MirBSD manpage: fields(3p)

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


     fields - compile-time class fields


             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 an 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;


     The "fields" pragma enables compile-time verified class

     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.

     Only valid for perl before 5.9.0:

perl v5.8.8                2005-02-05                           1

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

     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

     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 warn-
     ing if used together with the "-w" switch.

     Only valid for perls before 5.9.0:

     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  perl before 5.9.0:  fields::new() creates and blesses a
         pseudo-hash comprised of the fields declared using the
         "fields" pragma into the specified class.

          perl 5.9.0 and higher:  fields::new() creates and
         blesses a restricted-hash comprised of the fields
         declared using the "fields" pragma into the specified

         This function is usable with or without pseudo-hashes.
         It is the recommended way to construct a fields-based

         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;


perl v5.8.8                2005-02-05                           2

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

          before perl 5.9.0:

         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

         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.  Exam-

             my $tag = fields::phash(name => "Joe",
                                     rank => "captain",
                                     ser_num => 42);

             my $pseudohash = fields::phash(%args);

          perl 5.9.0 and higher:

         Pseudo-hashes have been removed from Perl as of 5.10.
         Consider using restricted hashes or fields::new()
         instead.  Using fields::phash() will cause an error.



perl v5.8.8                2005-02-05                           3

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