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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 the perlobj manpage, a basic reference in
     using objects, and the perltoot manpage, 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
     (the perlsub manpage), references (the perlref manpage et.
     seq.), and packages (the perlmod manpage), 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:
         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 subs' 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:


         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:

         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':



         { 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.  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 (pronounced "is a" not "ice-
     uh"), 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'.  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 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 allow it as an implicitly named package variable:

         package Cow;
         use vars qw(@ISA);
         @ISA = qw(Animal);

     If you're bringing in the class from outside, via an
     object-oriented module, you change:

         package Cow;
         use Animal;
         use vars qw(@ISA);
         @ISA = qw(Animal);

     into just:

         package Cow;
         use base qw(Animal);

     And that's pretty darn compact.

     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 didn't even need
     to say that a `Mouse' was an `Animal' at all, since all of
     the methods needed for `speak' are completely defined with
     `Mouse'.

     But we've now duplicated some of the code from
     `Animal->speak', and this can once again be a maintenance
     headache.  So, can we avoid that?  Can we say somehow that a
     `Mouse' does everything any other `Animal' does, but add in
     the extra comment?  Sure!

     First, we can invoke the `Animal::speak' method 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 have to include the `$class' parameter (almost
     surely the value of `"Mouse"') as the first parameter to
     `Animal::speak', since we've stopped using the method arrow.
     Why did we stop?  Well, if we invoke `Animal->speak' there,
     the first parameter to the method will be `"Animal"' not
     `"Mouse"', and when time comes for it to call for the
     `sound', it won't have the right class to come back to this
     package.

     Invoking `Animal::speak' directly is a mess, however.  What
     if `Animal::speak' didn't exist before, and was being
     inherited from a class mentioned in `@Animal::ISA'?  Because
     we are no longer using the method arrow, we get one and only
     one chance to hit the right subroutine.

     Also note that the `Animal' classname is now hardwired into
     the subroutine selection.  This is a mess if someone
     maintains the code, changing `@ISA' for <Mouse> and didn't
     notice `Animal' there in `speak'.  So, this is probably not
     the right way to go.

     Starting the search from a different place

     A better solution is to tell Perl to search from a higher
     place in the inheritance chain:


         # 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";
           }
         }

     Ahh.  This works.  Using this syntax, we start with `Animal'
     to find `speak', and use all of `Animal''s inheritance chain
     if not found immediately.  And yet the first parameter will
     be `$class', so the found `speak' method will get `Mouse' as
     its first entry, and eventually work its way back to
     `Mouse::sound' for the details.

     But this isn't the best solution.  We still have to keep the
     `@ISA' and the initial search package coordinated.  Worse,
     if `Mouse' had multiple entries in `@ISA', we wouldn't
     necessarily know which one had actually defined `speak'.
     So, is there an even better way?

     The SUPER way of doing things

     By changing the `Animal' class to the `SUPER' class in that
     invocation, we get a search of all of our super classes
     (classes listed in `@ISA') automatically:

         # 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";
           }
         }

     So, `SUPER::speak' means look in the current package's
     `@ISA' for `speak', invoking the first one found.

     Where we're at so far...

     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 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, and provide a nice way to reuse implementations with
     variations.  This is at the core of what objects provide,
     but objects also provide instance data, which we haven't
     even begun to cover.

     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 I add a subroutine, all horses automatically
     share it.  That's great for making horses the same, but how
     do we capture the distinctions about an individual horse?
     For example, suppose I want to give my first horse a name.
     There's got to be a way to keep its name separate from the
     other horses.


     We can do that by drawing a new distinction, called an
     "instance".  An "instance" is generally created by a class.
     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 $talking = \$name;

     So now `$talking' is a reference to what will be the
     instance-specific data (the name).  The final step in
     turning this into a real instance is with a special operator
     called `bless':

       bless $talking, Horse;

     This operator stores information about the package named
     `Horse' into the thing pointed at by the reference.  At this
     point, we say `$talking' is an instance of `Horse'.  That
     is, it's a specific horse.  The reference is otherwise
     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 names
     of packages (classes).  So, let's get the sound that
     `$talking' makes:

       my $noise = $talking->sound;

     To invoke `sound', Perl first notes that `$talking' is a
     blessed reference (and thus an instance).  It then
     constructs an argument list, in this case from just
     `($talking)'.  (Later we'll see that arguments will take
     their place following the instance variable, just like with
     classes.)

