PERLOBJ(1) Perl Programmers Reference Guide PERLOBJ(1)NAMEperlobj - Perl objects
DESCRIPTION
First you need to understand what references are in Perl.
See the perlref manpage for that. Second, if you still
find the following reference work too complicated, a tuto
rial on object-oriented programming in Perl can be found
in the perltoot manpage and the perltootc manpage.
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 com
pared 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 construc
tors.) 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 conve
nience, 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 construc
tors 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 the
Modules: Creation, Use, and Abuse entry in the perlmodlib
manpage), 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 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. Ref
erences 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 imple
ments 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 miss
ing 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 the
section on "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 look
ing 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 com
plains.
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;
$self->{ __PACKAGE__ . ".count"}++;
}
A Method is Simply a Subroutine
Unlike say C++, Perl doesn't provide any special syntax
for method definition. (It does provide a little syntax
for method invocation though. More on that later.) A
method expects its first argument to be the object (refer
ence) or package (string) it is being invoked on. There
are two ways of calling methods, which we'll call class
methods and instance methods.
A class method expects a class name as the first argument.
It provides functionality for the class as a whole, not
for any individual object belonging to the class. Con
structors are often class methods, but see the perltoot
manpage and the perltootc manpage for alternatives. Many
class methods simply ignore their first argument, because
they already know what package they're in and don't care
what package they were invoked via. (These aren't neces
sarily the same, because class methods follow the inheri
tance tree just like ordinary instance methods.) Another
typical use for class methods is to look up an object by
name:
sub find {
my ($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" 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 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 = 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 indi
rect object syntax is less cluttered, but it has the same
ambiguity as ordinary list operators. Indirect object
method calls are usually 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++ pro
grammers 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. Confusingly, how
ever, this rule applies only when the indirect object is a
bareword package name, not when it's a scalar, a BLOCK, or
a "Package::" qualified package name. In those cases, the
arguments are parsed in the same way as an indirect object
list operator like print, so
new Critter:: ('Bam' x 2), 1.4, 45
is the same as
Critter::->new(('Bam' x 2), 1.4, 45)
For more reasons why the indirect object syntax is ambigu
ous, see the section on "WARNING" below.
There are times when you wish to specify which class's
method to use. Here you can call your method as an ordi
nary subroutine call, being sure to pass the requisite
first argument explicitly:
$fred = MyCritter::find("Critter", "Fred");
MyCritter::display($fred, 'Height', 'Weight');
Unlike method calls, function calls don't consider inheri
tance. 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 vari
able containing the method name or a reference to the
function.
$method = $fast ? "findfirst" : "findbest";
$fred->$method(@args); # call by name
if ($coderef = $fred->can($parent . "::findbest")) {
$self->$coderef(@args); # call by coderef
}
WARNING
While indirect object syntax may well be appealing to
English speakers and to C++ programmers, be not seduced!
It suffers from two grave problems.
The first problem is that 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. (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.
The left side of ``->'' is not so limited, because it's an
infix operator, not a postfix operator.
As if that weren't bad enough, think about this: Perl must
guess at compile time whether "name" and "move" above are
functions or methods. Usually Perl gets it right, but
when it doesn't it, you get a function call compiled as a
method, or vice versa. This can introduce subtle bugs
that are hard to unravel. For example, calling a method
"new" in indirect notation--as C++ programmers are so wont
to do--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 if it messes up just isn't worth the years of debug
ging it would likely take you to track such subtle bugs
down.
The infix arrow notation using ``"->"'' doesn't suffer
from either of these disturbing ambiguities, so we recom
mend you use it exclusively.
Default UNIVERSAL methods
The "UNIVERSAL" package automatically contains the follow
ing methods that are 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. 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( [NEED] )
"VERSION" returns the version number of the class
(package). If the NEED argument is given then it will
check that the current version (as defined by the
$VERSION variable in the given package) not less than
NEED; it will die if this is not the case. This
method is normally called as a class method. This
method is called automatically by the "VERSION" form
of "use".
use A 1.2 qw(some imported subs);
# implies:
A->VERSION(1.2);
NOTE: "can" directly uses Perl's internal code for method
lookup, and "isa" uses a very similar method and cache-ing
strategy. This may cause strange effects if the Perl code
dynamically changes @ISA in any package.
You may add other methods to the UNIVERSAL class via Perl
or XS code. You do not need to "use UNIVERSAL" to make
these methods available to your program. This is neces
sary only if you wish to have "isa" available as a plain
subroutine in the current package.
Destructors
When the last reference to an object goes away, the object
is automatically destroyed. (This may even be after you
exit, if you've stored references in global variables.)
If you want to capture control just before the object is
freed, you may define a DESTROY method in your class. It
will automatically be called at the appropriate moment,
and you can do any extra cleanup you need to do. Perl
passes a reference to the object under destruction as the
first (and only) argument. Beware that the reference is a
read-only value, and cannot be modified by manipulating
"$_[0]" within the destructor. The object itself (i.e.
the thingy the reference points to, namely "${$_[0]}",
"@{$_[0]}", "%{$_[0]}" etc.) is not similarly constrained.
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 methodol
ogy, 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:
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 com
plete mark-and-sweep style of garbage collection is per
formed, 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 pro
gram 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. 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.
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
circularities in the self-referential structure.
SEE ALSO
A kinder, gentler tutorial on object-oriented programming
in Perl can be found in the perltoot manpage, the perl
bootc manpage and the perltootc manpage. You should also
check out the perlbot manpage for other object tricks,
traps, and tips, as well as the perlmodlib manpage for
some style guides on constructing both modules and
classes.
2001-03-18 perl v5.6.1 PERLOBJ(1)
