X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/38242c004862c1bab670bd395f573231149ffa1a..af36000c88ef5604a340513b05466c8690459612:/pod/perlobj.pod diff --git a/pod/perlobj.pod b/pod/perlobj.pod index f31ce2c..da27c5a 100644 --- a/pod/perlobj.pod +++ b/pod/perlobj.pod @@ -1,545 +1,843 @@ +=encoding utf8 + +=for comment +Consistent formatting of this file is achieved with: + perl ./Porting/podtidy pod/perlobj.pod + =head1 NAME +X X -perlobj - Perl objects +perlobj - Perl object reference =head1 DESCRIPTION -First you need to understand what references are in Perl. -See L 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 and L. +This document provides a reference for Perl's object orientation +features. If you're looking for an introduction to object-oriented +programming in Perl, please see L. + +In order to understand Perl objects, you first need to understand +references in Perl. See L for details. -If you're still with us, then -here are three very simple definitions that you should find reassuring. +This document describes all of Perl's OO features from the ground up. +If you're just looking to write some object-oriented code of your own, +you are probably better served by using one of the object systems from +CPAN described in L. + +If you're looking to write your own object system, or you need to +maintain code which implements objects from scratch then this document +will help you understand exactly how Perl does object orientation. + +There are a few basic principles which define object oriented Perl: =over 4 =item 1. -An object is simply a reference that happens to know which class it -belongs to. +An object is simply a reference that knows which class it belongs to. =item 2. -A class is simply a package that happens to provide methods to deal -with object references. +A class is simply a package. A class provides methods that expect to +operate on objects. =item 3. -A method is simply a subroutine that expects an object reference (or -a package name, for class methods) as the first argument. +A method is simply a subroutine that expects an object reference (or a +package name, for class methods) as the first argument. =back -We'll cover these points now in more depth. +Let's look at each of these principles in depth. =head2 An Object is Simply a Reference +X X X X -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 C 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 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 = {}; - 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 -L), -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 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. +Unlike many other languages which support object orientation, Perl does +not provide any special syntax for constructing an object. A +constructor is just a subroutine that returns a reference which has +been "blessed" into a class. -=head2 A Class is Simply a Package +Here is a simple constructor: + + package File; + + sub new { + my $class = shift; + + return bless {}, $class; + } + +The name C isn't special. We could name our constructor something +else: + + sub load { + my $class = shift; -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. + return bless {}, $class; + } -There is a special array within each package called @ISA, which says -where else to look for a method if you can't find it in the current -package. This is how Perl implements inheritance. Each element of the -@ISA array is just the name of another package that happens to be a -class package. The classes are searched (depth first) for missing -methods in the order that they occur in @ISA. The classes accessible -through @ISA are known as base classes of the current class. +The modern convention for OO modules is to use C as the +constructor, but you are free to use the name if that works best for +your class. -All classes implicitly inherit from class C as their -last base class. Several commonly used methods are automatically -supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for -more details. +The C<{}> code creates an empty anonymous hash reference. The C +function takes that reference and associates it with the class in +C<$class>. In the simplest case, the C<$class> variable will end up +containing the string "Critter". -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. +We can also call bless on a named variable: -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. + sub new { + my $class = shift; -If none of that works, Perl finally gives up and complains. + my $self = {}; + bless $self, $class; -If you want to stop the AUTOLOAD inheritance say simply + return $self; + } - sub AUTOLOAD; +Once we've blessed C<$self> we can start calling methods on it. This is +useful if you want to break out object initialization to its own +method: -and the call will die using the name of the sub being called. + sub new { + my $class = shift; -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. + my $self = {}; + bless $self, $class; - sub bump { - my $self = shift; - $self->{ __PACKAGE__ . ".count"}++; - } + $self->_initialize(); + + return $self; + } + +Since the object is also a hash reference, you can treat it as one, +using it store data associated with the object. Typically, code inside +the class can treat the hash as reference, while code outside the class +should always treat the object as opaque. This is called +B. Encapsulation means that the user of an object does +not have to know how it is implemented. The user simply calls +documented methods on the object. + +=head2 A Class is Simply a Package +X X X<@ISA> X + +Perl does not provide any special syntax for class definitions. A +package is simply a namespace containing variables and subroutines. The +only difference is that in a class, the subroutines expect an object or +class as the first argument. + +Each package contains a special array reference called C<@ISA>. The +C<@ISA> array contains a list of that class's parent classes, if any. +This array is examined when Perl does method resolution, which we will +cover later. + +It is possible to manually set C<@ISA>, and you may see this in older +Perl code. For new code, we recommend that you use the L pragma +to declare your parents. This pragma will take care of setting C<@ISA>. +It will also load the parent classes and make sure that the package +doesn't inherit from itself. + +All classes inherit from the L class implicitly. The +L class is implemented by the Perl core, and provides +several default methods, such as C, C, and C. +X X X + +Perl I provides method inheritance as a built-in feature. +Attribute inheritance is left up the class to implement. See the +L section for details. =head2 A Method is Simply a Subroutine +X + +Perl does not provide any special syntax for defining a method. A +method is simply a regular subroutine, and is declared with C. +What makes a method special is that it expects to receive either an +object or a class name as its first argument. + +Perl I provide special syntax for method invocation, the C<< -> +>> operator. We will cover this in more detail later. -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 L and L 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"; - } - } +Most methods you write will expect to operate on objects: + + sub save { + my $self = shift; + + open my $fh, '>', $self->path() + or die $!; + print {$fh} $self->data() + or die $!; + close $fh + or die $!; + } =head2 Method Invocation +X X X X<< -> >> + +Calling a method on an object is written as C<< $object->method >>. The +left hand side of the method invocation (or arrow) operator is the +object (or class name), and the right hand side is the method name. + + my $pod = File->new( 'perlobj.pod', $data ); + $pod->save(); + +The C<< -> >> operator is also used when dereferencing a reference. +It's the same operator, but it has two different uses. + +When you call a method, the thing on the left side of the arrow is +passed as the first argument to the method. That means when we call C<< +Critter->new() >>, the C method receives the string C<"Critter"> +as its first argument. When we call C<< $fred->speak() >>, the C<$fred> +object is passed as the first argument to C + +Perl knows what package the method is in by looking at the left side of +the arrow. If the left hand side is a package name, it looks for the +method in the package. If the left hand side is an object, then Perl +can tell what class that object has been blessed into. + +If the left hand side is neither a package name nor an object, then the +method call will cause an error, but see the section on L for more nuances. + +=head2 Inheritance +X + +We already talked about the special C<@ISA> array and the L +pragma. + +When a class inherits from another class, any methods defined in the +parent class are available in the child class. If you attempt to call a +method on an object that isn't defined in its own class, Perl will also +look for that method in any parent classes it may have. + + package File::MP3; + use parent 'File'; # sets @File::MP3::ISA = ('File'); + + my $mp3 = File::MP3->new( 'Andvari.mp3', $data ); + $mp3->save(); + +Since we didn't define a C method in the C class, +Perl will look at the C class's parent classes to find the +C method. If Perl could not find a C method anywhere in +the inheritance hierarchy, it would die. + +In this case, it finds it in the C class. Note that the object +passed to C in this case is still a C object, even +though the method is found in the C class. + +We can override a parent's method in a child class. When we do so, we +can still call the parent class's method with the C +pseudo-class. + + sub save { + my $self = shift; + + say 'Prepare to rock'; + $self->SUPER::save(); + } + +The C modifier can I be used for method calls. You can't +use it for regular subroutine calls: + + # XXX - This will not work! + SUPER::save($thing); + # This won't work either! + SUPER->save($thing); + +=head3 How SUPER is Resolved +X + +The C pseudo-class is resolved from the package where the call +is made. It is I resolved based on the object's class. This is +important, because it lets an inheritance hierarchy several levels deep +all call their parent methods. + + package A; + + sub new { + return bless {}, shift; + } + + sub speak { + my $self = shift; + + $self->SUPER::speak(); + + say 'A'; + } + + package B; + + use parent 'A'; + + sub speak { + my $self = shift; + + $self->SUPER::speak(); + + say 'B'; + } + + package C; + + use parent 'B'; + + sub speak { + my $self = shift; + + $self->SUPER::speak(); + + say 'C'; + } + + my $c = C->new(); + $c->speak(); + +In this example, we will get the following outupt: + + A + B + C + +This demonstrates how C is resolved. Even though the object is +blessed into the C class, the C method in