In order to understand Perl objects, you first need to understand
references in Perl. See L<perlref> for details.
-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<perlootut>.
+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<perlootut>.
If you're looking to write your own object system, or you need to
maintain code which implements objects from scratch then this document
=item 1.
-An object is simply a reference that knows which class it belongs to.
+An object is simply a data structure that knows to which class it
+belongs.
=item 2.
=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 a reference to an object
+(or a package name, for class methods) as the first argument.
=back
Let's look at each of these principles in depth.
-=head2 An Object is Simply a Reference
+=head2 An Object is Simply a Data Structure
X<object> X<bless> X<constructor> X<new>
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.
+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<bless> function,
+which is typically used within the I<constructor> subroutine of the
+class.
Here is a simple constructor:
The name C<new> isn't special. We could name our constructor something
else:
+ package File;
+
sub load {
my $class = shift;
return bless {}, $class;
}
-The modern convention for OO modules is to use C<new> as the
-constructor, but you are free to use the name if that works best for
-your class.
+The modern convention for OO modules is to always use C<new> as the
+name for the constructor, but there is no requirement to do so. Any
+subroutine that blesses a data structure into a class is a valid
+constructor in Perl.
-The C<{}> code creates an empty anonymous hash reference. The C<bless>
-function takes that reference and associates it with the class in
-C<$class>. In the simplest case, the C<$class> variable will end up
-containing the string "Critter".
+In the previous examples, the C<{}> code creates a reference to an
+empty anonymous hash. The C<bless> function then takes that reference
+and associates the hash with the class in C<$class>. In the simplest
+case, the C<$class> variable will end up containing the string "File".
-We can also call bless on a named variable:
+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;
return $self;
}
-Once we've blessed C<$self> we can start calling methods on it. This is
-useful if you want to break out object initialization to its own
-method:
+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;
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
+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>. 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<be> 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<referent>). This is best
+demonstrated with this code:
+
+ use Scalar::Util 'blessed';
+
+ my $foo = {};
+ my $bar = $foo;
+
+ bless $foo, 'Class';
+ print blessed( $bar ) // 'not blessed'; # prints "Class"
+
+ $bar = "some other value";
+ print blessed( $bar ) // 'not blessed'; # prints "not blessed"
+
+When we call C<bless> 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<blessed( $bar )> returns
+false. At that point C<$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<class> X<package> X<@ISA> X<inheritance>
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<parent> 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.
+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<don't> 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.
+
+Calling methods from a package means it must be loaded, of course, so
+you will often want to load a module and add it to C<@ISA> at the same
+time. You can do so in a single step using the L<parent> pragma.
+(In older code you may encounter the L<base> pragma, which is nowadays
+discouraged except when you have to work with the equally discouraged
+L<fields> pragma.)
+
+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<UNIVERSAL> class implicitly. The
L<UNIVERSAL> class is implemented by the Perl core, and provides
several default methods, such as C<isa()>, C<can()>, and C<VERSION()>.
-X<UNIVERSAL> X<base class> X<class, base>
+The C<UNIVERSAL> class will I<never> appear in a package's C<@ISA>
+variable.
Perl I<only> provides method inheritance as a built-in feature.
Attribute inheritance is left up the class to implement. See the
-L</Attributes> section for details.
+L</Writing Accessors> section for details.
=head2 A Method is Simply a Subroutine
X<method>
sub save {
my $self = shift;
- open my $fh, '>', $self->path()
- or die $!;
- print {$fh} $self->data()
- or die $!;
- close $fh
- or die $!;
+ open my $fh, '>', $self->path() or die $!;
+ print {$fh} $self->data() or die $!;
+ close $fh or die $!;
}
=head2 Method Invocation
X<invocation> X<method> X<arrow> 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
+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.
+The C<< -> >> syntax is also used when dereferencing a reference. It
+looks like the same operator, but these are two different operations.
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<new()> 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<speak()>
+variable is passed as the first argument to C<speak()>.
+
+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.
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.
+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.
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</Method Call
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
+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.
Since we didn't define a C<save()> method in the C<File::MP3> class,
Perl will look at the C<File::MP3> class's parent classes to find the
-C<save()> method. If Perl could not find a C<save()> method anywhere in
-the inheritance hierarchy, it would die.
