=head1 DESCRIPTION
-First of all, you need to understand what references are in Perl.
+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>.
+in Perl can be found in L<perltoot> and L<perltootc>.
If you're still with us, then
here are three very simple definitions that you should find reassuring.
package Critter;
sub spawn { bless {} }
-In fact, this might even be preferable, because the C++ programmers won't
+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,
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 {
return $self;
}
-Or if you expect people to call not just C<CLASS-E<gt>new()> but also
-C<$obj-E<gt>new()>, then use something like this. The initialize()
+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:
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.) If you find yourself having to
-do this, the parent class is probably misbehaving, though.
+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()
supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
more details.
-If a missing method is found in one of the base classes, it is cached
+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 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
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 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.)
+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 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
+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.
+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
=head2 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
+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.
- print STDERR "help!!!\n";
+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:
-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.
+ my $fred = Critter::find("Critter", "Fred");
+ Critter::display($fred, "Height", "Weight");
- $fred = find Critter "Fred";
- 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.
-These could be combined into one statement by using a BLOCK in the
-indirect object slot:
+If you need to, you I<can> force Perl to start looking in some other package:
- display {find Critter "Fred"} 'Height', 'Weight';
+ my $barney = MyCritter->Critter::find("Barney");
+ $barney->Critter::display("Height", "Weight");
-For C++ fans, there's also a syntax using -E<gt> notation that does exactly
-the same thing. The parentheses are required if there are any arguments.
+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>.
- $fred = Critter->find("Fred");
- $fred->display('Height', 'Weight');
+As a special case of the above, you may use the C<SUPER> pseudo-class to
+tell Perl to start looking for the method in the packages named in the
+current class's C<@ISA> list.
-or in one statement,
+ package MyCritter;
+ use base 'Critter'; # sets @MyCritter::ISA = ('Critter');
- Critter->find("Fred")->display('Height', 'Weight');
+ sub display {
+ my ($self, @args) = @_;
+ $self->SUPER::display("Name", @args);
+ }
-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
+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:
- new Critter ('Barney', 1.5, 70)
+ Critter->find("Fred")->display("Height", "Weight");
-are assumed to surround ALL the arguments of the method call, regardless
-of what comes after. Saying
+and so is the following:
- new Critter ('Bam' x 2), 1.4, 45
+ my $fred = (reverse "rettirC")->find(reverse "derF");
-would be equivalent to
+=head2 Indirect Object Syntax
- Critter->new('Bam' x 2), 1.4, 45
+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:
-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
+ print STDERR "help!!!\n";
- new Critter:: ('Bam' x 2), 1.4, 45
+The same syntax can be used to call either object or class methods.
-is the same as
+ my $fred = find Critter "Fred";
+ display $fred "Height", "Weight";
- Critter::->new(('Bam' x 2), 1.4, 45)
+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.
-For more reasons why the indirect object syntax is ambiguous, see
-L<"WARNING"> below.
+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.
-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:
+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.
- $fred = MyCritter::find("Critter", "Fred");
- MyCritter::display($fred, 'Height', 'Weight');
+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:
-Note however, that this does not do any 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:
+ move $obj->{FIELD}; # probably wrong!
+ move $ary[$i]; # probably wrong!
+
+Those actually parse as the very surprising:
- $fred = Critter->MyCritter::find("Fred");
- $fred->MyCritter::display('Height', 'Weight');
+ $obj->move->{FIELD}; # Well, lookee here
+ $ary->move([$i]); # Didn't expect this one, eh?
-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:
+Rather than what you might have expected:
- $self->SUPER::display('Height', 'Weight');
+ $obj->{FIELD}->move(); # You should be so lucky.
+ $ary[$i]->move; # Yeah, sure.
-Please note that the C<SUPER::> construct is meaningful I<only> within the
-class.
+To get the correct behavior with indirect object syntax, you would have
+to use a block around the indirect object:
-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:
+ move {$obj->{FIELD}};
+ move {$ary[$i]};
- $method = $fast ? "findfirst" : "findbest";
- $fred->$method(@args);
+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
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
-
- use UNIVERSAL qw(isa);
+You can also call C<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(isa($ref, 'ARRAY')) {
+ if(UNIVERSAL::isa($ref, 'ARRAY')) {
#...
}
+To determine if a reference is a blessed object, you can write
+
+ print "It's an object\n" if UNIVERSAL::isa($val, 'UNIVERSAL');
+
=item can(METHOD)
C<can> checks to see if its object has a method called C<METHOD>,
if it does then a reference to the sub is returned, if it does not then
I<undef> is returned.
+C<UNIVERSAL::can> can also be called as a subroutine with two arguments.
+It'll always return I<undef> if its first argument isn't an object or a
+class name. So here's another way to check if a reference is a
+blessed object
+
+ print "It's still an object\n" if UNIVERSAL::can($val, 'can');
+
+You can also use the C<blessed> function of Scalar::Util:
+
+ use Scalar::Util 'blessed';
+
+ my $blessing = blessed $suspected_object;
+
+C<blessed> returns the name of the package the argument has been
+blessed into, or C<undef>.
+
=item VERSION( [NEED] )
C<VERSION> returns the version number of the class (package). If the
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> in order 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.
+You do not need to C<use UNIVERSAL> to make these methods
+available to your program (and you should not do so).
=head2 Destructors
of your choosing get called. Explicitly calling DESTROY is also possible,
but is usually never needed.
-Do not confuse the foregoing with how objects I<CONTAINED> in the current
+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 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 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!
-
-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 ``-E<gt>'' 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 to track such subtle bugs down.
-
-The infix arrow notation using ``C<-E<gt>>'' doesn't suffer from either
-of these disturbing ambiguities, so we recommend you use it exclusively.
-
=head2 Summary
That's about all there is to it. Now you need just to go off and buy a
=head2 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
+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
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
+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.
=head1 SEE ALSO
A kinder, gentler tutorial on object-oriented programming in Perl can
-be found in L<perltoot>.
-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.
+be found in L<perltoot>, L<perlbootc> 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.
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