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 comprised 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