the C class +can still call C. + +There are cases where this package-based resolution can be a problem. +If you copy a subroutine from one package to another, C +resolution will be done based on the original package. + +=head3 Multiple Inheritance +X + +Multiple inheritance often indicates a design problem, but Perl always +give you enough rope to hang yourself with. + +To declare multiple parents, you simply need to pass multiple class +names to C: + + package MultiChild; + + use parent 'Parent1', 'Parent2'; + +=head3 Method Resolution Order +X X + +Method resolution order only matters in the case of multiple +inheritance. In the case of single inheritance, Perl simply looks up +the inheritance chain to find a method: + + Grandparent + | + Parent + | + Child + +If we call a method on a C object and that method is not defined +in the C class, Perl will look for that method. in the C +class and then the C class. + +If Perl cannot find the method in any of these classes, it will die +with an error message. + +When a class has multiple parents, the method lookup order becomes more +complicated. + +By default, Perl does a depth-first left-to-right search for a method. +That means it starts with the first parent in the C<@ISA> array, and +then searches all of its parents and so on. If it fails to find the +method, it then goes to the next parent in the original class's C<@ISA> +array and searches from there. + + PaternalGrandparent MaternalGrandparent + \ / + Father Mother + \ / + Child + +So given the diagram above, Perl will search C, C, +C, C, and finally C + +It is possible to ask for a different method resolution order with the +L pragma. + + package Child; + + use mro 'c3'; + use parent 'Father', 'Mother'; + +This pragma lets you switch to the "C3" resolution order. In simple +terms, this is a breadth-first order, so Perl will search C, +C, C, C, and finally +C. + +The C3 order also lets you call methods in sibling classes with the +C pseudo-class. See the L documentation for more details on +this feature. + +=head3 Method Resolution Caching + +When Perl searches for a method, it caches the lookup so that future +calls to the method do not need to search for it again. Changing a +class's parent class or adding subroutines to a class will invalidate +the cache for that class. + +The L pragma provides some functions for manipulating the method +cache directly. -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: +=head2 Writing Constructors +X - my $fred = Critter->find("Fred"); - $fred->display("Height", "Weight"); +As we mentioned earlier, Perl provides no special constructor syntax. +This means that a class must implement its own constructor. A +constructor is simply a class method that returns a new object. -You should already be familiar with the use of the C<< -> >> operator with -references. In fact, since C<$fred> above is a reference to an object, -you could think of the method call as just another form of -dereferencing. +The constructor can also accept additional parameters that define the +object. Let's write a real constructor for the C class we used +earlier: -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: + package File; - my $fred = Critter::find("Critter", "Fred"); - Critter::display($fred, "Height", "Weight"); + sub new { + my $class = shift; + my ( $path, $data ) = @_; -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. + my $self = bless { + path => $path, + data => $data, + }, $class; -If you need to, you I force Perl to start looking in some other package: + return $self; + } - my $barney = MyCritter->Critter::find("Barney"); - $barney->Critter::display("Height", "Weight"); +As you can see, we've stored the path and file data in the object +itself. Remember, under the hood, this object is still just a hash +reference. Later, we'll write accessors to manipulate this data. -Here C is presumably a subclass of C 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. +For our File::MP3 class, we can check to make sure that the path we're +given ends with ".mp3": -As a special case of the above, you may use the C pseudo-class to -tell Perl to start looking for the method in the packages named in the -current class's C<@ISA> list. + package File::MP3; - package MyCritter; - use base 'Critter'; # sets @MyCritter::ISA = ('Critter'); + sub new { + my $class = shift; + my ( $path, $data ) = @_; - sub display { - my ($self, @args) = @_; - $self->SUPER::display("Name", @args); - } + die "You cannot create a File::MP3 without an mp3 extension\n" + unless $path =~ /\.mp3\z/; -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: + return $class->SUPER::new(@_); + } - Critter->find("Fred")->display("Height", "Weight"); +This constructor lets its parent class do the actual object +construction. -and so is the following: +=head2 Writing Accessors +X - my $fred = (reverse "rettirC")->find(reverse "derF"); +As with constructors, Perl provides no special accessor declaration +syntax, so classes must write them by hand. There are two common types +of accessors, read-only and read-write. -=head2 Indirect Object Syntax +A simple read-only accessor simply gets the value of a single +attribute: -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: + sub path { + my $self = shift; - print STDERR "help!!!