phpman > man > 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 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