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 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 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"
       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 speci-
               fied 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 partic-
               ularly useful for creating a pseudo-hash from subroutine argu-
               ments:

                   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(3)