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PERLOBJ(1) Perl Programmers Reference Guide PERLOBJ(1)
NAME
perlobj - Perl objects
DESCRIPTION
First you need to understand what references are in Perl. See perlref
for that. Second, if you still find the following 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 construc-
tors. A constructor is merely a subroutine that returns a reference to
something "blessed" into a class, generally the class that the subrou-
tine 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, con-
structors 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 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 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 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:
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 ref-
erence as an ordinary reference. Outside the class package, the refer-
ence 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 cur-
rently 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 defi-
nitions. 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 accessible
through @ISA are known as base classes of the current class.
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 cur-
rent 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, setting 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 reasonable
workaround is to prepend your fieldname in the hash with the package
name.
sub bump {
my $self = shift;
$self->{ __PACKAGE__ . ".count"}++;
}
A Method is Simply a Subroutine
Unlike say C++, Perl doesn't provide any special syntax for method def-
inition. (It does provide a little syntax for method invocation
though. More on that later.) A method expects its first argument to
be the object (reference) or package (string) it is being invoked on.
There are two ways of calling methods, which we'll call class methods
and instance methods.
A class method expects a class name as the first argument. It provides
functionality for the class as a whole, not for any individual object
belonging to the class. Constructors are often class methods, but see
perltoot and perltooc for alternatives. Many class methods simply
ignore their first argument, because they already know what package
they're in and don't care what package they were invoked via. (These
aren't necessarily the same, because class methods follow the inheri-
tance tree just like ordinary instance methods.) Another typical use
for class methods is to look up an object by name:
sub find {
my ($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" vari-
able, 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 dereferenc-
ing.
Whatever is on the left side of the arrow, whether a reference 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");
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 pack-
age. 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 pack-
age:
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:
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 ref-
erence 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. Usu-
ally 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 call-
ing 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 lim-
ited to a name, a scalar variable, or a block, because it would have to
do too much lookahead otherwise, just like any other postfix derefer-
ence 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:
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[$i]};
Even then, you still have the same potential problem if there happens
to be a function named "move" in the current package. The "->" nota-
tion suffers from neither of these disturbing ambiguities, so we recom-
mend you use it exclusively. However, you may still end up having to
read code using the indirect object notation, so it's important to be
familiar with it.
Default UNIVERSAL methods
The "UNIVERSAL" package automatically contains the following methods
that are inherited by all other classes:
isa(CLASS)
"isa" returns true if its object is blessed into a subclass of
"CLASS"
You can also call "UNIVERSAL::isa" as a subroutine with two argu-
ments. Of course, this will do the wrong thing if someone has
overridden "isa" in a class, so don't do it.
If you need to determine whether you've received a valid invocant,
use the "blessed" function from Scalar::Util:
if (blessed($ref) && $ref->isa( 'Some::Class')) {
# ...
}
"blessed" returns the name of the package the argument has been
blessed into, or "undef".
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.
"UNIVERSAL::can" can also be called as a subroutine with two argu-
ments. It'll always return undef if its first argument isn't an
object or a class name. The same caveats for calling "UNIVER-
SAL::isa" directly apply here, too.
VERSION( [NEED] )
"VERSION" returns the version number of the class (package). If
the NEED argument is given then it will check that the current ver-
sion (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 normally called as a class method. This method is called auto-
matically 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 pack-
age.
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 automati-
cally 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 use "eval {}" and localise
$? if you use "system" or backticks.
If you arrange to re-bless the reference before the destructor 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 choosing get called. Explicitly calling DESTROY is
also possible, but is usually never needed.
Do not confuse the previous discussion with how objects CONTAINED 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 elsewhere.
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 ref-
erence 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 $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 pro-
gram 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 embed-
ded or a multithreadable language. For example, this program demon-
strates 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 /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 destruc-
ted. 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 implemented at a
future date.
In the meantime, the best solution is to create a non-recursive con-
tainer class that holds a pointer to the self-referential data struc-
ture. Define a DESTROY method for the containing object's class that
manually breaks the circularities 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.
perl v5.8.8 2006-01-07 PERLOBJ(1)
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