fields - phpMan

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