=head1 NAME
+X<object> X<OOP>
perlobj - Perl objects
=head1 DESCRIPTION
-First of all, you need to understand what references are in Perl. See
-L<perlref> for that.
+First you need to understand what references are in Perl.
+See L<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 L<perltoot> and L<perltooc>.
-Here are three very simple definitions that you should find reassuring.
+If you're still with us, then
+here are three very simple definitions that you should find reassuring.
=over 4
=item 3.
A method is simply a subroutine that expects an object reference (or
-a package name, for static methods) as the first argument.
+a package name, for class methods) as the first argument.
=back
We'll cover these points now in more depth.
=head2 An Object is Simply a Reference
+X<object> X<bless> X<constructor> X<new>
Unlike say C++, Perl doesn't provide any special syntax for
constructors. A constructor is merely a subroutine that returns a
-reference that has been "blessed" into a class, generally the
+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 C<{}> 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, since the referenced object itself knows that
-it has been blessed, and its reference to it could have been returned
-directly, like this:
+That word C<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 C<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 C<{}> 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 = {};
return $self;
}
-In fact, you often see such a thing in more complicated constructors
+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 = {}
+ my $self = {};
bless $self;
$self->initialize();
- $self;
+ return $self;
+ }
+
+If you care about inheritance (and you should; see
+L<perlmodlib/"Modules: Creation, Use, and Abuse">),
+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 C<< CLASS->new() >> but also
+C<< $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
-only be accessed through the class's methods.
+be accessed only through the class's methods.
-A constructor may rebless 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 only
-belong to one class at a time. (Although of course it's free to
-inherit methods from many classes.)
+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 simply uses the reference in order to find the object. Consider
+function uses the reference to find the object. Consider
the following example:
$a = {};
bless $a, BLAH;
print "\$b is a ", ref($b), "\n";
-This reports $b as being a BLAH, so obviously bless()
+This reports $b as being a BLAH, so obviously bless()
operated on the object and not on the reference.
=head2 A Class is Simply a Package
+X<class> X<package> X<@ISA> X<inheritance>
Unlike say C++, Perl doesn't provide any special syntax for class
-definitions. You just use a package as a class by putting method
+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
+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
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
+All classes implicitly inherit from class C<UNIVERSAL> as their
+last base class. Several commonly used methods are automatically
+supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
+more details.
+X<UNIVERSAL> X<base class> X<class, base>
+
+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 a method isn't found, but an AUTOLOAD routine is found, then
-that is called on behalf of the missing method.
+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.
+X<AUTOLOAD>
+
+If none of that works, Perl finally gives up and complains.
+
+If you want to stop the AUTOLOAD inheritance say simply
+X<AUTOLOAD>
-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. If that doesn't work, Perl finally gives up and
-complains.
+ sub AUTOLOAD;
-Perl classes only do 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.
+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.
+X<inheritance, method> X<inheritance, data>
+
+ sub bump {
+ my $self = shift;
+ $self->{ __PACKAGE__ . ".count"}++;
+ }
=head2 A Method is Simply a Subroutine
+X<method>
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 static and virtual, in honor of
-the two C++ method types they most closely resemble.
-
-A static 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 static
-methods. Many static methods simply ignore their first argument, since
-they already know what package they're in, and don't care what package
-they were invoked via. (These aren't necessarily the same, since
-static methods follow the inheritance tree just like ordinary virtual
-methods.) Another typical use for static methods is to look up an
+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 L<perltoot> and L<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 {
$objtable{$name};
}
-A virtual method expects an object reference as its first argument.
+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.
}
=head2 Method Invocation
+X<invocation> X<method> X<arrow> X<< -> >>
+
+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");
+ $fred->display("Height", "Weight");
+
+You should already be familiar with the use of the C<< -> >> operator with
+references. In fact, since C<$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 I<can> force Perl to start looking in some other package:
+
+ my $barney = MyCritter->Critter::find("Barney");
+ $barney->Critter::display("Height", "Weight");
+
+Here C<MyCritter> is presumably a subclass of C<Critter> that defines
+its own versions of find() and display(). We haven't specified what
+those methods do, but that doesn't matter above since we've forced Perl
+to start looking for the subroutines in C<Critter>.
