fields(3) Perl Programmers Reference Guide fields(3)NAMEfields - compile-time class fieldsSYNOPSIS
{
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 Foo $var = Foo::->new;
$var->{foo} = 42;
# this will generate a compile-time 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;
}
}
DESCRIPTION
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.
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 sub
classes. Inherited fields can be overridden but will gen
erate a warning if used together with the "-w" switch.
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 fields::new() creates and blesses a pseudo-hash
comprised of the fields declared using the
"fields" pragma into the specified class. 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 fields::phash() can be used to create and initial
ize a plain (unblessed) pseudo-hash. This func
tion 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 ele
ments 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 argu
ments:
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
the base manpage, the Pseudo-hashes: Using an array as a
hash entry in the perlref manpage
2001-03-18 perl v5.6.1 fields(3)
