X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/cb1a09d0194fed9b905df7b04a4bc031d354609d..caa66803130d6377a39ed599f49eb8aef88d4f1d:/pod/perlobj.pod diff --git a/pod/perlobj.pod b/pod/perlobj.pod index 59c6f12..61e636b 100644 --- a/pod/perlobj.pod +++ b/pod/perlobj.pod @@ -1,410 +1,1047 @@ +=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 of all, you need to understand what references are in Perl. See -L for that. +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. + +This document describes all of Perl's object-oriented (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. -Here are three very simple definitions that you should find reassuring. +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 data structure that knows to which class it +belongs. =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 static methods) as the first argument. +A method is simply a subroutine that expects a reference to an object +(or 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 - -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 {} } - -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: - - sub new { - my $self = {}; - bless $self; - return $self; - } - -In fact, 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 $self; - } - -Or if you expect people to call not just Cnew()> but also -C<$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 -only be accessed 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 only -belong to one class at a time. (Although of course it's free to -inherit methods from many classes.) - -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 -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. +Let's look at each of these principles in depth. -=head2 A Class is Simply a Package +=head2 An Object is Simply a Data Structure +X X X X + +Unlike many other languages which support object orientation, Perl does +not provide any special syntax for constructing an object. Objects are +merely Perl data structures (hashes, arrays, scalars, filehandles, +etc.) that have been explicitly associated with a particular class. + +That explicit association is created by the built-in C function, +which is typically used within the I subroutine of the +class. + +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: + + package File; + + sub load { + my $class = shift; + + return bless {}, $class; + } + +The modern convention for OO modules is to always use C as the +name for the constructor, but there is no requirement to do so. Any +subroutine that blesses a data structure into a class is a valid +constructor in Perl. + +In the previous examples, the C<{}> code creates a reference to an +empty anonymous hash. The C function then takes that reference +and associates the hash with the class in C<$class>. In the simplest +case, the C<$class> variable will end up containing the string "File". + +We can also use a variable to store a reference to the data structure +that is being blessed as our object: + + sub new { + my $class = shift; + + my $self = {}; + bless $self, $class; + + return $self; + } + +Once we've blessed the hash referred to by C<$self> we can start +calling methods on it. This is useful if you want to put object +initialization in its own separate method: + + sub new { + my $class = shift; + + my $self = {}; + bless $self, $class; + + $self->_initialize(); + + return $self; + } + +Since the object is also a hash, you can treat it as one, using it to +store data associated with the object. Typically, code inside the class +can treat the hash as an accessible data structure, 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. + +Note, however, that (unlike most other OO languages) Perl does not +ensure or enforce encapsulation in any way. If you want objects to +actually I opaque you need to arrange for that yourself. This can +be done in a variety of ways, including using L<"Inside-Out objects"> +or modules from CPAN. + +=head3 Objects Are Blessed; Variables Are Not + +When we bless something, we are not blessing the variable which +contains a reference to that thing, nor are we blessing the reference +that the variable stores; we are blessing the thing that the variable +refers to (sometimes known as the I). This is best +demonstrated with this code: -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 into the class. - -There is a special array within each package called @ISA which says -where else to look for a method if you can't find it in the current -package. This is how Perl implements inheritance. Each element of the -@ISA array is just the name of another package that happens to be a -class package. The classes are searched (depth first) for missing -methods in the order that they occur in @ISA. The classes accessible -through @ISA are known as base classes of the current class. - -If a missing method is found in one of the base classes, 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 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. - -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. + use Scalar::Util 'blessed'; + + my $foo = {}; + my $bar = $foo; + + bless $foo, 'Class'; + print blessed( $bar ); # prints "Class" + + $bar = "some other value"; + print blessed( $bar ); # prints undef + +When we call C on a variable, we are actually blessing the +underlying data structure that the variable refers to. We are not +blessing the reference itself, nor the variable that contains that +reference. That's why the second call to C returns +false. At that point C<$bar> is no longer storing a reference to an +object. + +You will sometimes see older books or documentation mention "blessing a +reference" or describe an object as a "blessed reference", but this is +incorrect. It isn't the reference that is blessed as an object; it's +the thing the reference refers to (i.e. the referent). + +=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 may expect a +reference to an object or the name of a class as the first argument. +This is purely a matter of convention, so a class may contain both +methods and subroutines which I operate on an object or class. + +Each package contains a special array 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. Much older code also uses the L pragma. 