+
+As a special case of the above, you may use the C<SUPER> pseudo-class to
+tell Perl to start looking for the method in the packages named in the
+current class's C<@ISA> list.
+X<SUPER>
+
+ package MyCritter;
+ use base 'Critter'; # sets @MyCritter::ISA = ('Critter');
+
+ sub display {
+ my ($self, @args) = @_;
+ $self->SUPER::display("Name", @args);
+ }
+
+It is important to note that C<SUPER> refers to the superclass(es) of the
+I<current package> and not to the superclass(es) of the object. Also, the
+C<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:
+X<SUPER>
+
+ 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:
-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
+ my $fred = (reverse "rettirC")->find(reverse "derF");
- print STDERR "help!!!\n";
+The right side of the arrow typically is the method name, but a simple
+scalar variable containing either the method name or a subroutine
+reference can also be used.
-This same syntax can be used to call either static or virtual methods.
-We'll use the two methods defined above, the static method to lookup
-an object reference and the virtual method to print out its attributes.
+=head2 Indirect Object Syntax
+X<indirect object syntax> X<invocation, indirect> X<indirect>
- $fred = find Critter "Fred";
- display $fred 'Height', 'Weight';
+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:
-These could be combined into one statement by using a BLOCK in the
-indirect object slot:
+ print STDERR "help!!!\n";
- display {find Critter "Fred"} 'Height', 'Weight';
+The same syntax can be used to call either object or class methods.
-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.
+ my $fred = find Critter "Fred";
+ display $fred "Height", "Weight";
- $fred = Critter->find("Fred");
- $fred->display('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.
-or in one statement,
+But what if there are no arguments? In that case, Perl must guess what
+you want. Even worse, it must make that guess I<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.
- Critter->find("Fred")->display('Height', 'Weight');
+For example, a call to a method C<new> in indirect notation -- as C++
+programmers are wont to make -- can be miscompiled into a subroutine
+call if there's already a C<new> function in scope. You'd end up
+calling the current package's C<new> as a subroutine, rather than the
+desired class's method. The compiler tries to cheat by remembering
+bareword C<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 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 parens of
+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 C<print> and C<printf>.)
+This can lead to horribly confusing precedence problems, as in these
+next two lines:
- new Critter ('Barney', 1.5, 70)
+ move $obj->{FIELD}; # probably wrong!
+ move $ary[$i]; # probably wrong!
-are assumed to surround ALL the arguments of the method call, regardless
-of what comes after. Saying
+Those actually parse as the very surprising:
- new Critter ('Bam' x 2), 1.4, 45
+ $obj->move->{FIELD}; # Well, lookee here
+ $ary->move([$i]); # Didn't expect this one, eh?
-would be equivalent to
+Rather than what you might have expected:
- Critter->new('Bam' x 2), 1.4, 45
+ $obj->{FIELD}->move(); # You should be so lucky.
+ $ary[$i]->move; # Yeah, sure.
-which is unlikely to do what you want.
+To get the correct behavior with indirect object syntax, you would have
+to use a block around the indirect object:
-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:
+ move {$obj->{FIELD}};
+ move {$ary[$i]};
- $fred = MyCritter::find("Critter", "Fred");
- MyCritter::display($fred, 'Height', 'Weight');
+Even then, you still have the same potential problem if there happens to
+be a function named C<move> in the current package. B<The C<< -> >>
+notation suffers from neither of these disturbing ambiguities, so we
+recommend you use it exclusively.> However, you may still end up having
+to read code using the indirect object notation, so it's important to be
+familiar with it.