+C<save()> method. If Perl cannot find a C<save()> method anywhere in
+the inheritance hierarchy, it will die.
-In this case, it finds it in the C<File> class. Note that the object
-passed to C<save()> in this case is still a C<File::MP3> object, even
-though the method is found in the C<File> class.
+In this case, it finds a C<save()> method in the C<File> class. Note
+that the object passed to C<save()> in this case is still a
+C<File::MP3> object, even though the method is found in the C<File>
+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<SUPER>
}
The C<SUPER> modifier can I<only> be used for method calls. You can't
-use it for regular subroutine calls:
+use it for regular subroutine calls or class methods:
+
+ SUPER::save($thing); # FAIL: looks for save() sub in package SUPER
+
+ SUPER->save($thing); # FAIL: looks for save() method in class
+ # SUPER
+
+ $thing->SUPER::save(); # Okay: looks for save() method in parent
+ # classes
- # XXX - This will not work!
- SUPER::save($thing);
- # This won't work either!
- SUPER->save($thing);
=head3 How SUPER is Resolved
X<SUPER>
The C<SUPER> pseudo-class is resolved from the package where the call
is made. It is I<not> resolved based on the object's class. This is
-important, because it lets an inheritance hierarchy several levels deep
-all call their parent methods.
+important, because it lets methods at different levels within a deep
+inheritance hierarchy each correctly call their respective parent
+methods.
package A;
sub speak {
my $self = shift;
- $self->SUPER::speak();
-
say 'A';
}
package B;
- use parent 'A';
+ use parent -norequire, 'A';
sub speak {
my $self = shift;
package C;
- use parent 'B';
+ use parent -norequire, 'B';
sub speak {
my $self = shift;
my $c = C->new();
$c->speak();
-In this example, we will get the following outupt:
+In this example, we will get the following output:
A
B
This demonstrates how C<SUPER> is resolved. Even though the object is
blessed into the C<C> class, the C<speak()> method in the C<B> class
-can still call C<SUPER::speak()>.
+can still call C<SUPER::speak()> and expect it to correctly look in the
+parent class of C<B> (i.e the class the method call is in), not in the
+parent class of C<C> (i.e. the class the object belongs to).
-There are cases where this package-based resolution can be a problem.
-If you copy a subroutine from one package to another, C<SUPER>
+There are rare cases where this package-based resolution can be a
+problem. If you copy a subroutine from one package to another, C<SUPER>
resolution will be done based on the original package.
=head3 Multiple Inheritance
X<multiple inheritance>
Multiple inheritance often indicates a design problem, but Perl always
-give you enough rope to hang yourself with.
+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<use parent>:
the inheritance chain to find a method:
Grandparent
- |
+ |
Parent
- |
+ |
Child
If we call a method on a C<Child> object and that method is not defined
-in the C<Child> class, Perl will look for that method
. in the C<Parent>
-class and then the C<Grandparent> class.
+in the C<Child> class, Perl will look for that method in the C<Parent>
+class and then, if necessary, in the C<Grandparent> class.
If Perl cannot find the method in any of these classes, it will die
with an error message.
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.
+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<Child>, C<Father>,
-C<PaternalGrandparent>, C<Mother>, and finally C<MaternalGrandparent>
+C<PaternalGrandparent>, C<SharedGreatGrandParent>, C<Mother>, and
+finally C<MaternalGrandparent>. This may be a problem because now we're
+looking in C<SharedGreatGrandParent> I<before> we've checked all its
+derived classes (i.e. before we tried C<Mother> and
+C<MaternalGrandparent>).
It is possible to ask for a different method resolution order with the
L<mro> pragma.
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<Child>,
-C<Father>, C<Mother>, C<PaternalGrandparent>, and finally
-C<MaternalGrandparent>.
+terms, "C3" order ensures that shared parent classes are never searched
+before child classes, so Perl will now search: C<Child>, C<Father>,
+C<PaternalGrandparent>, C<Mother> C<MaternalGrandparent>, and finally
+C<SharedGreatGrandParent>. Note however that this is not
+"breadth-first" searching: All the C<Father> ancestors (except the
+common ancestor) are searched before any of the C<Mother> ancestors are
+considered.