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. + +However the parent classes are set, the package's C<@ISA> variable will +contain a list of those parents. This is simply a list of scalars, each +of which is a string that corresponds to a package name. + +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. +The C class will I appear in a package's C<@ISA> +variable. + +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. -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 -object by name: - - sub find { - my ($class, $name) = @_; - $objtable{$name}; - } - -A virtual 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"; - } - } +Perl I provide special syntax for method invocation, the C<< -> +>> operator. We will cover this in more detail later. + +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 >>. -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 +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. - print STDERR "help!!!\n"; + my $pod = File->new( 'perlobj.pod', $data ); + $pod->save(); -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. +The C<< -> >> syntax is also used when dereferencing a reference. It +looks like the same operator, but these are two different operations. - $fred = find Critter "Fred"; - display $fred 'Height', 'Weight'; +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> +variable is passed as the first argument to C. -These could be combined into one statement by using a BLOCK in the -indirect object slot: +Just as with any Perl subroutine, all of the arguments passed in C<@_> +are aliases to the original argument. This includes the object itself. +If you assign directly to C<$_[0]> you will change the contents of the +variable that holds the reference to the object. We recommend that you +don't do this unless you know exactly what you're doing. - display {find Critter "Fred"} 'Height', 'Weight'; +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 that package. If the left hand side is an object, then Perl +looks for the method in the package that the object has been blessed +into. -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 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. - $fred = Critter->find("Fred"); - $fred->display('Height', 'Weight'); +=head2 Inheritance +X -or in one statement, +We already talked about the special C<@ISA> array and the L +pragma. - Critter->find("Fred")->display('Height', 'Weight'); +When a class inherits from another class, any methods defined in the +parent class are available to 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. -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 + package File::MP3; + use parent 'File'; # sets @File::MP3::ISA = ('File'); - new Critter ('Barney', 1.5, 70) + my $mp3 = File::MP3->new( 'Andvari.mp3', $data ); + $mp3->save(); -are assumed to surround ALL the arguments of the method call, regardless -of what comes after. Saying +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 cannot find a C method anywhere in +the inheritance hierarchy, it will die. - new Critter ('Bam' x 2), 1.4, 45 +In this case, it finds a C method 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. -would be equivalent to +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. - Critter->new('Bam' x 2), 1.4, 45 + sub save { + my $self = shift; -which is unlikely to do what you want. + say 'Prepare to rock'; + $self->SUPER::save(); + } -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: +The C modifier can I be used for method calls. You can't +use it for regular subroutine calls or class methods: - $fred = MyCritter::find("Critter", "Fred"); - MyCritter::display($fred, 'Height', 'Weight'); + SUPER::save($thing); # FAIL: looks for save() sub in package SUPER -Note however, that this does not do any inheritance. If you merely -wish to specify that Perl should I looking for a method in a -particular package, use an ordinary method call, but qualify the method -name with the package like this: + SUPER->save($thing); # FAIL: looks for save() method in class + # SUPER - $fred = Critter->MyCritter::find("Fred"); - $fred->MyCritter::display('Height', 'Weight'); + $thing->SUPER::save(); # Okay: looks for save() method in parent + # classes -If you're trying to control where the method search begins I you're -executing in the class itself, then you may use the SUPER pseudoclass, -which says to start looking in your base class's @ISA list without having -to explicitly name it: - $self->SUPER::display('Height', 'Weight'); +=head3 How SUPER is Resolved +X -Please note that the C construct is I meaningful within the -class. +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 methods at different levels within a deep +inheritance hierarchy each correctly call their respective parent +methods. + + package A; + + sub new { + return bless {}, shift; + } + + sub speak { + my $self = shift; + + say 'A'; + } + + package B; + + use parent -norequire, 'A'; + + sub speak { + my $self = shift; + + $self->SUPER::speak(); + + say 'B'; + } + + package C; + + use parent -norequire, '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 output: + + 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 and expect it to correctly look in the +parent class of C (i.e the class the method call is in), not in the +parent class of C (i.e. the class the object belongs to). + +There are rare 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 +gives you enough rope to hang yourself with if you ask for it. + +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, if necessary, in 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, grandparents, etc. 