\n"; + return $self->{path}; + } -The same syntax can be used to call either object or class methods. +A read-write accessor will allow the caller to set the value as well as +get it: - my $fred = find Critter "Fred"; - display $fred "Height", "Weight"; + sub path { + my $self = shift; -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. + if (@_) { + $self->{path} = shift; + } -But what if there are no arguments? In that case, Perl must guess what -you want. Even worse, it must make that guess I. -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. + return $self->{path}; + } -For example, a call to a method C in indirect notation -- as C++ -programmers are wont to make -- can be miscompiled into a subroutine -call if there's already a C function in scope. You'd end up -calling the current package's C as a subroutine, rather than the -desired class's method. The compiler tries to cheat by remembering -bareword Cs, but the grief when it messes up just isn't worth the -years of debugging it will take you to track down such subtle bugs. +=head2 An Aside About Smarter and Safer Code -There is another problem with this syntax: the indirect object is -limited to a name, a scalar variable, or a block, because it would have -to do too much lookahead otherwise, just like any other postfix -dereference in the language. (These are the same quirky rules as are -used for the filehandle slot in functions like C and C.) -This can lead to horribly confusing precedence problems, as in these -next two lines: +Our constructor and accessors are not very smart. They don't check that +a C<$path> is defined, nor do they check that a C<$path> is a valid +filesystem path. - move $obj->{FIELD}; # probably wrong! - move $ary[$i]; # probably wrong! +Doing these checks by hand can quickly become tedious. Writing a bunch +of accessors by hand is also incredibly tedioud. There are a lot of +modules on CPAN that can help you write safer and more concise code, +including the modules we recommend in L. -Those actually parse as the very surprising: +=head2 Method Call Variations +X - $obj->move->{FIELD}; # Well, lookee here - $ary->move([$i]); # Didn't expect this one, eh? +Perl supports several other ways to call methods besides the typical +C<< $object->method() >> pattern we've seen so far. -Rather than what you might have expected: +=head3 Method Names as Strings - $obj->{FIELD}->move(); # You should be so lucky. - $ary[$i]->move; # Yeah, sure. +Perl lets you use a scalar variable containing a string as a method +name: -To get the correct behavior with indirect object syntax, you would have -to use a block around the indirect object: + my $file = File->new( $path, $data ); - move {$obj->{FIELD}}; - move {$ary[$i]}; + my $method = 'save'; + $file->$method(); -Even then, you still have the same potential problem if there happens to -be a function named C in the current package. B >> -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. +This works exactly like calling C<< $file->save() >>. This can be very +useful for writing dynamic code. For example, it allows you to pass a +method name to be called as a parameter to another method. -=head2 Default UNIVERSAL methods +=head3 Class Names as Strings -The C package automatically contains the following methods that -are inherited by all other classes: +Perl also lets you use a scalar containing a string as a class name: + + my $class = 'File'; + + my $file = $class->new( $path, $data ); + +Again, this allows for very dynamic code. + +=head3 Subroutine References as Methods + +You can also call subroutine reference as a method: + + my $sub = sub { + my $self = shift; + + $self->save(); + }; + + $file->$sub(); + +This is exactly equivalent to writing C<< $sub->($file) >>. You may see +this idiom in the wild combined with a call to C: + + if ( my $meth = $object->can('foo') ) { + $object->$meth(); + } + +=head2 Invoking Class Methods +X + +Because Perl allows you to use barewords for package names and +subroutine names, it can sometimes guess wrong about what you intend a +bareword to be. The construct C<< Class->new() >> can be interpreted as +C<< Class()->new() >>. In English, that reads as "call a subroutine +named Class(), then call new() as a method on the return value". + +You can force Perl to interpret this as a class method call in two +ways. First, you can append a C<::> to the class name: + + Class::->new() + +Perl will always interpret this as a method call. You can also quote +the class name: + + 'Class'->new() + +Of course, if the class name is in a scalar Perl will do the right +thing as well: + + my $class = 'Class'; + $class->new(); + +=head3 Indirect Object Syntax +X + +B + +Perl suports another method invocation syntax called "indirect object" +notation. + +This notation is quite common when used with file handles: + + print $fh $data; + +This is really like calling C<< $fh->print($data) >>. This syntax is +called "indirect" because the method comes before the object it is +being invoked on. + +This syntax can be used with any class or object method: + + my $file = new File $path, $data; + save $file; + +We recommend that you avoid this syntax, for several reasons. + +First, it can be confusing to read. In the above example, it's not +clear if C is a method or simply a subroutine that expects a file +object as its first argument. + +When used with class methods, the problem is even worse. Because Perl +allows subroutine names to be written as barewords, Perl has to guess +whether the bareword after