The C3 order also lets you call methods in sibling classes with the
C<next> pseudo-class. See the L<mro> documentation for more details on
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.
+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<File> class we used
}
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.
+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":
+For our C<File::MP3> class, we can check to make sure that the path
+we're given ends with ".mp3":
package File::MP3;
This constructor lets its parent class do the actual object
construction.
-=head2 Writing Accessors
+=head2 Attributes
+X<attribute>
+
+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<accessor>
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.
+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:
filesystem path.
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
+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<perlootut>.
=head2 Method Call Variations
X<method>
-Perl supports several other ways to call methods besides the typical
-C<< $object->method() >> pattern we've seen so far.
+Perl supports several other ways to call methods besides the C<<
+$object->method() >> usage we've seen so far.
+
+=head3 Method Names with a Fully Qualified Name
+
+Perl allows you to call methods using their fully qualified name (the
+package and method name):
+
+ my $mp3 = File::MP3->new( 'Regin.mp3', $data );
+ $mp3->File::save();
+
+When you a fully qualified method name like C<File::save>, the method
+resolution search for the C<save> method starts in the C<File> class,
+skipping any C<save> method the C<File::MP3> class may have defined. It
+still searches the C<File> class's parents if necessary.
+
+While this feature is most commonly used to explicitly call methods
+inherited from an ancestor class, there is no technical restriction
+that enforces this:
+
+ my $obj = Tree->new();
+ $obj->Dog::bark();
+
+This calls the C<bark> method from class C<Dog> on an object of class
+C<Tree>, even if the two classes are completely unrelated. Use this
+with great care.
+
+The C<SUPER> pseudo-class that was described earlier is I<not> the same
+as calling a method with a fully-qualified name. See the earlier
+L</Inheritance> section for details.
=head3 Method Names as Strings
=head3 Subroutine References as Methods
-You can also call subroutine reference as a method:
+You can also use a subroutine reference as a method:
my $sub = sub {
my $self = shift;
$object->$meth();
}
+=head3 Dereferencing 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<or> a subroutine
+reference.
+
+=head3 Method Calls on Filehandles
+
+Under the hood, Perl filehandles are instances of the C<IO::Handle> or
+C<IO::File> class. Once you have an open filehandle, you can call
+methods on it. Additionally, you can call methods on the C<STDIN>,
+C<STDOUT>, and C<STDERR> filehandles.
+
+ open my $fh, '>', 'path/to/file';
+ $fh->autoflush();
+ $fh->print('content');
+
+ STDOUT->autoflush();
+
=head2 Invoking Class Methods
X<invocation>
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:
+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<Class()> in the current
+namespace, Perl will always interpret C<< Class->new() >> as the second
+alternative: a call to C<new()> on the object returned by a call to
+C<Class()>
+
+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. You can also quote
-the class name:
+Perl will always interpret this as a method call.
+
+Alternatively, you can quote the class name:
'Class'->new()
=head3 Indirect Object Syntax
X<indirect object>
-B<Outside of the file handle case, use of this syntax is discouraged,
-as it can confuse the Perl interpreter. See below for more details.>
+B<Outside of the file handle case, use of this syntax is discouraged as
+it can confuse the Perl interpreter. See below for more details.>
-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.
+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:
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<save> is a method or simply a subroutine that expects a file
-object as its first argument.
+clear if C<save> is a method provided by the C<File> 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
File->new( $path, $data ) >> B<or> 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!
+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<bless>, C<blessed>, and C<ref>
-As we saw earlier, an object is simply a reference that has been
-blessed into a class with the C<bless> function. The C<bless> function
-has can take either one or two arguments:
+As we saw earlier, an object is simply a data structure that has been
+blessed into a class via the C<bless> function. The C<bless> function
+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.
+In the first form, the anonymous hash is being blessed into the class
+in C<$class>. In the second form, the anonymous hash is blessed into
+the current package.
