=head1 NAME
+X<object> X<OOP>
perlobj - Perl objects
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<perltootc>.
+in Perl can be found in L<perltoot> and L<perltooc>.
If you're still with us, then
here are three very simple definitions that you should find reassuring.
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
}
Or if you expect people to call not just C<< CLASS->new() >> but also
-C<< $obj->new() >>, then use something like this. The initialize()
-method used will be of whatever $class we blessed the
-object into:
+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;
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 use a package as a class by putting method
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
+class package. The classes are searched for missing methods in
+depth-first, left-to-right order by default (see L<mro> for alternative
+search order and other in-depth information). The classes accessible
through @ISA are known as base classes of the current class.
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.
+supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> or
+L<UNIVERSAL|UNIVERSAL> 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
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>
sub AUTOLOAD;
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;
}
=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
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<perltootc> for alternatives.
+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
}
=head2 Method Invocation
+X<invocation> X<method> X<arrow> X<< -> >>
-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
+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:
- print STDERR "help!!!\n";
+ my $fred = Critter->find("Fred");
+ $fred->display("Height", "Weight");
-This same syntax can be used to call either class or instance methods.
-We'll use the two methods defined above, the class method to lookup
-an object reference and the instance method to print out its attributes.
+You should already be familiar with the use of the C<< -> >> operator with
+references. In fact, since C<$fred> above is a reference to an object,
+you could think of the method call as just another form of
+dereferencing.
- $fred = find Critter "Fred";
- display $fred 'Height', 'Weight';
+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:
-These could be combined into one statement by using a BLOCK in the
-indirect object slot:
+ my $fred = Critter::find("Critter", "Fred");
+ Critter::display($fred, "Height", "Weight");
- display {find Critter "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.
-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.
+If you need to, you I<can> force Perl to start looking in some other package:
- $fred = Critter->find("Fred");
- $fred->display('Height', 'Weight');
+ my $barney = MyCritter->Critter::find("Barney");
+ $barney->Critter::display("Height", "Weight");
-or in one statement,
+Here C<MyCritter> is presumably a subclass of C<Critter> that defines
+its own versions of find() and display(). We haven't specified what
+those methods do, but that doesn't matter above since we've forced Perl
+to start looking for the subroutines in C<Critter>.
- Critter->find("Fred")->display('Height', 'Weight');
+As a special case of the above, you may use the C<SUPER> pseudo-class to
+tell Perl to start looking for the method in the packages named in the
+current class's C<@ISA> list.
+X<SUPER>
-There are times when one syntax is more readable, and times when the
-other syntax is more readable. The indirect object syntax is less
-cluttered, but it has the same ambiguity as ordinary list operators.
-Indirect object method calls are usually parsed using the same rule as list
-operators: "If it looks like a function, it is a function". (Presuming
-for the moment that you think two words in a row can look like a
-function name. C++ programmers seem to think so with some regularity,
-especially when the first word is "new".) Thus, the parentheses of
+ package MyCritter;
+ use base 'Critter'; # sets @MyCritter::ISA = ('Critter');
- new Critter ('Barney', 1.5, 70)
-
-are assumed to surround ALL the arguments of the method call, regardless
-of what comes after. Saying
-
- new Critter ('Bam' x 2), 1.4, 45
-
-would be equivalent to
+ sub display {
+ my ($self, @args) = @_;
+ $self->SUPER::display("Name", @args);
+ }
- Critter->new('Bam' x 2), 1.4, 45
+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>
-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 C<Package::> qualified package name.
-In those cases, the arguments are parsed in the same way as an
-indirect object list operator like print, so
+ something->SUPER::method(...); # OK
+ SUPER::method(...); # WRONG
+ SUPER->method(...); # WRONG
- new Critter:: ('Bam' x 2), 1.4, 45
+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:
-is the same as
+ Critter->find("Fred")->display("Height", "Weight");
- Critter::->new(('Bam' x 2), 1.4, 45)
+and so is the following:
-For more reasons why the indirect object syntax is ambiguous, see
-L<"WARNING"> below.
+ my $fred = (reverse "rettirC")->find(reverse "derF");
-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:
+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.
- $fred = MyCritter::find("Critter", "Fred");
- MyCritter::display($fred, 'Height', 'Weight');
+If the right side of the arrow is a scalar containing a reference
+to a subroutine, then this is equivalent to calling the referenced
+subroutine directly with the class name or object on the left side
+of the arrow as its first argument. No lookup is done and there is
+no requirement that the subroutine be defined in any package related
+to the class name or object on the left side of the arrow.
-Unlike method calls, function calls don't consider inheritance. If you wish
-merely 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:
+For example, the following calls to $display are equivalent:
- $fred = Critter->MyCritter::find("Fred");
- $fred->MyCritter::display('Height', 'Weight');
+ my $display = sub { my $self = shift; ... };
+ $fred->$display("Height", "Weight");
+ $display->($fred, "Height", "Weight");
-If you're trying to control where the method search begins I<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:
+=head2 Indirect Object Syntax
+X<indirect object syntax> X<invocation, indirect> X<indirect>
- $self->SUPER::display('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:
-Please note that the C<SUPER::> construct is meaningful I<only> within the
-class.
