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fields(3)
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 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 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
  the base manpage, the Pseudo-hashes: Using an array as a hash entry in the
  perlref manpage

Index for Section 3

Alphabetical listing for F

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