In the Pet
module:
package Pet; use strict; use fields qw(name weight _Pet_pid); my $PID = 0; sub new { my Pet $self = shift; unless (ref $self) { $self = fields::new($self); $self->{_Pet_pid} = "this is Pet's secret ID"; } $self->{name} = "Hey, you!"; $self->{weight} = 20; return $self; } 1;
In a separate program, demopet:
use Pet; my Pet $rock = new Pet; # typed lexical $rock->{name} = "quartz"; $rock->{weight} = "2kg"; $rock->{_Pet_pid} = 1233; # private attribute $rock->{color} = "blue"; # generates compile-time error
In the Dog
module:
package Dog; use strict; use base 'Pet'; # inherit fields and methods from Pet use fields qw(name pedigree); # override Pet name attribute, # add new pedigree attribute use fields qw(wag _Dog_private); # not shared with Pet sub new { my $class = shift; my $self = fields::new($class); $self->SUPER::new(); # init base fields $self->{pedigree} = "none"; # init own fields return $self; }
In a separate program, demodog:
use Dog;my Dog $spot = new Dog; # typed lexical $spot->{name} = "Theloneus"; # not inherited $spot->{weight} = "30lbs"; # inherited $spot->{pedigree} = "mutt"; # not inherited $spot->{color} = "brown"; # generates compile-time error $spot->{_Pet_pid} = 3324; # generates compile-time error
The fields
pragma provides a method of
declaring class fields that can be type checked at compile time. This
relies on a feature known as pseudohashes: if a typed lexical variable
(my Pet $rock
) is holding a reference (the
Pet
object) and is used to access a hash element
($rock->{name}
), and if there exists a package
with the same name as the declared type that has set up class fields
using the fields
pragma, then the operation is
turned into an array access at compile time, provided the field
specified is valid.
The related base
pragma will combine fields
from base classes and any fields declared using the
fields
pragma. This enables field inheritance to
work properly.
Field names that start with an underscore character are made private to the class and are not visible to subclasses. Inherited fields can be overridden but will generate a warning if warnings are enabled.
The effect of all this is that you can have objects with named fields which are as compact as arrays and as fast to access. This only works as long as the objects are accessed through properly typed lexical variables, though. If the variables are not typed, access is only checked at run time, so your program runs slower because it has to do both a hash access and an array access. In addition to field declarations, the following functions are supported:
new
The fields::new
function creates and
blesses a pseudohash into the specified class (which may also be
specified by passing an object of that class). The object is
created with the fields declared earlier for that class using
the fields
pragma. This makes it possible to
write a constructor like this:
package Critter::Sounds; use fields qw(cat dog bird);sub new { my Critter::Sounds $self = shift; $self = fields::new($self) unless ref $self; $self->{cat} = 'meow'; # scalar element @$self{'dog','bird'} = ('bark','tweet'), # slice return $self; }
phash
The fields::phash
function creates and
initializes a plain (unblessed) pseudohash. You should always
use this function to create pseudohashes instead of creating
them directly, in case we decide to change the
implementation.
If the first argument to phash
is a reference
to an array, the pseudohash will be created with keys from that array.
If a second argument is supplied, it must also be a reference to an
array whose elements will be used as the values. If the second array
contains less elements than the first, the trailing elements of the
pseudohash will not be initialized. This makes it particularly useful
for creating a pseudohash from subroutine arguments:
sub dogtag { my $tag = fields::phash([qw(name rank ser_num)], [@_]); }
Alternatively, you can pass a list key/value pairs that will be used to construct the pseudohash:
my $tag = fields::phash(name => "Joe", rank => "captain", ser_num => 42); my $pseudohash = fields::phash(%args);
For more on pseudohashes, see Section 8.3.5 in Chapter 8.
The current implementation keeps the declared fields in the
%FIELDS
hash of the calling package, but this may
change in future versions, so it's best to rely on this pragma's
interface to manage your fields.