+ print STDERR "help!!!\n";
-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.
+The same syntax can be used to call either object or class methods.
- $method = $fast ? "findfirst" : "findbest";
- $fred->$method(@args); # call by name
+ my $fred = find Critter "Fred";
+ display $fred "Height", "Weight";
- if ($coderef = $fred->can($parent . "::findbest")) {
- $self->$coderef(@args); # call by coderef
- }
+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.
-=head2 WARNING
+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.
-While indirect object syntax may well be appealing to English speakers and
-to C++ programmers, be not seduced! It suffers from two grave problems.
+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.
-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 C<print> and C<printf>.) This can lead to horribly
-confusing precedence problems, as in these next two lines:
+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:
move $obj->{FIELD}; # probably wrong!
move $ary[$i]; # probably wrong!
$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 I<at
-compile time> whether C<name> and C<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 C<new>
-in indirect notation--as C++ programmers are so wont to do--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 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 ``C<< -> >>'' doesn't suffer from either
-of these disturbing ambiguities, so we recommend you use it exclusively.
+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[$i]};
+
+Even then, you still have the same potential problem if there happens to
+be a function named C<move> in the current package. B<The C<< -> >>
+notation suffers from neither of these disturbing ambiguities, so we
+recommend you use it exclusively.> However, you may still end up having
+to read code using the indirect object notation, so it's important to be
+familiar with it.
=head2 Default UNIVERSAL methods
+X<UNIVERSAL>
The C<UNIVERSAL> package automatically contains the following methods that
are inherited by all other classes:
=over 4
=item isa(CLASS)
+X<isa>
C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
-C<isa> is also exportable and can be called as a sub with two arguments. This
-allows the ability to check what a reference points to. Example
+=item DOES(ROLE)
+X<DOES>
- use UNIVERSAL qw(isa);
-
- if(isa($ref, 'ARRAY')) {
- #...
- }
+C<DOES> returns I<true> if its object claims to perform the role C<ROLE>. By
+default, this is equivalent to C<isa>.
=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<undef> is returned.
=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 $VERSION variable in the given package) not less than
-NEED; it will die if this is not the case. This method is normally
-called as a class method. This method is called automatically by the
-C<VERSION> form of C<use>.
+NEED; it will die if this is not the case. This method is called automatically
+by the C<VERSION> form of C<use>.
- use A 1.2 qw(some imported subs);
+ use Package 1.2 qw(some imported subs);
# implies:
- A->VERSION(1.2);
+ Package->VERSION(1.2);
=back
-B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
-C<isa> uses a very similar method and cache-ing strategy. This may cause
-strange effects if the Perl code dynamically changes @ISA in any package.
-
-You may add other methods to the UNIVERSAL class via Perl or XS code.
-You do not need to C<use UNIVERSAL> to make these methods
-available to your program. This is necessary only if you wish to
-have C<isa> available as a plain subroutine in the current package.
-
=head2 Destructors
+X<destructor> X<DESTROY>
When the last reference to an object goes away, the object is
automatically destroyed. (This may even be after you exit, if you've
the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>,
C<%{$_[0]}> etc.) is not similarly constrained.
+Since DESTROY methods can be called at unpredictable times, it is
+important that you localise any global variables that the method may
+update. In particular, localise C<$@> if you use C<eval {}> and
+localise C<$?> if you use C<system> or backticks.
+
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
of your choosing get called. Explicitly calling DESTROY is also possible,
but is usually never needed.
+DESTROY is subject to AUTOLOAD lookup, just like any other method. Hence, if
+your class has an AUTOLOAD method, but does not need any DESTROY actions,
+you probably want to provide a DESTROY method anyway, to prevent an
+expensive call to AUTOLOAD each time an object is freed. As this technique
+makes empty DESTROY methods common, the implementation is optimised so that
+a DESTROY method that is an empty or constant subroutine, and hence could
+have no side effects anyway, is not actually called.
+X<AUTOLOAD> X<DESTROY>
+
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
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
node such as one might use in a sophisticated tree structure:
sub new_node {
- my $self = shift;
- my $class = ref($self) || $self;
- my $node = {};
+ my $class = shift;
+ my $node = {};
$node->{LEFT} = $node->{RIGHT} = $node;
$node->{DATA} = [ @_ ];
return bless $node => $class;
warn "time to die...";
exit;
-When run as F</tmp/test>, the following output is produced:
+When run as F</foo/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.
+ 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
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.
=head1 SEE ALSO
-A kinder, gentler tutorial on object-oriented programming in Perl
-can be found in L<perltoot> and L<perltootc>. 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.
+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