-The second form is discouraged, because it breaks the ability of a
-subclass to reuse the parent's constructor, but you may still run
+The second form is strongly discouraged, because it breaks the ability
+of a subclass to reuse the parent's constructor, but you may still run
across it in existing code.
-If you want to know whether a particular scalar is an object, you can
-use the C<blessed> function exported by L<Scalar::Util>, which is
-shipped with the Perl core.
+If you want to know whether a particular scalar refers to an object,
+you can use the C<blessed> function exported by L<Scalar::Util>, which
+is shipped with the Perl core.
use Scalar::Util 'blessed';
- if ( blessed($thing) ) { ... }
+ if ( defined 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<blessed>
-function returns false.
+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<blessed> function
+returns C<undef>.
-Similarly, Perl's built in C<ref> function treats a blessed reference
-specially. If you call C<ref($thing)> and C<$thing> is an object, it
-will return the name of the class that the object has been blessed
-into.
+Note that C<blessed($thing)> will also return false if C<$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<ref> function treats a reference to a
+blessed object specially. If you call C<ref($thing)> and C<$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, we
-recommend that you use C<blessed>, since C<ref> returns true values for
-all references, not just objects.
+If you simply want to check that a variable contains an object
+reference, we recommend that you use C<defined blessed($object)>, since
+C<ref> returns true values for all references, not just objects.
=head2 The UNIVERSAL Class
X<UNIVERSAL>
All classes automatically inherit from the L<UNIVERSAL> class, which is
-built in to the Perl core. This class provides a number of methods, all
+built-in to the Perl core. This class provides a number of methods, all
of which can be called on either a class or an object. You can also
choose to override some of these methods in your class. If you do so,
-we recommend that you follow the built in semantics described below.
+we recommend that you follow the built-in semantics described below.
=over 4
The C<isa> method returns I<true> if the object is a member of the
class in C<$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<DOES>
provided for use by object system extensions that implement roles, like
C<Moose> and C<Role::Tiny>.
+You can also override C<DOES> directly in your own classes. If you
+override this method, it should never throw an exception.
+
=item can($method)
X<can>
-The C<can> method checks to see if its object has a method called
-C<$method>. If it does, then a reference to the subroutine is returned.
-If it does not then C<undef> is returned.
-
-This will return true if the object's class or any of it's parent
-classes contain C<$method>.
+The C<can> method checks to see if the class or object it was called on
+has a method named C<$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<undef> is returned.
If your class responds to method calls via C<AUTOLOAD>, you may want to
-overload C<can> to respond true for methods which C<AUTOLOAD> handles.
+overload C<can> to return a subroutine reference for methods which your
+C<AUTOLOAD> method handles.
+
+If you override this method, it should never throw an exception.
=item VERSION($need)
X<VERSION>
X<AUTOLOAD>
If you call a method that doesn't exist in a class, Perl will throw an
-error. However, if that class or any of its parent classes defined an
-C<AUTOLOAD> method, that method will be called instead.
+error. However, if that class or any of its parent classes defines an
+C<AUTOLOAD> method, that C<AUTOLOAD> method is called instead.
-This is called as a regular method, and the caller will not know the
-difference. Whatever value your C<AUTOLOAD> method returns is given to
-the caller.
+C<AUTOLOAD> is called as a regular method, and the caller will not know
+the difference. Whatever value your C<AUTOLOAD> method returns is
+returned to the caller.
The fully qualified method name that was called is available in the
C<$AUTOLOAD> package global for your class. Since this is a global, if
-you want to refer to do it without a package name prefix, you need to
-declare it.
+you want to refer to do it without a package name prefix under C<strict
+'vars'>, you need to declare it.
- # XXX - this is a terrible way to implement accessors, but it makes for a
- # simple example.
+ # 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/.*:://;
-
- return if $called eq 'DESTROY';
+ # 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<our $AUTOLOAD> declaration, this code will not compile
under the L<strict> pragma.
As the comment says, this is not a good way to implement accessors.
-It's slow and too clever by far. See L<perlootut> for recommendations
-on OO coding in Perl.
+It's slow and too clever by far. However, you may see this as a way to
+provide accessors in older Perl code. See L<perlootut> for
+recommendations on OO coding in Perl.
