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Tim Bunce


perlobj - Perl objects







We'll cover these points now in more depth.

An Object is Simply a Reference


    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, because the referenced object itself knows that it has been blessed, and the 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;
        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
        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 $self = {};
        bless $self, $class
        return $self;


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 belong to only one class at a time. (Although of course it's free to inherit methods from many classes.)


    $a = {};
    $b = $a;
    bless $a, BLAH;


A Class is Simply a Package



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.


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 class and instance. (Sometimes you'll hear these called static and virtual, in honor of the two C++ method types they most closely resemble.)

A class 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 class methods. Many class methods simply ignore their first argument, because they already know what package they're in, and don't care what package they were invoked via. (These aren't necessarily the same, because class methods follow the inheritance tree just like ordinary instance methods.) Another typical use for class methods is to look up an object by name:

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


    sub display {
        my $self = shift;
        foreach $key (@keys) {

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 class or instance methods. We'll use the two methods defined above, the class method to lookup an 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 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->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 parentheses 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 wish merely 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 pseudo class, which says to start looking in your base class's @ISA list without having to name it explicitly:

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

Please note that the SUPER:: construct is meaningful only 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";

Default UNIVERSAL methods





    use UNIVERSAL qw(isa);





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





Perl doesn't do nested destruction for you. If your constructor re-blessed a reference from one of your base classes, your DESTROY may need to call DESTROY for any base classes that need it. But this applies to only re-blessed objects--an object reference that is merely CONTAINED in the current object will be freed and destroyed automatically when the current object is freed.


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 in the following, A and B are equivalent to each other, and C 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"}}



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.


        my $a;
        $a = \$a;


    sub new_node {
        my $self = shift;
        my $node = {};
        return bless $node => $class;




    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...";

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.


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 kinder, gentler tutorial on object-oriented programming in Perl can be found in perltoot. You should also check out perlbot for other object tricks, traps, and tips, as well as perlmodlib for some style guides on constructing both modules and classes.