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. + + SharedGreatGrandParent + / \ + PaternalGrandparent MaternalGrandparent + \ / + Father Mother + \ / + Child + +So given the diagram above, Perl will search C, C, +C, C, C, and +finally C. This may be a problem because now we're +looking in C I we've checked all its +derived classes (i.e. before we tried C and +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, "C3" order ensures that shared parent classes are never searched +before child classes, so Perl will now search: C, C, +C, C C, and finally +C. Note however that this is not +"breadth-first" searching: All the C ancestors (except the +common ancestor) are searched before any of the C ancestors are +considered. + +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. + +=head2 Writing Constructors +X + +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 reference to a new +object. + +The constructor can also accept additional parameters that define the +object. Let's write a real constructor for the C class we used +earlier: + + package File; + + sub new { + my $class = shift; + my ( $path, $data ) = @_; + + my $self = bless { + path => $path, + data => $data, + }, $class; + + return $self; + } + +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. +Later, we'll write accessors to manipulate this data. + +For our File::MP3 class, we can check to make sure that the path we're +given ends with ".mp3": + + package File::MP3; + + sub new { + my $class = shift; + my ( $path, $data ) = @_; + + die "You cannot create a File::MP3 without an mp3 extension\n" + unless $path =~ /\.mp3\z/; + + return $class->SUPER::new(@_); + } + +This constructor lets its parent class do the actual object +construction. + +=head2 Attributes +X + +An attribute is a piece of data belonging to a particular object. +Unlike most object-oriented languages, Perl provides no special syntax +or support for declaring and manipulating attributes. + +Attributes are often stored in the object itself. For example, if the +object is an anonymous hash, we can store the attribute values in the +hash using the attribute name as the key. + +While it's possible to refer directly to these hash keys outside of the +class, it's considered a best practice to wrap all access to the +attribute with accessor methods. + +This has several advantages. Accessors make it easier to change the +implementation of an object later while still preserving the original +API. + +An accessor lets you add additional code around attribute access. For +example, you could apply a default to an attribute that wasn't set in +the constructor, or you could validate that a new value for the +attribute is acceptable. + +Finally, using accessors makes inheritance much simpler. Subclasses can +use the accessors rather than having to know how a parent class is +implemented internally. + +=head3 Writing Accessors +X + +As with constructors, Perl provides no special accessor declaration +syntax, so classes must provide explicitly written accessor methods. +There are two common types of accessors, read-only and read-write. + +A simple read-only accessor simply gets the value of a single +attribute: + + sub path { + my $self = shift; + + return $self->{path}; + } + +A read-write accessor will allow the caller to set the value as well as +get it: + + sub path { + my $self = shift; + + if (@_) { + $self->{path} = shift; + } + + return $self->{path}; + } + +=head2 An Aside About Smarter and Safer Code + +Our constructor and accessors are not very smart. They don't check that +a C<$path> is defined, nor do they check that a C<$path> is a valid +filesystem path. + +Doing these checks by hand can quickly become tedious. Writing a bunch +of accessors by hand is also incredibly tedious. There are a lot of +modules on CPAN that can help you write safer and more concise code, +including the modules we recommend in L. + +=head2 Method Call Variations +X + +Perl supports several other ways to call methods besides the C<< +$object->method() >> usage we've seen so far. + +=head3 Method Names as Strings + +Perl lets you use a scalar variable containing a string as a method +name: + + my $file = File->new( $path, $data ); + + my $method = 'save'; + $file->$method(); + +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. + +=head3 Class Names as Strings -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: +Perl also lets you use a scalar containing a string as a class name: - $method = $fast ? "findfirst" : "findbest"; - $fred->$method(@args); + my $class = 'File'; + + my $file = $class->new( $path, $data ); + +Again, this allows for very dynamic code. + +=head3 Subroutine References as Methods + +You can also use a 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(); + } + +=head3 Deferencing Method Call + +Perl also lets you use a dereferenced scalar reference in a method +call. That's a mouthful, so let's look at some code: + + $file->${ \'save' }; + $file->${ returns_scalar_ref() }; + $file->${ \( returns_scalar() ) }; + $file->${ returns_ref_to_sub_ref() }; + +This works if the dereference produces a string I a subroutine +reference. + +=head3 Method Calls on Filehandles + +Under the hood, Perl filehandles are instances of the C or +C class. Once you have an open filehandle, you can call +methods on it. Additionally, you can call methods on the C, +C, and C filehandles. + + open my $fh, '>', 'path/to/file'; + $fh->autoflush(); + $fh->print('content'); + + STDOUT->autoflush(); + +=head2 Invoking Class Methods +X + +Because Perl allows you to use barewords for package names and +subroutine names, it sometimes interprets a bareword's meaning +incorrectly. For example, the construct C<< Class->new() >> can be +interpreted as either C<< 'Class'->new() >> or C<< Class()->new() >>. +In English, that second interpretation reads as "call a subroutine +named Class(), then call new() as a method on the return value of +Class()". If there is a subroutine named C in the current +namespace, Perl will always interpret C<< Class->new() >> as the second +alternative: a call to C on the object returned by a call to +C + +You can force Perl to use the first interpretation (i.e. as a method +call on the class named "Class") in two ways. First, you can append a +C<::> to the class name: + + Class::->new() + +Perl will always interpret this as a method call. + +Alternatively, you can 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 supports another method invocation syntax called "indirect object" +notation. 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 provided by the C class 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, what +barewords it has previously seen, and other input. 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. + +You can force Perl to interpret the bareword as a class name by +appending "::" to it, like we saw earlier: + + my $file = new File:: $path, $data; + +=head2 C, C, and C + +As we saw earlier, an object is simply a data structure that has been +blessed into a class via the C function. The C function +can take either one or two arguments: + + my $object = bless {}, $class; + my $object = bless {}; + +In the first form, the anonymous hash is being blessed into the class +in C<$class>. In the second form, the anonymous hash is blessed into +the current package. + +The second form is strongly 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 refers to an object, +you can use the C function exported by L, which +is shipped with the Perl core. + + use Scalar::Util 'blessed'; + + if ( defined blessed($thing) ) { ... } + +If C<$thing> refers to an object, then this function returns the name +of the package the object has been blessed into. If C<$thing> doesn't +contain a reference to a blessed object, the C function +returns C. + +Note that C will also return false if C<$thing> has +been blessed into a class named "0". This is a possible, but quite +pathological. Don't create a class named "0" unless you know what +you're doing. + +Similarly, Perl's built-in C function treats a reference to a +blessed object specially. If you call C and C<$thing> +holds a reference to an object, it will return the name of the class +that the object has been blessed into. + +If you simply want to check that a variable contains an object +reference, we recommend that you use C, since +C returns true values for all references, not just objects. + +=head2 The UNIVERSAL Class +X + +All classes automatically inherit from the L class, which is +built-in to the Perl core. This class provides a number of methods, all +of which can be called on either a class or an object. You can also +choose to override some of these methods in your class. If you do so, +we recommend that you follow the built-in semantics described below. + +=over 4 + +=item isa($class) +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>. + +If you override this method, it should never throw an exception. + +=item DOES($role) +X + +The C method returns I if its object claims to perform the +role C<$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 override C directly in your own classes. If you +override this method, it should never throw an exception. + +=item can($method) +X + +The C method checks to see if the class or object it was called on +has a method named C<$method>. This checks for the method in the class +and all of its parents. If the method exists, then a reference to the +subroutine is returned. If it does not then C is returned. + +If your class responds to method calls via C, you may want to +overload C to return a subroutine reference for methods which your +C method handles. + +If you override this method, it should never throw an exception. + +=item VERSION($need) +X + +The C method returns the version number of the class +(package). + +If the C<$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: + Package->VERSION(1.2); + +We recommend that you use this method to access another package's +version, rather than looking directly at C<$Package::VERSION>. The +package you are looking at could have overridden the C method. + +We also recommend using this method to check whether a module has a +sufficient version. The internal implementation uses the L +module to make sure that different types of version numbers are +compared correctly. + +=back + +=head2 AUTOLOAD +X + +If you call a method that doesn't exist in a class, Perl will throw an +error. However, if that class or any of its parent classes defines an +C method, that C method is called instead. + +C is called as a regular method, and the caller will not know +the difference. Whatever value your C method returns is +returned to the caller. + +The fully qualified method name that was called is available in the +C<$AUTOLOAD> package global for your class. Since this is a global, if +you want to refer to do it without a package name prefix under C, 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; + + # Remove qualifier from original method name... + my $called = $AUTOLOAD =~ s/.