PERLOBJ(1) Perl Programmers Reference Guide PERLOBJ(1)NAMEperlobj - Perl objects
DESCRIPTION
First you need to understand what references are in Perl.
See perlref for that. Second, if you still find the follow-
ing reference work too complicated, a tutorial on object-
oriented programming in Perl can be found in perltoot and
perltooc.
If you're still with us, then 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 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 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 {} }
That word "new" isn't special. You could have written a
construct this way, too:
package Critter;
sub spawn { bless {} }
This might even be preferable, because the C++ programmers
won't be tricked into thinking that "new" works in Perl as
it does in C++. It doesn't. We recommend that you name your
constructors whatever makes sense in the context of the
problem you're solving. For example, constructors in the Tk
extension to Perl are named after the widgets they create.
One thing that's different about Perl constructors compared
with those in C++ is that in Perl, they have to allocate
their own memory. (The other things is that they don't
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automatically call overridden base-class constructors.) The
"{}" allocates an anonymous hash containing no key/value
pairs, and returns it 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;
}
You often see such a thing in more complicated constructors
that wish to call methods in the class as part of the con-
struction:
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 perlmodlib), 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 the following.
(Note that using this to call new() on an instance does not
automatically perform any copying. If you want a shallow or
deep copy of an object, you'll have to specifically allow
for that.) The initialize() method used will be of whatever
$class we blessed the object into:
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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 be accessed only through the class's
methods.
Although a constructor can in theory re-bless a referenced
object currently belonging to another class, this is almost
certainly going to get you into trouble. 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.) If you find yourself having to do this, the
parent class is probably misbehaving, though.
A clarification: Perl objects are blessed. References are
not. Objects know which package they belong to. References
do not. The bless() function uses the reference 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 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 accessi-
ble through @ISA are known as base classes of the current
class.
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All classes implicitly inherit from class "UNIVERSAL" as
their last base class. Several commonly used methods are
automatically supplied in the UNIVERSAL class; see "Default
UNIVERSAL methods" for more details.
If a missing method is found in a base class, 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 neither the current class, its named base classes, nor
the UNIVERSAL class contains the requested method, these
three places are searched all over again, this time looking
for a method named AUTOLOAD(). If an AUTOLOAD is found,
this method is called on behalf of the missing method, set-
ting the package global $AUTOLOAD to be the fully qualified
name of the method that was intended to be called.
If none of that works, Perl finally gives up and complains.
If you want to stop the AUTOLOAD inheritance say simply
sub AUTOLOAD;
and the call will die using the name of the sub being
called.
Perl classes do method inheritance only. 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. The only
problem with this is that you can't sure that you aren't
using a piece of the hash that isn't already used. A rea-
sonable workaround is to prepend your fieldname in the hash
with the package name.
sub bump {
my $self = shift;
$self->{ __PACKAGE__ . ".count"}++;
}
A Method is Simply a Subroutine
Unlike say C++, Perl doesn't provide any special syntax for
method definition. (It does provide a little syntax for
method invocation though. More on that later.) A method
expects its first argument to be the object (reference) or
package (string) it is being invoked on. There are two ways
of calling methods, which we'll call class methods and
instance methods.
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A class method expects a class name as the first argument.
It provides functionality for the class as a whole, not for
any individual object belonging to the class. Constructors
are often class methods, but see perltoot and perltooc for
alternatives. Many class methods simply ignore their first
argument, because they already know what package they're in
and don't care what package they were invoked via. (These
aren't necessarily the same, because class methods follow
the inheritance tree just like ordinary instance methods.)
Another typical use for class methods is to look up an
object by name:
sub find {
my ($class, $name) = @_;
$objtable{$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
For various historical and other reasons, Perl offers two
equivalent ways to write a method call. The simpler and
more common way is to use the arrow notation:
my $fred = Critter->find("Fred");
$fred->display("Height", "Weight");
You should already be familiar with the use of the "->"
operator with references. In fact, since $fred above is a
reference to an object, you could think of the method call
as just another form of dereferencing.
Whatever is on the left side of the arrow, whether a refer-
ence or a class name, is passed to the method subroutine as
its first argument. So the above code is mostly equivalent
to:
my $fred = Critter::find("Critter", "Fred");
Critter::display($fred, "Height", "Weight");
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How does Perl know which package the subroutine is in? By
looking at the left side of the arrow, which must be either
a package name or a reference to an object, i.e. something
that has been blessed to a package. Either way, that's the
package where Perl starts looking. If that package has no
subroutine with that name, Perl starts looking for it in any
base classes of that package, and so on.
If you need to, you can force Perl to start looking in some
other package:
my $barney = MyCritter->Critter::find("Barney");
$barney->Critter::display("Height", "Weight");
Here "MyCritter" is presumably a subclass of "Critter" that
defines its own versions of find() and display(). We
haven't specified what those methods do, but that doesn't
matter above since we've forced Perl to start looking for
the subroutines in "Critter".
As a special case of the above, you may use the "SUPER"
pseudo-class to tell Perl to start looking for the method in
the packages named in the current class's @ISA list.
package MyCritter;
use base 'Critter'; # sets @MyCritter::ISA = ('Critter');
sub display {
my ($self, @args) = @_;
$self->SUPER::display("Name", @args);
}
It is important to note that "SUPER" refers to the
superclass(es) of the current package and not to the
superclass(es) of the object. Also, the "SUPER" pseudo-class
can only currently be used as a modifier to a method name,
but not in any of the other ways that class names are nor-
mally used, eg:
something->SUPER::method(...); # OK
SUPER::method(...); # WRONG
SUPER->method(...); # WRONG
Instead of a class name or an object reference, you can also
use any expression that returns either of those on the left
side of the arrow. So the following statement is valid:
Critter->find("Fred")->display("Height", "Weight");
and so is the following:
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my $fred = (reverse "rettirC")->find(reverse "derF");
The right side of the arrow typically is the method name,
but a simple scalar variable containing either the method
name or a subroutine reference can also be used.
