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PERLOBJ

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
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NAME

perlobj - Perl objects

DESCRIPTION

First of all, you need to understand what references are in Perl.See the perlref manpage for that. Second, if you still find the followingreference work too complicated, a tutorial on object-oriented programmingin Perl can be found in the perltoot manpage.

If you're still with us, thenhere are three very simple definitions that you should find reassuring.

1.: An object is simply a reference that happens to know which class itbelongs to.
2.: A class is simply a package that happens to provide methods to dealwith object references.
3.: A method is simply a subroutine that expects an object reference (ora package name, for class methods) as the first argument.

We'll cover these points now in more depth.

An Object is Simply a Reference

Unlike say C++, Perl doesn't provide any special syntax forconstructors. A constructor is merely a subroutine that returns areference to something ``blessed'' into a class, generally theclass that the subroutine is defined in. Here is a typicalconstructor:

    package Critter;    sub new { bless {} }

The {} constructs a reference to an anonymous hash containing nokey/value pairs. The bless() takes that reference and tells the objectit references that it's now a Critter, and returns the reference.This is for convenience, because the referenced object itself knows thatit has been blessed, and the reference to it could have been returneddirectly, like this:

    sub new {        my $self = {};        bless $self;        return $self;    }

In fact, you often see such a thing in more complicated constructorsthat wish to call methods in the class as part of the construction:

    sub new {        my $self = {}        bless $self;        $self->initialize();        return $self;    }

If you care about inheritance (and you should; seethe section on Modules: Creation, Use, and Abuse in the perlmod manpage),then you want to use the two-arg form of blessso that your constructors may be inherited:

    sub new {        my $class = shift;        my $self = {};        bless $self, $class        $self->initialize();        return $self;    }

Or if you expect people to call not just CLASS->new() but also$obj->new(), then use something like this. The initialize()method used will be of whatever $class we blessed theobject into:

    sub new {        my $this = shift;        my $class = ref($this) || $this;        my $self = {};        bless $self, $class        $self->initialize();        return $self;    }

Within the class package, the methods will typically deal with thereference as an ordinary reference. Outside the class package,the reference is generally treated as an opaque value that maybe accessed only through the class's methods.

A constructor may re-bless a referenced object currently belonging toanother class, but then the new class is responsible for all cleanuplater. The previous blessing is forgotten, as an object may belongto only one class at a time. (Although of course it's free toinherit methods from many classes.)

A clarification: Perl objects are blessed. References are not. Objectsknow which package they belong to. References do not. The bless()function uses the reference to find the object. Considerthe following example:

    $a = {};    $b = $a;    bless $a, BLAH;    print "\$b is a ", ref($b), "\n";

This reports $b as being a BLAH, so obviously bless()operated on the object and not on the reference.

A Class is Simply a Package

Unlike say C++, Perl doesn't provide any special syntax for classdefinitions. You use a package as a class by putting methoddefinitions into the class.

There is a special array within each package called @ISA which sayswhere else to look for a method if you can't find it in the currentpackage. This is how Perl implements inheritance. Each element of the@ISA array is just the name of another package that happens to be aclass package. The classes are searched (depth first) for missingmethods in the order that they occur in @ISA. The classes accessiblethrough @ISA are known as base classes of the current class.

If a missing method is found in one of the base classes, it is cachedin the current class for efficiency. Changing @ISA or defining newsubroutines invalidates the cache and causes Perl to do the lookup again.

If a method isn't found, but an AUTOLOAD routine is found, thenthat is called on behalf of the missing method.

If neither a method nor an AUTOLOAD routine is found in @ISA, then onelast try is made for the method (or an AUTOLOAD routine) in a classcalled UNIVERSAL. (Several commonly used methods are automaticallysupplied in the UNIVERSAL class; see the section on Default UNIVERSAL methods formore details.) If that doesn't work, Perl finally gives up andcomplains.

Perl classes do only method inheritance. Data inheritance is leftup to the class itself. By and large, this is not a problem in Perl,because most classes model the attributes of their object usingan anonymous hash, which serves as its own little namespace to becarved up by the various classes that might want to do somethingwith the object.

A Method is Simply a Subroutine

Unlike say C++, Perl doesn't provide any special syntax for methoddefinition. (It does provide a little syntax for method invocationthough. More on that later.) A method expects its first argumentto be the object or package it is being invoked on. There are just twotypes of methods, which we'll call class and instance.(Sometimes you'll hear these called static and virtual, in honor ofthe two C++ method types they most closely resemble.)