*:://r; + + # Is there an attribute of that name? + die "No such attribute: $called" + unless exists $self->{$called}; + + # If so, return it... + return $self->{$called}; + } + + sub DESTROY { } # see below + +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. However, you may see this as a way to +provide accessors in older Perl code. 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 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 in the current object will be freed and destroyed -automatically when the current object is freed. - -=head2 WARNING - -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. The left side of -> is not so -limited, because it's an infix operator, not a postfix operator. - -That means that below, A and B are equivalent to each other, and C and D -are equivalent, but AB and CD are different: - - A: method $obref->{"fieldname"} - B: (method $obref)->{"fieldname"} - C: $obref->{"fieldname"}->method() - D: method {$obref->{"fieldname"}} - -=head2 Summary - -That's about all there is to it. Now you just need 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. For this reason, 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 and in fact -are destructed in a separate pass before ordinary refs just to try 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. - -A more complete garbage collection strategy will be implemented -at a future date. +destroyed. If you only have one reference to an object stored in a +lexical scalar, the object is destroyed when that scalar goes out of +scope. If you store the object in a package 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 +define a C method in your class. This method will always be +called by Perl at the appropriate time, unless the method is empty. + +This is called just like any other method, with the object as the first +argument. It does not receive any additional arguments. However, the +C<$_[0]> variable will be read-only in the destructor, so you cannot +assign a value to it. + +If your C method throws an error, this error will be ignored. +It will not be sent to C and it will not cause the program to +die. However, if your destructor is running inside an C block, +then the error will change the value of C<$@>. + +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<$@>, and if you +use C or backticks you should localize C<$?>. + +If you define an C in your class, then Perl will call your +C to handle the C method. You can prevent this by +defining an empty C, like we did in the autoloading example. +You can also check the value of C<$AUTOLOAD> and return without doing +anything when called to handle C. + +=head3 Global Destruction + +The order in which objects are destroyed during the global destruction +before the program exits is unpredictable. This means that any objects +contained by your object may already have been destroyed. You should +check that a contained object is defined before calling a method on it: + + 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 global destruction 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 +the global destruction phase on older versions of Perl, you can use the +C module on CPAN. + +If your C method issues a warning during global destruction, +the Perl interpreter will append the string " during global +destruction" the warning. + +During global destruction, Perl will always garbage collect objects +before unblessed references. See L +for more information about global destruction. + +=head2 Non-Hash Objects + +All the examples so far have shown objects based on a blessed hash. +However, it's possible to bless any type of data structure or referent, +including scalars, globs, and subroutines. 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; + + use strict; + use warnings; + + 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 + +In the past, the Perl community experimented with a technique called +"inside-out objects". An inside-out object stores its data outside of +the object's reference, indexed on a unique property of the object, +such as its memory address, rather than in the object itself. This has +the advantage of enforcing the encapsulation of object attributes, +since their data is not stored in the object itself. + +This technique was popular for a while (and was recommended in Damian +Conway's I), but never achieved universal +adoption. 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. + +Here is a simple example of the technique, using the +L core module. This module was added to the core +to support inside-out object implementations. + + package Time; + + use strict; + use warnings; + + use Hash::Util::FieldHash 'fieldhash'; + + fieldhash my %time_for; + + sub new { + my $class = shift; + + my $self = bless \( my $object ), $class; + + $time_for{$self} = time; + + return $self; + } + + sub epoch { + my $self = shift; + + return $time_for{$self}; + } + + my $time = Time->new; + print $time->epoch; + +=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 -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. +A kinder, gentler tutorial on object-oriented programming in Perl can +be found in L. You should also check out L for +some style guides on constructing both modules and classes. +