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perl5.003_02a
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NAME

perlobj - Perl objects

DESCRIPTION

First of all, you need to understand what references are in Perl. See perlref for that.

Here are three very simple definitions that you should find reassuring.

  1. An object is simply a reference that happens to know which class it belongs to.

  2. A class is simply a package that happens to provide methods to deal with object references.

  3. A method is simply a subroutine that expects an object reference (or a package name, for static 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 for constructors. A constructor is merely a subroutine that returns a reference to something "blessed" into a class, generally the class that the subroutine is defined in. Here is a typical constructor:

package Critter;
sub new { bless {} }

The {} constructs a reference to an anonymous hash containing no key/value pairs. The bless() takes that reference and tells the object it references that it's now a Critter, and returns the reference. This is for convenience, since the referenced object itself knows that it has been blessed, and its reference to it could have been returned directly, like this:

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

In fact, you often see such a thing in more complicated constructors that 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; see "Modules: Creation, Use and Abuse" in perlmod), then you want to use the two-arg form of bless so 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 the object 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 the reference as an ordinary reference. Outside the class package, the reference is generally treated as an opaque value that may only be accessed through the class's methods.

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

A clarification: Perl objects are blessed. References are not. Objects know which package they belong to. References do not. The bless() function simply uses the reference in order to find the object. Consider the 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 class definitions. You just use a package as a class by putting method definitions into the class.

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

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

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

If neither a method nor an AUTOLOAD routine is found in @ISA, then one last try is made for the method (or an AUTOLOAD routine) in a class called UNIVERSAL. (Several commonly used methods are automatically supplied in the UNIVERSAL class; see "Default UNIVERSAL methods" for more details.) If that doesn't work, Perl finally gives up and complains.

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

A Method is Simply a Subroutine

Unlike say C++, Perl doesn't provide any special syntax for method definition. (It does provide a little syntax for method invocation though. More on that later.) A method expects its first argument to be the object or package it is being invoked on. There are just two types of methods, which we'll call static and virtual, in honor of the two C++ method types they most closely resemble.

A static method expects a class name as the first argument. It provides functionality for the class as a whole, not for any individual object belonging to the class. Constructors are typically static methods. Many static methods simply ignore their first argument, since they already know what package they're in, and don't care what package they were invoked via. (These aren't necessarily the same, since static methods follow the inheritance tree just like ordinary virtual methods.) Another typical use for static methods is to look up an object by name:

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

A virtual 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 already familiar with, and the other of which will look familiar. Perl 4 already had an "indirect object" syntax that you use when you say

print STDERR "help!!!\n";

This same syntax can be used to call either static or virtual methods. We'll use the two methods defined above, the static method to lookup an object reference and the virtual 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 the indirect object slot:

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

For C++ fans, there's also a syntax using -> notation that does exactly the 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 the other syntax is more readable. The indirect object syntax is less cluttered, but it has the same ambiguity as ordinary list operators. Indirect object method calls are parsed using the same rule as list operators: "If it looks like a function, it is a function". (Presuming for the moment that you think two words in a row can look like a function name. C++ programmers seem to think so with some regularity, especially when the first word is "new".) Thus, the parens of

new Critter ('Barney', 1.5, 70)

are assumed to surround ALL the arguments of the method call, regardless of 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 subroutine call, 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 merely wish to specify that Perl should START looking for a method in a particular package, use an ordinary method call, but qualify the method name 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're executing in the class itself, then you may use the SUPER pseudoclass, which says to start looking in your base class's @ISA list without having to explicitly name it:

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

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

Sometimes you want to call a method when you don't know the method name ahead of time. You can use the arrow form, replacing the method name with 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 that are inherited by all other classes:

isa ( CLASS )

isa returns true if its object is blessed into a sub-class of CLASS

isa is also exportable and can be called as a sub with two arguments. This allows 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 then undef is returned.

VERSION ( [ VERSION ] )

VERSION returns the VERSION number of the class (package). If an argument is given then it will check that the current version is not less that the given argument. This method is normally called as a static method. This method is also called when the VERSION form of use is used.

use A 1.2 qw(some imported subs);

A->require_version( 1.2 );
class ()

class returns the class name of its object.

is_instance ()

is_instance returns true if its object is an instance of some class, false if its object is the class (package) itself. Example

A->is_instance();       # False

$var = 'A';
$var->is_instance();    # False

$ref = bless [], 'A';
$ref->is_instance();    # True

NOTE: can directly uses Perl's internal code for method lookup, and isa uses a very similar method and cache-ing strategy. This may cause strange 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.

Destructors

When the last reference to an object goes away, the object is automatically destroyed. (This may even be after you exit, if you've stored references in global variables.) If you want to capture control just before the object is freed, you may define a DESTROY method in your 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 constructor reblessed a reference from one of your base classes, your DESTROY may need to call DESTROY for any base classes that need it. But this only applies to reblessed objects--an object reference that is merely CONTAINED in the current object will be freed and destroyed automatically 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 any other postfix dereference in the language. The left side of -> is not so limited, because it's an infix operator, not a postfix operator.

That means that below, A and B are equivalent to each other, and C and D are equivalent, but AB and CD 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 just need to go off and buy a book about object-oriented design methodology, and bang your forehead with it for the next six months or so.

Two-Phased Garbage Collection

For most purposes, Perl uses a fast and simple reference-based garbage collection system. For this reason, there's an extra dereference going on at some level, so if you haven't built your Perl executable using your C compiler's -O flag, performance will suffer. If you have built Perl with cc -O, then this probably won't matter.

A more serious concern is that unreachable memory with a non-zero reference count will not normally get freed. Therefore, this is a bad idea:

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

Even thought $a should go away, it can't. When building recursive data structures, you'll have to break the self-reference yourself explicitly if you don't care to leak. For example, here's a self-referential node 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 you break their self reference yourself. (In other words, this is not to be construed as a feature, and you shouldn't depend on it.)

Almost.

When an interpreter thread finally shuts down (usually when your program exits), then a rather costly but complete mark-and-sweep style of garbage collection is performed, and everything allocated by that thread gets destroyed. This is essential to support Perl as an embedded or a multithreadable language. For example, this program demonstrates Perl's two-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 thread garbage collector reaching the unreachable.

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

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

SEE ALSO

You should also check out perlbot for other object tricks, traps, and tips, as well as perlmod for some style guides on constructing both modules and classes.