     Now for the fun part: Perl takes the class in which the
     instance was blessed, in this case `Horse', and uses that to
     locate the subroutine to invoke the method.  In this case,
     `Horse::sound' is found directly (without using
     inheritance), yielding the final subroutine invocation:

       Horse::sound($talking)

     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' list to try to find the method in one of the
     superclasses, just as for a class method.  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;
         }
       }

     Now we call for the name:

       print $talking->name, " says ", $talking->sound, "\n";

     Inside `Horse::name', the `@_' array contains just
     `$talking', which the `shift' stores into `$self'.  (It's
     traditional to shift the first parameter off into a variable
     named `$self' for instance methods, so stay with that unless
     you have strong reasons otherwise.)  Then, `$self' gets de-
     referenced as a scalar ref, yielding `Mr. Ed', and we're
     done with that.  The result is:

       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 let the Horse class
     build a new horse:



       { package Horse;
         @ISA = qw(Animal);
         sub sound { "neigh" }
         sub name {
           my $self = shift;
           $$self;
         }
         sub named {
           my $class = shift;
           my $name = shift;
           bless \$name, $class;
         }
       }

     Now with the new `named' method, we can build a horse:

       my $talking = 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 `$name', it also returns the
     reference to `$name', so that's fine as a return value.  And
     that's how to build a horse.

     We've called the constructor `named' here, so that it
     quickly denotes the constructor's argument as the name for
     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 it
     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 $talking = Horse->named("Mr. Ed");
       $talking->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.

     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 `undef' 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 # it's an instance, return name
           : "an unnamed $either"; # it's a class, return generic
       }

     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 $talking = Horse->named("Mr. Ed");
       print Horse->name, "\n"; # prints "an unnamed Horse\n"
       print $talking->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 # it's an instance, return name
             : "an unnamed $either"; # it's a class, return generic
         }
         sub speak {
           my $either = shift;
           print $either->name, " goes ", $either->sound, "\n";
         }
         sub eat {
           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 $talking = Horse->named("Mr. Ed");
       $talking->eat("hay");
       Sheep->eat("grass");

     which prints:

       Mr. Ed eats hay.
       an unnamed 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($talking, "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,
     and that's now messed up because it's expecting a scalar
     reference.  Not to worry, because that's pretty easy to fix
     up:

       ## in Animal
       sub name {
         my $either = shift;
         ref $either ?
           $either->{Name} :
           "an unnamed $either";
       }

     And of course `named' still builds a scalar sheep, so let's
     fix that as well:

       ## in Animal
       sub named {
         my $class = shift;
         my $name = shift;
         my $self = { Name => $name, Color => $class->default_color };
         bless $self, $class;
       }

     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 initial color.  For a sheep, we might define it as
     white:

       ## in Sheep
       sub default_color { "white" }

     And then to keep from having to define one for each
     additional class, we'll define a "backstop" method that
     serves as the "default default", directly in `Animal':

       ## in Animal
       sub default_color { "brown" }

     Now, 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 $talking = Horse->named("Mr. Ed");
       $talking->set_color("black-and-white");
       print $talking->name, " is colored ", $talking->color, "\n";

     which results in:

       Mr. Ed is colored black-and-white


     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,
     subclasses that add instance data, per-class data,
     overloading, "isa" and "can" tests, `UNIVERSAL' class, and
     so on.  That's for the rest of the Perl documentation to
     cover.  Hopefully, this gets you started, though.




SEE ALSO
     For more information, see the perlobj manpage (for all the
     gritty details about Perl objects, now that you've seen the
     basics), the perltoot manpage (the tutorial for those who
     already know objects), the perlbot manpage (for some more
     tricks), and books such as Damian Conway's excellent Object
     Oriented Perl.



COPYRIGHT
     Copyright (c) 1999, 2000 by Randal L. Schwartz and
     Stonehenge Consulting Services, Inc.  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.

     Portions of this text have been derived from materials
     originally appearing in Linux Magazine and used with
     permission.









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