PERLOBJ(1) PERLOBJ(1)
NAME
perlobj - 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 {} }
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:
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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 perlmod 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 $self;
}
Within the class package, the methods will typically deal with the
reference as an ordinary reference. Outside the class package, the
reference is generally treated as an opaque value that may be accessed
only through the class's methods.
A constructor may re-bless a referenced object currently belonging to
another class, but then the new class is responsible for all cleanup
later. The previous blessing is forgotten, as an object may belong to
only one class at a time. (Although of course it's free to inherit
methods from many classes.)
A clarification: Perl objects are blessed. References are not. Objects
know which package they belong to. References do not. The bless()
function uses the reference to find the object. Consider the following
example:
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$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.
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 the section on Default UNIVERSAL
methods for more details.) If that doesn't work, Perl finally gives up
and complains.
Perl classes do only method inheritance. 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.
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.)
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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 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) = @_;
$objtable{$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 $fred 'Height', 'Weight';
These could be combined into one statement by using a BLOCK in the
indirect object slot:
display {find Critter "Fred"} 'Height', 'Weight';
For C++ fans, there's also a syntax using -> notation that does exactly
the same thing. The parentheses are required if there are any arguments.
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$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 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
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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.
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.
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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 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.
WARNING
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"}}
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 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.
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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;
}
sub DESTROY {
my $self = shift;
warn "DESTROYING $self";
}
package main;
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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.
SEE ALSO
A kinder, gentler tutorial on object-oriented programming 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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