the method is a class name or subroutine +name. In other words, Perl can resolve the syntax as either C<< +File->new( $path, $data ) >> B C<< new( File( $path, $data ) ) >>. + +To parse this code, Perl uses a heuristic based on what package names +it has seen, what subroutines exist in the current package, and what +barewords it has seen previously. Needless to say, heuristics can +produce very surprising results! + +Older documentation (and some CPAN modules) encouraged this syntax, +particularly for constructors, so you may still find it in the wild. +However, we encourage you to avoid using it in new code. + +=head2 C, C, and C + +As we saw earlier, an object is simply a reference that has been +blessed into a class with the C function. The C function +has can take either one or two arguments: + + my $object = bless {}, $class; + my $object = bless {}; + +In the first form, the anonymous hash reference is being blessed into +the class in C<$class>. In the second form, the reference is blessed +into the current package. + +The second form is discouraged, because it breaks the ability of a +subclass to reuse the parent's constructor, but you may still run +across it in existing code. + +If you want to know whether a particular scalar is an object, you can +use the C function exported by L, which is +shipped with the Perl core. + + use Scalar::Util 'blessed'; + + if ( blessed($thing) ) { ... } + +If the C<$thing> has been blessed, then this function returns the name +of the package the object has been blessed into. Note that the example +above will return false if C<$thing> has been blessed into a class +named "0". If the C<$thing> is not a blessed reference, the C +function returns false. + +Similarly, Perl's built in C function treats a blessed reference +specially. If you call C and C<$thing> is an object, it +will return the name of the class that the object has been blessed +into. + +If you simply want to check that a variable contains an object, we +recommend that you use C, since C returns true values for +all references, not just objects. + +=head2 The UNIVERSAL Class +X + +All classes automatically inherit from the L class, which is +built in to the Perl core. This class provides a number of methods, all +of which can be called on either a class or an object. You can also +choose to override some of these methods in your class. If you do so, +we recommend that you follow the built in semantics described below. =over 4 -=item isa(CLASS) +=item isa($class) +X + +The C method returns I if the object is a member of the +class in C<$class>, or a member of a subclass of C<$class>. + +=item DOES($role) +X -C returns I if its object is blessed into a subclass of C +The C method returns I if its object claims to perform the +role C<$role>. By default, this is equivalent to C. This method is +provided for use by object system extensions that implement roles, like +C and C. -You can also call C as a sub with two arguments. The -first does not need to be an object or even a reference. This -allows the ability to check what a reference points to, or whether -something is a reference of a given type. Example +=item can($method) +X - if(UNIVERSAL::isa($ref, 'ARRAY')) { - #... - } +The C method checks to see if its object has a method called +C<$method>. If it does, then a reference to the subroutine is returned. +If it does not then C is returned. -=item can(METHOD) +This will return true if the object's class or any of it's parent +classes contain C<$method>. -C checks to see if its object has a method called C, -if it does then a reference to the sub is returned, if it does not then -I is returned. +If your class responds to method calls via C, you may want to +overload C to respond true for methods which C handles. -=item VERSION( [NEED] ) +=item VERSION($need) +X -C 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 $VERSION variable in the given package) not less than -NEED; it will die if this is not the case. This method is normally -called as a class method. This method is called automatically by the -C form of C. +The C method returns the version number of the class +(package). - use A 1.2 qw(some imported subs); +If the C<$need> argument is given then it will check that the current +version (as defined by the $VERSION variable in the package) is greater +than or equal to C<$need>; it will die if this is not the case. This +method is called automatically by the C form of C. + + use Package 1.2 qw(some imported subs); # implies: - A->VERSION(1.2); + Package->VERSION(1.2); + +We recommend that you use this method to access another package's +version, rather than looking directly at C<$Package::VERSION>. The +package you are looking at could have overridden the C method. + +We also recommend using this method to check whether a module has a +sufficient version. The internal implementation uses the L +module to make sure that different types of version numbers are +compared correctly. =back -B C directly uses Perl's internal code for method lookup, and -C uses a very similar method and cache-ing strategy. This may cause -strange effects if the Perl code dynamically changes @ISA in any package. +=head2 AUTOLOAD +X + +If you call a method that doesn't exist in a class, Perl will throw an +error. However, if that class or any of its parent classes defined an +C method, that method will be called instead. + +This is called as a regular method, and the caller will not know the +difference. Whatever value your C method returns is given to +the caller. + +The fully qualified method name that was called is available in the +C<$AUTOLOAD> package global for your class. Since this is a global, if +you want to refer to do it without a package name prefix, you need to +declare it. + + # XXX - this is a terrible way to implement accessors, but it makes for a + # simple example. + our $AUTOLOAD; + sub AUTOLOAD { + my $self = shift; + + ( my $called = $AUTOLOAD ) =~ s/.