A class method expects a class name as the first argument. Itprovides functionality for the class as a whole, not for any individualobject belonging to the class. Constructors are typically classmethods. Many class methods simply ignore their first argument, becausethey already know what package they're in, and don't care what packagethey were invoked via. (These aren't necessarily the same, becauseclass methods follow the inheritance tree just like ordinary instancemethods.) Another typical use for class methods is to look up anobject by name:

    sub find {        my ($class, $name) = @_;        $objtable{$name};    }

An instance method expects an object reference as its first argument.Typically it shifts the first argument into a ``self'' or ``this'' variable,and then uses that as an ordinary reference.

    sub display {        my $self = shift;        my @keys = @_ ? @_ : sort keys %$self;        foreach $key (@keys) {            print "\t$key => $self->{$key}\n";        }    }

Method Invocation

There are two ways to invoke a method, one of which you're alreadyfamiliar with, and the other of which will look familiar. Perl 4already had an ``indirect object'' syntax that you use when you say

    print STDERR "help!!!\n";

This same syntax can be used to call either class or instance methods.We'll use the two methods defined above, the class method to lookupan object reference and the instance method to print out its attributes.

    $fred = find Critter "Fred";    display $fred 'Height', 'Weight';

These could be combined into one statement by using a BLOCK in theindirect object slot:

    display {find Critter "Fred"} 'Height', 'Weight';

For C++ fans, there's also a syntax using -> notation that does exactlythe same thing. The parentheses are required if there are any arguments.

    $fred = Critter->find("Fred");    $fred->display('Height', 'Weight');

or in one statement,

    Critter->find("Fred")->display('Height', 'Weight');

There are times when one syntax is more readable, and times when theother syntax is more readable. The indirect object syntax is lesscluttered, but it has the same ambiguity as ordinary list operators.Indirect object method calls are parsed using the same rule as listoperators: ``If it looks like a function, it is a function''. (Presumingfor the moment that you think two words in a row can look like afunction name. C++ programmers seem to think so with some regularity,especially when the first word is ``new''.) Thus, the parentheses of

    new Critter ('Barney', 1.5, 70)

are assumed to surround ALL the arguments of the method call, regardlessof what comes after. Saying

    new Critter ('Bam' x 2), 1.4, 45

would be equivalent to

    Critter->new('Bam' x 2), 1.4, 45

which is unlikely to do what you want.

There are times when you wish to specify which class's method to use.In this case, you can call your method as an ordinary subroutinecall, being sure to pass the requisite first argument explicitly:

    $fred =  MyCritter::find("Critter", "Fred");    MyCritter::display($fred, 'Height', 'Weight');

Note however, that this does not do any inheritance. If you wishmerely to specify that Perl should START looking for a method in aparticular package, use an ordinary method call, but qualify the methodname with the package like this:

    $fred = Critter->MyCritter::find("Fred");    $fred->MyCritter::display('Height', 'Weight');

If you're trying to control where the method search begins and you'reexecuting in the class itself, then you may use the SUPER pseudo class,which says to start looking in your base class's @ISA list without havingto name it explicitly:

    $self->SUPER::display('Height', 'Weight');

Please note that the SUPER:: construct is meaningful only within theclass.

Sometimes you want to call a method when you don't know the method nameahead of time. You can use the arrow form, replacing the method namewith a simple scalar variable containing the method name:

    $method = $fast ? "findfirst" : "findbest";    $fred->$method(@args);

Default UNIVERSAL methods

The UNIVERSAL package automatically contains the following methods thatare inherited by all other classes:

isa(CLASS)

isa returns true if its object is blessed into a subclass of CLASS

isa is also exportable and can be called as a sub with two arguments. Thisallows the ability to check what a reference points to. Example

    use UNIVERSAL qw(isa);

    if(isa($ref, 'ARRAY')) {        ...    }

can(METHOD)

can checks to see if its object has a method called METHOD,if it does then a reference to the sub is returned, if it does not thenundef is returned.

VERSION( [NEED] )

VERSION returns the version number of the class (package). If theNEED argument is given then it will check that the current version (asdefined by the $VERSION variable in the given package) not less thanNEED; it will die if this is not the case. This method is normallycalled as a class method. This method is called automatically by theVERSION form of use.

    use A 1.2 qw(some imported subs);    # implies:    A->VERSION(1.2);

NOTE: can directly uses Perl's internal code for method lookup, andisa uses a very similar method and cache-ing strategy. This may causestrange effects if the Perl code dynamically changes @ISA in any package.

