perlobj - Perl objects
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 [Toc] [Back]
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 "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 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 $a, BLAH;
print " is a ", ref($b), "0;
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 [Toc] [Back]
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
"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;
$self->{ __PACKAGE__ . ".count"}++;
}
A Method is Simply a Subroutine [Toc] [Back]
Unlike say C++, Perl doesn't provide any special syntax
for method definition. (It does provide a little syntax
for method invocation though. More on that later.) A
method expects its first argument to be the object (reference)
or package (string) it is being invoked on. There
are two ways of calling methods, which we'll call class
methods and instance methods.
A class method expects a class name as the first argument.
It provides functionality for the class as a whole, not
for any individual object belonging to the class. Constructors
are often class methods, but see perltoot and
perltooc for alternatives. Many class methods simply
ignore their first argument, because they already know
what package they're in and don't care what package they
were invoked via. (These aren't necessarily the same,
because class methods follow the inheritance tree just
like ordinary instance methods.) Another typical use for
class methods is to look up an object by name:
sub find {
my ($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 $key (@keys) {
print "$key => $self->{$key}0;
}
}
Method Invocation [Toc] [Back]
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 dereferencing.
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 package. 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 package:
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 normally
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");
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!!!0;
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. Usually 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 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 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 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.
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[$i]};
Even then, you still have the same potential problem if
there happens to be a function named "move" in the current
package. The "->" notation suffers from neither of these
disturbing ambiguities, so we recommend you use it exclu-
sively. However, you may still end up having to read code
using the indirect object notation, so it's important to
be familiar with it.
Default UNIVERSAL methods [Toc] [Back]
The "UNIVERSAL" package automatically contains the following
methods that are inherited by all other classes:
isa(CLASS)
"isa" returns true if its object is blessed into a
subclass of "CLASS"
You can also call "UNIVERSAL::isa" as a subroutine
with two arguments. The first does not need to be an
object or even a reference. This allows you to check
what a reference points to, or whether something is a
reference of a given type. Example
if(UNIVERSAL::isa($ref, 'ARRAY')) {
#...
}
To determine if a reference is a blessed object, you
can write
print "It's an object0 if UNIVERSAL::isa($val,
'UNIVERSAL');
can(METHOD)
"can" checks to see if its object has a method called
"METHOD", if it does then a reference to the sub is
returned, if it does not then undef is returned.
"UNIVERSAL::can" can also be called as a subroutine
with two arguments. It'll always return undef if its
first argument isn't an object or a class name. So
here's another way to check if a reference is a
blessed object
print "It's still an object0 if UNIVERSAL::can($val, 'can');
You can also use the "blessed" function of
Scalar::Util:
use Scalar::Util 'blessed';
my $blessing = blessed $suspected_object;
"blessed" returns the name of the package the argument
has been blessed into, or "undef".
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 (and you should
not do so).
Destructors [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
For most purposes, Perl uses a fast and simple, referencebased
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 = ;
}
Even thought $a should go away, it can't. When building
recursive data structures, you'll have to break the selfreference
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 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 = est;
warn "CREATING " . est;
return bless est;
}
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 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. 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 nonrecursive
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.
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 v5.8.5 2002-11-06 12 [ Back ] |