Indirect Object Syntax
The other way to invoke a method is by using the so-called
"indirect object" notation. This syntax was available in
Perl 4 long before objects were introduced, and is still
used with filehandles like this:
print STDERR "help!!!\n";
The same syntax can be used to call either object or class
methods.
my $fred = find Critter "Fred";
display $fred "Height", "Weight";
Notice that there is no comma between the object or class
name and the parameters. This is how Perl can tell you want
an indirect method call instead of an ordinary subroutine
call.
But what if there are no arguments? In that case, Perl must
guess what you want. Even worse, it must make that guess at
compile time. Usually Perl gets it right, but when it
doesn't you get a function call compiled as a method, or
vice versa. This can introduce subtle bugs that are hard to
detect.
For example, a call to a method "new" in indirect notation
-- as C++ programmers are wont to make -- can be miscompiled
into a subroutine call if there's already a "new" function
in scope. You'd end up calling the current package's "new"
as a subroutine, rather than the desired class's method.
The compiler tries to cheat by remembering bareword
"require"s, but the grief when it messes up just isn't worth
the years of debugging it will take you to track down such
subtle bugs.
There is another problem with this syntax: the 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.
(These are the same quirky rules as are used for the
filehandle slot in functions like "print" and "printf".)
This can lead to horribly confusing precedence problems, as
in these next two lines:
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move $obj->{FIELD}; # probably wrong!
move $ary[$i]; # probably wrong!
Those actually parse as the very surprising:
$obj->move->{FIELD}; # Well, lookee here
$ary->move([$i]); # Didn't expect this one, eh?
Rather than what you might have expected:
$obj->{FIELD}->move(); # You should be so lucky.
$ary[$i]->move; # Yeah, sure.
To get the correct behavior with indirect object syntax, you
would have to use a block around the indirect object:
move {$obj->{FIELD}};
move {$ary[$i]};
Even then, you still have the same potential problem if
there happens to be a function named "move" in the current
package. The "->" notation suffers from neither of these
disturbing ambiguities, so we recommend you use it
exclusively. However, you may still end up having to read
code using the indirect object notation, so it's important
to be familiar with it.
Default UNIVERSAL methods
The "UNIVERSAL" package automatically contains the following
methods that are inherited by all other classes:
isa(CLASS)
"isa" returns true if its object is blessed into a sub-
class of "CLASS"
You can also call "UNIVERSAL::isa" as a subroutine with
two arguments. Of course, this will do the wrong thing
if someone has overridden "isa" in a class, so don't do
it.
If you need to determine whether you've received a valid
invocant, use the "blessed" function from Scalar::Util:
if (blessed($ref) && $ref->isa( 'Some::Class')) {
# ...
}
"blessed" returns the name of the package the argument
has been blessed into, or "undef".
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"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.
"UNIVERSAL::can" can also be called as a subroutine with
two arguments. It'll always return undef if its first
argument isn't an object or a class name. The same
caveats for calling "UNIVERSAL::isa" directly apply
here, too.
VERSION( [NEED] )
"VERSION" returns the version number of the class (pack-
age). If the NEED argument is given then it will check
that the current version (as defined by the $VERSION
variable in the given package) not less than NEED; it
will die if this is not the case. This method is nor-
mally called as a class method. This method is called
automatically by the "VERSION" 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, 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. You do not need to "use UNIVERSAL" to make these
methods available to your program (and you should not do
so).
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 passes a
reference to the object under destruction as the first (and
only) argument. Beware that the reference is a read-only
value, and cannot be modified by manipulating $_[0] within
the destructor. The object itself (i.e. the thingy the
reference points to, namely "${$_[0]}", "@{$_[0]}",
"%{$_[0]}" etc.) is not similarly constrained.
Since DESTROY methods can be called at unpredictable times,
it is important that you localise any global variables that
the method may update. In particular, localise $@ if you
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use "eval {}" and localise $? if you use "system" or back-
ticks.
If you arrange to re-bless the reference before the destruc-
tor returns, perl will again call the DESTROY method for the
re-blessed object after the current one returns. This can
be used for clean delegation of object destruction, or for
ensuring that destructors in the base classes of your choos-
ing get called. Explicitly calling DESTROY is also possi-
ble, but is usually never needed.
Do not confuse the previous discussion with how objects CON-
TAINED in the current one are destroyed. Such objects will
be freed and destroyed automatically when the current object
is freed, provided no other references to them exist else-
where.
Summary
That's about all there is to it. Now you need just 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. That means 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:
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sub new_node {
my $class = shift;
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 multithread-
able 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;
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When run as /foo/test, the following output is produced:
starting program at /foo/test line 18.
CREATING SCALAR(0x8e5b8) at /foo/test line 7.
CREATING SCALAR(0x8e57c) at /foo/test line 7.
leaving block at /foo/test line 23.
DESTROYING Subtle=SCALAR(0x8e5b8) at /foo/test line 13.
just exited block at /foo/test line 26.
time to die... at /foo/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. Objects are destructed in a separate pass before
ordinary refs just 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.
See "PERL_DESTRUCT_LEVEL" in perlhack for more information.
A more complete garbage collection strategy will be imple-
mented at a future date.
In the meantime, the best solution is to create a non-
recursive container class that holds a pointer to the self-
referential data structure. Define a DESTROY method for the
containing object's class that manually breaks the circular-
ities in the self-referential structure.
SEE ALSO
A kinder, gentler tutorial on object-oriented programming in
Perl can be found in perltoot, perlboot and perltooc. 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.
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