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

       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 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;

       The "fields" pragma enables compile-time and run-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

       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.

       Only valid for Perl 5.8.x and earlier: Field names that start with an
       underscore character are made private to the class and are not visible
       to subclasses.

       Also, in Perl 5.8.x and earlier, 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 fields::new() 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;

           This function only works in Perl 5.8.x and earlier.  Pseudo-hashes
           were removed from Perl as of 5.10.  Consider using restricted
           hashes or fields::new() instead (which itself uses restricted
           hashes under 5.10+).  See Hash::Util.  Using fields::phash() under
           5.10 or higher will cause an error.

           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 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);

       base, Hash::Util

perl v5.26.1                      2020-10-19                     fields(3perl)
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