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

       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 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

       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 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 inheri-
       tance 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"

       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;

            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 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.  Examples:

               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 v5.8.6                       2001-09-21                         fields(3)