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


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 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;
               }
           }

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.

       Only valid for perl before 5.9.0:

       If a typed lexical variable holding a reference is used to
       access a hash element and a package with the same name as



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


       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 sub-
       classes.  Inherited fields can be overridden but will gen-
       erate a warning 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
           class.

           This function is usable with or without pseudo-hashes.
           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
            before perl 5.9.0:

           fields::phash() can be used to create and initialize a



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


           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 ele-
           ments 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-
           ples:

               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.

SEE ALSO
       base





















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