-Note however, that this does not do any inheritance. If you merely
-wish to specify that Perl should I<START> looking for a method in a
-particular package, use an ordinary method call, but qualify the method
-name with the package like this:
+=head2 Default UNIVERSAL methods
+X<UNIVERSAL>
- $fred = Critter->MyCritter::find("Fred");
- $fred->MyCritter::display('Height', 'Weight');
+The C<UNIVERSAL> package automatically contains the following methods that
+are inherited by all other classes:
+
+=over 4
+
+=item isa(CLASS)
+X<isa>
+
+C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
+
+You can also call C<UNIVERSAL::isa> as a subroutine with two arguments. Of
+course, this will do the wrong thing if someone has overridden C<isa> in a
+class, so don't do it.
+
+If you need to determine whether you've received a valid invocant, use the
+C<blessed> function from L<Scalar::Util>:
+X<invocant> X<blessed>
+
+ if (blessed($ref) && $ref->isa( 'Some::Class')) {
+ # ...
+ }
+
+C<blessed> returns the name of the package the argument has been
+blessed into, or C<undef>.
+
+=item can(METHOD)
+X<can>
+
+C<can> checks to see if its object has a method called C<METHOD>,
+if it does then a reference to the sub is returned, if it does not then
+I<undef> is returned.
+
+C<UNIVERSAL::can> can also be called as a subroutine with two arguments. It'll
+always return I<undef> if its first argument isn't an object or a class name.
+The same caveats for calling C<UNIVERSAL::isa> directly apply here, too.
+
+=item VERSION( [NEED] )
+X<VERSION>
+
+C<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 $VERSION variable in the given package) not less than
+NEED; it will die if this is not the case. This method is normally
+called as a class method. This method is called automatically by the
+C<VERSION> form of C<use>.
+
+ use A 1.2 qw(some imported subs);
+ # implies:
+ A->VERSION(1.2);
+
+=back
+
+B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
+C<isa> uses a very similar method and cache-ing strategy. This may cause
+strange effects if the Perl code dynamically changes @ISA in any package.
+
+You may add other methods to the UNIVERSAL class via Perl or XS code.
+You do not need to C<use UNIVERSAL> to make these methods
+available to your program (and you should not do so).
=head2 Destructors
+X<destructor> X<DESTROY>
When the last reference to an object goes away, the object is
automatically destroyed. (This may even be after you exit, if you've
stored references in global variables.) If you want to capture control
just before the object is freed, you may define a DESTROY method in
your class. It will automatically be called at the appropriate moment,
-and you can do any extra cleanup you need to do.
-
-Perl doesn't do nested destruction for you. If your constructor
-reblessed a reference from one of your base classes, your DESTROY may
-need to call DESTROY for any base classes that need it. But this only
-applies to reblessed objects--an object reference that is merely
-I<CONTAINED> in the current object will be freed and destroyed
-automatically when the current object is freed.
+and you can do any extra cleanup you need to do. Perl passes a reference
+to the object under destruction as the first (and only) argument. Beware
+that the reference is a read-only value, and cannot be modified by
+manipulating C<$_[0]> within the destructor. The object itself (i.e.
+the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>,
+C<%{$_[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 I<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.
=head2 Summary
-That's about all there is to it. Now you just need to go off and buy a
+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.
+=head2 Two-Phased Garbage Collection
+X<garbage collection> X<GC> X<circular reference>
+X<reference, circular> X<DESTROY> X<destructor>
+
+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 C<-O> flag, performance
+will suffer. If you I<have> built Perl with C<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 I<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 $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 = \$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 F</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
+C<-DDEBUGGING> was enabled during perl build time.
+See L<perlhack/PERL_DESTRUCT_LEVEL> 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 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.
+
=head1 SEE ALSO
-You should also check out L<perlbot> for other object tricks, traps, and tips.
+A kinder, gentler tutorial on object-oriented programming in Perl can
+be found in L<perltoot>, L<perlboot> and L<perltooc>. You should
+also check out L<perlbot> for other object tricks, traps, and tips, as
+well as L<perlmodlib> for some style guides on constructing both
+modules and classes.
https://perl5.git.perl.org / perl5.git / blobdiff