fields(3p) Perl Programmers Reference Guide fields(3p)
fields - compile-time class fields
{
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 an error
$var->{zap} = 42;
# 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;
}
}
The "fields" pragma enables compile-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.
Only valid for perl before 5.9.0:
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fields(3p) Perl Programmers Reference Guide fields(3p)
If a typed lexical variable 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 operation is turned into an array access at compile
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.
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 warn-
ing if used together with the "-w" switch.
Only valid for perls before 5.9.0:
The effect of all this is that you can have objects with
named fields which are as compact and as fast arrays to
access. This only works as long as the objects are accessed
through properly typed variables. If the objects are not
typed, access is only checked at run time.
The following functions are supported:
new perl before 5.9.0: fields::new() creates and blesses a
pseudo-hash comprised of the fields declared using the
"fields" pragma into the specified class.
perl 5.9.0 and higher: fields::new() creates and
blesses a restricted-hash comprised of the fields
declared using the "fields" pragma into the specified
class.
This function is usable with or without pseudo-hashes.
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
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fields(3p) Perl Programmers Reference Guide fields(3p)
before perl 5.9.0:
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. Exam-
ples:
my $tag = fields::phash(name => "Joe",
rank => "captain",
ser_num => 42);
my $pseudohash = fields::phash(%args);
perl 5.9.0 and higher:
Pseudo-hashes have been removed from Perl as of 5.10.
Consider using restricted hashes or fields::new()
instead. Using fields::phash() will cause an error.
base
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