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Perl Documentation

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:

SEE ALSO

base, Hash::Util