PERLOBJ(1) Perl Programmers Reference Guide PERLOBJ(1)NAMEperlobj - Perl objects
DESCRIPTION
First of all, 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 tutorial on object-oriented programming in
Perl can be found in the perltoot 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 {} }
In fact, 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 automatically call overridden base-class
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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;
}
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 the
section on Modules: Creation, Use, and Abuse 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
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class package, the reference is generally treated as an
opaque value that may be accessed only 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 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 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 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 neither the current class, its named base classes, nor
the UNIVERSAL class contains the requested method, these
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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.
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;
$self->{ __PACKAGE__ . ".count"}++;
}
A Method is Simply a Subroutine
Unlike say C++, Perl doesn't provide any special syntax
for method definition. (It does provide a little syntax
for method invocation though. More on that later.) A
method expects its first argument to be the object
(reference) or package (string) it is being invoked on.
There are just two types of methods, which we'll call
class and instance. (Sometimes you'll hear these called
static and virtual, in honor of the two C++ method types
they most closely resemble.)
A class method expects a class name as the first argument.
It provides functionality for the class as a whole, not
for any individual object belonging to the class.
Constructors are typically class methods. Many class
methods simply ignore their first argument, because they
already know what package they're in, and don't care what
package they were invoked via. (These aren't necessarily
the same, because class methods follow the inheritance
tree just like ordinary instance methods.) Another
typical use for class methods is to look up an object by
name:
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sub find {
my ($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
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indirect 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++
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. Confusingly,
however, 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
ambiguous, see the section on WARNING below.
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');
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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";
$fred->$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
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.
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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 in order to
make these methods available to your program. This is
necessary 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 foregoing 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.
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
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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 requires, but the grief if
it messes up just isn't worth the years of debugging it
would likely take you to to track such subtle bugs down.
The infix arrow notation using ``->'' doesn't suffer from
either of these disturbing ambiguities, so we recommend
you use it exclusively.
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. For this reason, there's
an extra dereference going on at some level, so if you
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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;
}
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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.
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
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in Perl can be found in the perltoot 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.
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