You may add other methods to the UNIVERSAL class via Perl or XS code.You do not need to use UNIVERSAL in order to make these methodsavailable to your program. This is necessary only if you wish tohave isa available as a plain subroutine in the current package.

Destructors

When the last reference to an object goes away, the object isautomatically destroyed. (This may even be after you exit, if you'vestored references in global variables.) If you want to capture controljust before the object is freed, you may define a DESTROY method inyour class. It will automatically be called at the appropriate moment,and you can do any extra cleanup you need to do.

Perl doesn't do nested destruction for you. If your constructorre-blessed a reference from one of your base classes, your DESTROY mayneed to call DESTROY for any base classes that need it. But this appliesto only re-blessed objects---an object reference that is merelyCONTAINED in the current object will be freed and destroyedautomatically when the current object is freed.

WARNING

An indirect object is limited to a name, a scalar variable, or a block,because it would have to do too much lookahead otherwise, just like anyother postfix dereference in the language. The left side of -> is not solimited, because it's an infix operator, not a postfix operator.

That means that in the following, A and B are equivalent to each other, andC and D are equivalent, but A/B and C/D are different:

    A: method $obref->{"fieldname"}    B: (method $obref)->{"fieldname"}    C: $obref->{"fieldname"}->method()    D: method {$obref->{"fieldname"}}

Summary

That's about all there is to it. Now you need just to go off and buy abook about object-oriented design methodology, and bang your foreheadwith it for the next six months or so.

Two-Phased Garbage Collection

For most purposes, Perl uses a fast and simple reference-basedgarbage collection system. For this reason, there's an extradereference going on at some level, so if you haven't builtyour Perl executable using your C compiler's -O flag, performancewill suffer. If you have built Perl with cc -O, then thisprobably won't matter.

A more serious concern is that unreachable memory with a non-zeroreference count will not normally get freed. Therefore, this is a badidea:

    {        my $a;        $a = \$a;    }

Even thought $a should go away, it can't. When building recursive datastructures, you'll have to break the self-reference yourself explicitlyif you don't care to leak. For example, here's a self-referentialnode such as one might use in a sophisticated tree structure:

    sub new_node {        my $self = shift;        my $class = ref($self) || $self;        my $node = {};        $node->{LEFT} = $node->{RIGHT} = $node;        $node->{DATA} = [ @_ ];        return bless $node => $class;    }

If you create nodes like that, they (currently) won't go away unless youbreak their self reference yourself. (In other words, this is not to beconstrued as a feature, and you shouldn't depend on it.)

Almost.

When an interpreter thread finally shuts down (usually when your programexits), then a rather costly but complete mark-and-sweep style of garbagecollection is performed, and everything allocated by that thread getsdestroyed. This is essential to support Perl as an embedded or amultithreadable language. For example, this program demonstrates Perl'stwo-phased garbage collection:

    #!/usr/bin/perl    package Subtle;

    sub new {        my $test;        $test = \$test;        warn "CREATING " . \$test;        return bless \$test;    }

    sub DESTROY {        my $self = shift;        warn "DESTROYING $self";    }

    package main;

    warn "starting program";    {        my $a = Subtle->new;        my $b = Subtle->new;        $$a = 0;  # break selfref        warn "leaving block";    }

    warn "just exited block";    warn "time to die...";    exit;

When run as /tmp/test, the following output is produced:

    starting program at /tmp/test line 18.    CREATING SCALAR(0x8e5b8) at /tmp/test line 7.    CREATING SCALAR(0x8e57c) at /tmp/test line 7.    leaving block at /tmp/test line 23.    DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.    just exited block at /tmp/test line 26.    time to die... at /tmp/test line 27.    DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.

Notice that ``global destruction'' bit there? That's the threadgarbage collector reaching the unreachable.

Objects are always destructed, even when regular refs aren't and in factare destructed in a separate pass before ordinary refs just to try toprevent object destructors from using refs that have been themselvesdestructed. Plain refs are only garbage-collected if the destruct levelis greater than 0. You can test the higher levels of global destructionby setting the PERL_DESTRUCT_LEVEL environment variable, presuming-DDEBUGGING was enabled during perl build time.

A more complete garbage collection strategy will be implementedat a future date.

This document was created byman2html,using the manual pages.

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