*:://; -You may add other methods to the UNIVERSAL class via Perl or XS code. -You do not need to C to make these methods -available to your program (and you should not do so). + return if $called eq 'DESTROY'; + + die "No such attribute: $called" + unless exists $self->{$called}; + + return $self->{$called}; + } + +Without the C declaration, this code will not compile +under the L pragma. + +As the comment says, this is not a good way to implement accessors. +It's slow and too clever by far. See L for recommendations +on OO coding in Perl. + +If your class does have an C method, we strongly recommend +that you override C in your class as well. Your overridden C +method should return a subroutine reference for any method that your +C responds to. =head2 Destructors +X X + +When the last refernce to an object goes away, the object is destroyed. +If you only have one reference to an object stored in a lexical scalar, +the object is destroyed when that lexical goes out of scope. If you +store the object in a global, that object may not go out of scope until +the program exits. + +If you want to do something when the object is destroyed, you can +defined a C method in your class. This method will always be +called by Perl at the appropriate time. + +This is called just like any other method, with the object as the first +argument. It does not receive any additional arguments. + +Because C methods can be called at any time, you should +localize any global variables you might update in your C. In +particular, if you use C you should localize C<$@>, and if you +use C or backticks, you should localize C<$?>. + +As we saw in the C example, if you define an C +method, this will be called for the C method, so you need to +handle this case in your C. -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 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 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 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 - -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 built Perl with C, 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 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 F, 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 -C<-DDEBUGGING> was enabled during perl build time. -See L 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. +The order in which objects are destroyed when the program exits is +unpredictable. This means that any objects contained by your object may +already have been destroyed. This means you should check that they are +defined before calling methods on them: + + sub DESTROY { + my $self = shift; + + $self->{handle}->close() if $self->{handle}; + } + +You can use the C<${^GLOBAL_PHASE}> variable to detect if you are +currently in the program exit phase: + + sub DESTROY { + my $self = shift; + + return if ${^GLOBAL_PHASE} eq 'DESTRUCT'; + + $self->{handle}->close(); + } + +Note that this variable was added in Perl 5.14.0. If you want to detect +program exit on older versions of Perl, you can use the +C module on CPAN. + +=head2 Non-Hashref Objects + +All the examples so far have shown objects based on a blessed hash +reference. However, it's possible to bless any type of reference, +including scalar refs, glob refs, and code refs. You may see this sort +of thing when looking at code in the wild. + +Here's an example of a module as a blessed scalar: + + package Time; + + sub new { + my $class = shift; + + my $time = time; + return bless \$time, $class; + } + + sub epoch { + my $self = shift; + return ${ $self }; + } + + my $time = Time->new(); + print $time->epoch(); + +=head2 Inside-Out objects + +The Perl community in the past has experimented with a technique +referred to as "inside-out objects". Without going into the details, +this was a method of enforcing data hiding for an object. An inside-out +object stores its data in a lexical class variable rather than in the +object itself. + +This technique was popular for a while, and was recommended in Damian +Conway's Perl Best Practices. The L module on CPAN +provides a comprehensive implementation of this technique, and you may +see it or other inside-out modules in the wild. + +=head2 Pseudo-hashes + +The pseudo-hash feature was an experimental feature introduced in +earlier versions of Perl and removed in 5.10.0. A pseudo-hash is an +array reference which can be accessed using named keys like a hash. You +may run in to some code in the wild which uses it. See the L +pragma for more information. =head1 SEE ALSO A kinder, gentler tutorial on object-oriented programming in Perl can -be found in L, L and L. You should -also check out L for other object tricks, traps, and tips, as -well as L for some style guides on constructing both -modules and classes. +be found in L. You should also check out L for +some style guides on constructing both modules and classes. +