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

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

NNAAMMEE #

 fields - compile-time class fields

SSYYNNOOPPSSIISS #

     {
         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 a run-time error
     $var->{zap} = 42;

     # this will generate a compile-time error
     my Foo $foo = Foo->new;
     $foo->{zap} = 24;

     # 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;
         }
     }

DDEESSCCRRIIPPTTIIOONN #

 The "fields" pragma enables compile-time and run-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
 nnoott 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 ("my Class $var") 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 hash key is verified at
 compile time.  If the variables are not typed, access is only checked at
 run 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.  Inherited fields can be overridden but
 will generate a warning if warnings are enabled.

 OOnnllyy vvaalliidd ffoorr PPeerrll 55..88..xx aanndd eeaarrlliieerr:: Field names that start with an
 underscore character are made private to the class and are not visible to
 subclasses.

 Also, iinn PPeerrll 55..88..xx aanndd eeaarrlliieerr, this pragma uses pseudo-hashes, the
 effect being that you can have objects with named fields which are as
 compact and as fast arrays to access, as long as the objects are accessed
 through properly typed variables.

 The following functions are supported:

 new ffiieellddss::::nneeww(()) creates and blesses a hash comprised of the fields
     declared using the "fields" pragma into the specified class.  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
     TThhiiss ffuunnccttiioonn oonnllyy wwoorrkkss iinn PPeerrll 55..88..xx aanndd eeaarrlliieerr..  Pseudo-hashes
     were removed from Perl as of 5.10.  Consider using restricted hashes
     or ffiieellddss::::nneeww(()) instead (which itself uses restricted hashes under
     5.10+).  See Hash::Util.  Using ffiieellddss::::pphhaasshh(()) under 5.10 or higher
     will cause an error.

     ffiieellddss::::pphhaasshh(()) 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)], [@_]);
         }

     ffiieellddss::::pphhaasshh(()) 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);

SSEEEE AALLSSOO #

 base, Hash::Util

perl v5.36.3 2019-02-13 fields(3p)