usu-
           ally 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 really doing is manipulating a credit card
           account, a completely abstract entity with some data attached to it
           (balance, APR, etc).  But for ease of access, you have a credit
           card 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 mean the credit card account you're accessing.

       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 writ-
       ing it down on different slips of paper, or whether you go to the trou-
       ble 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 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 only by having a bank approve
       your application for a credit card account -- at which point they let
       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 repre-
       sentation 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
       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 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 con-
       scious 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 representa-
       tion 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 string representa-
       tion of the object value from $x, not the object value itself -- when
       "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 "join" their
       string representations.

       Unfortunately, this distinction between object values and their string
       representations doesn't really fit into the analogy of credit card num-
       bers, because credit card numbers really 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 representa-
       tions:

       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 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 manipu-
       late 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 accessable 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 particular class does its calculations, but simi-
       lar limitations may occur with objects from other classses.

       But often, even wanting to save an object is basically wrong -- what
       would saving an ftp 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 pur-
       poses, so that you can make a new object later and just set those val-
       ues for it.

       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-ori-
       ented style.  This article is about the basics of object-oriented
       interfaces, and it'd be going far afield to explain what object-ori-
       ented 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 pro-
       grammers 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 hin-
       drance, 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 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 file-
       handles.  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 upload-
       ing files to one server at a time, but it wouldn't allow you to easily
       write programs that make need to use 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.

       [end body of article]

       [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:]

       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 any-
       where else.  It's just my shorthand for what would properly be called
       an "object reference" or "reference to a blessed item".  In fact, peo-
       ple usually say "object" when they properly mean a reference to that
       object.

       * I mentioned creating objects with constructors, but I didn't mention
       destroying them with destructor -- 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 mod-
       ules won't require the user to know about destructors.

       * I said that class method syntax has to have the class name, as in
       $session = Net::FTP->new($host).  Actually, you can instead use any
       expression that returns a class name: $ftp_class = 'Net::FTP'; $session
       = $ftp_class->new($host).  Moreover, instead of the method name for
       object- or class-method calls, you can use a scalar holding the method
       name: $foo->$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 "perltoot" documentation, or Damian Conway's
       exhaustive and clear book Object Oriented Perl (Manning Publications
       1999, ISBN 1-884777-79-1).

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perl v5.8.8                       2006-11-15     HTML::Tree::AboutObjects(3pm)