PERLCLASS(1) Perl Programmers Reference Guide PERLCLASS(1)
NAME
perlclass - Perl class syntax reference
SYNOPSIS
use v5.38;
use feature 'class';
class My::Example 1.234 {
field $x;
ADJUST {
$x = "Hello, world";
}
method print_message {
say $x;
}
}
My::Example->new->print_message;
DESCRIPTION
This document describes the syntax of the Perl's "class" feature, which
provides native keywords supporting object-oriented programming
paradigm.
History
Since Perl 5, support for objects revolved around the concept of
blessing references with a package name. Such reference could then be
used to call subroutines from the package it was blessed with (or any
of its parents). This system, while bare-bones, was flexible enough to
allow creation of multiple more advanced, community-driven systems for
object orientation.
Class feature is a core implementation of class syntax which is
familiar to what one would find in other programming languages. It
isn't a "bless" wrapper, but a completely new system built right into
the perl interpreter.
KEYWORDS
Enabling the "class" feature allows the usage of the following new
keywords in the scope of current package:
class
class NAME BLOCK
class NAME VERSION BLOCK
class NAME;
class NAME VERSION;
The "class" keyword declares a new package which is intended to be a
class. All other keywords from the "class" feature should be used in
scope of this declaration.
class WithVersion 1.000 {
# class definition goes here
}
Classes can be declared in either block or statement syntax. If a block
is used, the body of the block contains the implementation of the
class. If the statement form is used, the remainder of the file is used
up until the next "class" or "package" statement.
"class" and "package" declarations are similar, but classes
automatically get a constructor named "new" - You don't have to (and
should not) write one. Additionally, in the class BLOCK you are
allowed to declare fields and methods.
field
field VARIABLE_NAME;
field VARIABLE_NAME = EXPR;
field VARIABLE_NAME : ATTRIBUTES;
field VARIABLE_NAME : ATTRIBUTES = EXPR;
Fields are variables which are visible in the scope of the class - more
specifically within "method" and "ADJUST" blocks. Each class instance
get their own storage of fields, independent of each other.
A field behaves like a normal lexically scoped variable. It has a sigil
and is private to the class (though creation of an accessor method will
make it accessible from the outside). The main difference is that
different instances access different values in the same scope.
class WithFields {
field $scalar = 42;
field @array = qw(this is just an array);
field %hash = (species => 'Martian', planet => 'Mars');
}
Fields may optionally have initializing expressions. If present, the
expression will be evaluated within the constructor of each object
instance. During each evaluation, the expression can use the value of
any previously-set field, as well as see any other variables in scope.
class WithACounter {
my $next_count = 1;
field $count = $next_count++;
}
When combined with the ":param" field attribute, the defaulting
expression can use any of the "=", "//=" or "||=" operators.
Expressions using "=" will apply whenever the caller did not pass the
corresponding parameter to the constructor at all. Expressions using
"//=" will also apply if the caller did pass the parameter but the
value was undefined, and expressions using "||=" will apply if the
value was false.
method
method METHOD_NAME SIGNATURE BLOCK
method METHOD_NAME BLOCK
method SIGNATURE BLOCK
method BLOCK
Methods are subroutines intended to be called in the context of class
objects.
A variable named $self populated with the current object instance will
automatically be created in the lexical scope of "method".
Methods always act as if "use feature 'signatures'" is in effect, but
$self will not appear in the arguments list as far as the signature is
concerned.
class WithMethods {
field $greetings;
ADJUST {
$greetings = "Hello";
}
method greet($name = "someone") {
say "$greetings, $name";
}
}
Just like regular subroutines, methods can be anonymous:
class AnonMethodFactory {
method get_anon_method {
return method {
return 'this is an anonymous method';
};
}
}
ATTRIBUTES
Specific aspects of the keywords mentioned above are managed using
attributes. Attributes all start with a colon, and one or more of them
can be appended after the item's name, separated by a space.
Class attributes
:isa
Classes may inherit from one superclass, by using the ":isa" class
attribute.
class Example::Base { ... }
class Example::Subclass :isa(Example::Base) { ... }
Inherited methods are visible and may be invoked. Fields are always
lexical and therefore not visible by inheritance.
The ":isa" attribute may request a minimum version of the base class;
it is applied similar to "use" - if the provided version is too low it
will fail at compile time.
class Example::Subclass :isa(Example::Base 2.345) { ... }
The ":isa" attribute will attempt to "require" the named module if it
is not already loaded.
Field attributes
:param
A scalar field with a ":param" attribute will take its value from a
named parameter passed to the constructor. By default the parameter
will have the same name as the field (minus its leading "$" sigil), but
a different name can be specified in the attribute.
field $x :param;
field $y :param(the_y_value);
If there is no defaulting expression then the parameter is required by
the constructor; the caller must pass it or an exception is thrown.
With a defaulting expression this becomes optional.
Method attributes
None yet.
OBJECT LIFECYCLE
Construction
Each object begins its life with a constructor call. The constructor is
always named "new" and is invoked like a method call on the class name:
my $object = My::Class->new(%arguments);
During the construction, class fields are compared to %arguments hash
and populated where possible.
Adjustment
Object adjustment can be performed during the construction to run user-
defined code. It is done with the help of "ADJUST" blocks, which are
called in order of declaration.
They are similar to "BEGIN" blocks, which run during the compilation of
a package. However, they also have access to $self lexical (object
instance) and all object fields created up to that point.
Lifetime
After the construction phase, object is ready to be used.
Using "blessed" ("Scalar::Util::blessed" or "builtin::blessed") on the
object will return the name of the class, while "reftype"
("Scalar::Util::reftype" or "builtin::reftype") will return the string
'OBJECT'.
Destruction
Just like with other references, when object reference count reaches
zero it will automatically be destroyed.
TODO
This feature is still experimental and very incomplete. The following
list gives some overview of the kinds of work still to be added or
changed:
o Roles
Some syntax for declaring a role (likely a "role" keyword), and for
consuming a role into a class (likely a :does() attribute).
o Parameters to ADJUST blocks
Some syntax for declaring that an "ADJUST" block can consume named
parameters, which become part of the class constructor's API. This
might be inspired by a similar plan to add named arguments to
subroutine signatures.
class X {
ADJUST (:$alpha, :$beta = 123) {
...
}
}
my $obj = X->new(alpha => 456);
o ADJUST blocks as true blocks
Currently, every ADJUST block is wrapped in its own CV that gets
invoked with the full ENTERSUB overhead. It should be possible to
use the same mechanism that makes all field initializer expressions
appear within the same CV on ADJUST blocks as well, merging them
all into a single CV per class. This will make it faster to invoke
if a class has more than one of them.
o Accessor generator attributes
Attributes to request that accessor methods be generated for
fields. Likely ":reader" and ":writer".
class X {
field $name :reader;
}
Equivalent to
class X {
field $name;
method name { return $name; }
}
o Metaprogramming
An extension of the metaprogramming API (currently proposed by
RFC0022 <https://github.com/Perl/RFCs/pull/25>) which adds
knowledge of classes, methods, fields, ADJUST blocks, and other
such class-related details.
o Extension Customisation
Ways in which out-of-core modules can interact with the class
system, including an ability for them to provide new class or field
attributes.
AUTHORS
Paul Evans
Bartosz Jarzyna
perl v5.38.2 2023-11-28 PERLCLASS(1)
perl 5.38.2 - Generated Fri Nov 29 10:39:22 CST 2024