If your class does have an C<AUTOLOAD> method, we strongly recommend
that you override C<can> in your class as well. Your overridden C<can>
=head2 Destructors
X<destructor> X<DESTROY>
-When the last refernce to an object goes away, the object is destroyed.
-If you only have one reference to an object stored in a lexical scalar,
-the object is destroyed when that lexical goes out of scope. If you
-store the object in a global, that object may not go out of scope until
-the program exits.
+When the last reference to an object goes away, the object is
+destroyed. If you only have one reference to an object stored in a
+lexical scalar, the object is destroyed when that scalar goes out of
+scope. If you store the object in a package global, that object may not
+go out of scope until the program exits.
If you want to do something when the object is destroyed, you can
-defined a C<DESTROY> method in your class. This method will always be
-called by Perl at the appropriate time.
+define a C<DESTROY> method in your class. This method will always be
+called by Perl at the appropriate time, unless the method is empty.
This is called just like any other method, with the object as the first
-argument. It does not receive any additional arguments.
+argument. It does not receive any additional arguments. However, the
+C<$_[0]> variable will be read-only in the destructor, so you cannot
+assign a value to it.
+
+If your C<DESTROY> method throws an error, this error will be ignored.
+It will not be sent to C<STDERR> and it will not cause the program to
+die. However, if your destructor is running inside an C<eval {}> block,
+then the error will change the value of C<$@>.
Because C<DESTROY> methods can be called at any time, you should
localize any global variables you might update in your C<DESTROY>. In
particular, if you use C<eval {}> you should localize C<$@>, and if you
-use C<system> or backticks, you should localize C<$?>.
+use C<system> or backticks you should localize C<$?>.
+
+If you define an C<AUTOLOAD> in your class, then Perl will call your
+C<AUTOLOAD> to handle the C<DESTROY> method. You can prevent this by
+defining an empty C<DESTROY>, 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<DESTROY>.
-As we saw in the C<AUTOLOAD> example, if you define an C<AUTOLOAD>
-method, this will be called for the C<DESTROY> method, so you need to
-handle this case in your C<AUTOLOAD>.
+=head3 Global Destruction
-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:
+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 $self = shift;
}
You can use the C<${^GLOBAL_PHASE}> variable to detect if you are
-currently in the program exit phase:
+currently in the global destruction phase:
sub DESTROY {
my $self = shift;
}
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
+the global destruction phase on older versions of Perl, you can use the
C<Devel::GlobalDestruction> module on CPAN.
-=head2 Non-Hashref Objects
+If your C<DESTROY> method issues a warning during global destruction,
+the Perl interpreter will append the string " during global
+destruction" to the warning.
+
+During global destruction, Perl will always garbage collect objects
+before unblessed references. See L<perlhacktips/PERL_DESTRUCT_LEVEL>
+for more information about global destruction.
+
+=head2 Non-Hash 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.
+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:
=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 in a lexical
-class variable, indexed on a unique property of the object, such as its
-memory address, rather than in the object itself.
+"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<Perl Best Practices>), but never achieved wide adoption due
-to additional complexity. The L<Object::InsideOut> module on CPAN
-provides a comprehensive implementation of this technique, and you may
-see it or other inside-out modules in the wild.
+Conway's I<Perl Best Practices>), but never achieved universal
+adoption. The L<Object::InsideOut> 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<Hash::Util::FieldHash> core module. This module was added to the core
to support inside-out object implementations.
- package Time::InsideOut;
+ package Time;
use strict;
use warnings;
use Hash::Util::FieldHash 'fieldhash';
- fieldhash my %TIME;
+ fieldhash my %time_for;
sub new {
my $class = shift;
- my $self = bless \( my $empty ), $class;
- $TIME{$self} = time;
- $self;
+ my $self = bless \( my $object ), $class;
+
+ $time_for{$self} = time;
+
+ return $self;
}
sub epoch {
my $self = shift;
- $TIME{$self};
+ return $time_for{$self};
}
- my $time = Time::InsideOut->new;
+ my $time = Time->new;
print $time->epoch;
=head2 Pseudo-hashes
https://perl5.git.perl.org / perl5.git / blobdiff