fields(3pm)						 Perl Programmers Reference Guide					       fields(3pm)

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 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 ver-
       sions.  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 subclasses.  Inherited fields can
       be overridden but will generate 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 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, "Pseudo-hashes: Using an array as a hash" in perlref

perl v5.8.0							    2002-06-01							       fields(3pm)