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PERLOBJ(1)
NAME
perlobj - 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 tutorial 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 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 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. 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 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,
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;
$self->{ __PACKAGE__ . ".count"}++;
}
A Method is Simply a Subroutine
Unlike say C++, Perl doesn't provide any special syntax for method
definition. (It does provide a little syntax for method invocation though.
More on that later.) A method expects its first argument to be the object
(reference) or package (string) it is being invoked on. There are two ways
of calling methods, which we'll call class methods and instance methods.
A class method expects a class name as the first argument. It provides
functionality for the class as a whole, not for any individual object
belonging to the class. Constructors are often class methods, but see the
perltoot manpage and the perltootc manpage for alternatives. Many class
methods simply ignore their first argument, because they already know what
package they're in and don't care what package they were invoked via.
(These aren't necessarily the same, because class methods follow the
inheritance tree just like ordinary instance methods.) Another typical use
for class methods is to look up an object by name:
sub find {
my ($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.
$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 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. Here
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');
Unlike method calls, function calls don't consider 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
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 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 debugging 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 recommend you use it exclusively.
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 $VERSION variable in the given package) not less than
NEED; it will die if this is not the case. This method is normally
called as a class method. This method is called automatically by the
"VERSION" form of "use".
use A 1.2 qw(some imported subs);
# implies:
A->VERSION(1.2);
NOTE: "can" directly uses Perl's internal code for method lookup, and "isa"
uses a very similar method and cache-ing strategy. This may cause strange
effects if the Perl code dynamically changes @ISA in any package.
You may add other methods to the UNIVERSAL class via Perl or XS code. You
do not need to "use UNIVERSAL" to make these methods available to your
program. This is necessary only if you wish to have "isa" available as a
plain subroutine in the current package.
Destructors
When the last reference to an object goes away, the object is automatically
destroyed. (This may even be after you exit, if you've stored references
in global variables.) If you want to capture control just before the
object is freed, you may define a DESTROY method in your class. It will
automatically be called at the appropriate moment, and you can do any extra
cleanup you need to do. Perl passes a reference to the object under
destruction as the first (and only) argument. Beware that the reference is
a read-only value, and cannot be modified by manipulating "$_[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 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 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;
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 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 in Perl can be
found in the perltoot manpage, the perlbootc 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.
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