fields(3perl) - phpMan

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

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 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;
               }
           }

DESCRIPTION
       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 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 ("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;
               }

       phash
           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);

SEE ALSO
       base, Hash::Util

perl v5.34.0                                2023-11-23                              fields(3perl)

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