fields(3perl) Perl Programmers Reference Guide fields(3perl)
NAME
fields - compile-time class fields
SYNOPSIS
{
package Foo;
use fields qw(foo bar _Foo_private);
sub new {
my Foo $self = shift;
unless (ref $self) {
$self = fields::new($self);
$self->{_Foo_private} = "this is Foo's secret";
}
$self->{foo} = 10;
$self->{bar} = 20;
return $self;
}
}
my $var = Foo->new;
$var->{foo} = 42;
# this will generate a run-time error
$var->{zap} = 42;
# this will generate a compile-time error
my Foo $foo = Foo->new;
$foo->{zap} = 24;
# subclassing
{
package Bar;
use base 'Foo';
use fields qw(baz _Bar_private); # not shared with Foo
sub new {
my $class = shift;
my $self = fields::new($class);
$self->SUPER::new(); # init base fields
$self->{baz} = 10; # init own fields
$self->{_Bar_private} = "this is Bar's secret";
return $self;
}
}
DESCRIPTION
The "fields" pragma enables compile-time and run-time verified class
fields.
NOTE: The current implementation keeps the declared fields in the
%FIELDS hash of the calling package, but this may change in future
versions. Do not update the %FIELDS hash directly, because it must be
created at compile-time for it to be fully useful, as is done by this
pragma.
If a typed lexical variable (my Class $var) holding a reference is used
to access a hash element and a package with the same name as the type
has declared class fields using this pragma, then the hash key is
verified at compile time. If the variables are not typed, access is
only checked at run time.
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. Inherited fields can be overridden but
will generate a warning if warnings are enabled.
Only valid for Perl 5.8.x and earlier: Field names that start with an
underscore character are made private to the class and are not visible
to subclasses.
Also, in Perl 5.8.x and earlier, this pragma uses pseudo-hashes, the
effect being that you can have objects with named fields which are as
compact and as fast arrays to access, as long as the objects are
accessed through properly typed variables.
The following functions are supported:
new fields::new() creates and blesses a hash of the fields declared
using the "fields" pragma into the specified class. It is the
recommended way to construct a fields-based object.
This makes it possible to write a constructor like this:
package Critter::Sounds;
use fields qw(cat dog bird);
sub new {
my $self = shift;
$self = fields::new($self) unless ref $self;
$self->{cat} = 'meow'; # scalar element
@$self{'dog','bird'} = ('bark','tweet'); # slice
return $self;
}
phash
This function only works in Perl 5.8.x and earlier. Pseudo-hashes
were removed from Perl as of 5.10. Consider using restricted hashes
or fields::new() instead (which itself uses restricted hashes under
5.10+). See Hash::Util. Using fields::phash() under 5.10 or higher
will cause an error.
fields::phash() can be used to create and initialize a plain
(unblessed) pseudo-hash. This function should always be used
instead of creating pseudo-hashes directly.
If the first argument is a reference to an array, the pseudo-hash
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 pseudo-hash
will not be initialized. This makes it particularly useful for
creating a pseudo-hash from subroutine arguments:
sub dogtag {
my $tag = fields::phash([qw(name rank ser_num)], [@_]);
}
fields::phash() also accepts a list of key-value pairs that will be
used to construct the pseudo hash. Examples:
my $tag = fields::phash(name => "Joe",
rank => "captain",
ser_num => 42);
my $pseudohash = fields::phash(%args);
SEE ALSO
base, Hash::Util
perl v5.40.1 2026-08-30 fields(3perl)
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