fields(3pm)

1fields(3pm)            Perl Programmers Reference Guide            fields(3pm)
2
3
4

NAME

6       fields - compile-time class fields
7

SYNOPSIS

9           {
10               package Foo;
11               use fields qw(foo bar _Foo_private);
12               sub new {
13                   my Foo $self = shift;
14                   unless (ref $self) {
15                       $self = fields::new($self);
16                       $self->{_Foo_private} = "this is Foo's secret";
17                   }
18                   $self->{foo} = 10;
19                   $self->{bar} = 20;
20                   return $self;
21               }
22           }
23
24           my $var = Foo->new;
25           $var->{foo} = 42;
26
27           # this will generate a run-time error
28           $var->{zap} = 42;
29
30           # this will generate a compile-time error
31           my Foo $foo = Foo->new;
32           $foo->{zap} = 24;
33
34           # subclassing
35           {
36               package Bar;
37               use base 'Foo';
38               use fields qw(baz _Bar_private);        # not shared with Foo
39               sub new {
40                   my $class = shift;
41                   my $self = fields::new($class);
42                   $self->SUPER::new();                # init base fields
43                   $self->{baz} = 10;                  # init own fields
44                   $self->{_Bar_private} = "this is Bar's secret";
45                   return $self;
46               }
47           }
48

DESCRIPTION

50       The "fields" pragma enables compile-time and run-time verified class
51       fields.
52
53       NOTE: The current implementation keeps the declared fields in the
54       %FIELDS hash of the calling package, but this may change in future
55       versions.  Do not update the %FIELDS hash directly, because it must be
56       created at compile-time for it to be fully useful, as is done by this
57       pragma.
58
59       If a typed lexical variable ("my Class $var") holding a reference is
60       used to access a hash element and a package with the same name as the
61       type has declared class fields using this pragma, then the hash key is
62       verified at compile time.  If the variables are not typed, access is
63       only checked at run time.
64
65       The related "base" pragma will combine fields from base classes and any
66       fields declared using the "fields" pragma.  This enables field
67       inheritance to work properly.  Inherited fields can be overridden but
68       will generate a warning if warnings are enabled.
69
70       Only valid for Perl 5.8.x and earlier: Field names that start with an
71       underscore character are made private to the class and are not visible
72       to subclasses.
73
74       Also, in Perl 5.8.x and earlier, this pragma uses pseudo-hashes, the
75       effect being that you can have objects with named fields which are as
76       compact and as fast arrays to access, as long as the objects are
77       accessed through properly typed variables.
78
79       The following functions are supported:
80
81       new fields::new() creates and blesses a hash comprised of the fields
82           declared using the "fields" pragma into the specified class.  It is
83           the recommended way to construct a fields-based object.
84
85           This makes it possible to write a constructor like this:
86
87               package Critter::Sounds;
88               use fields qw(cat dog bird);
89
90               sub new {
91                   my $self = shift;
92                   $self = fields::new($self) unless ref $self;
93                   $self->{cat} = 'meow';                      # scalar element
94                   @$self{'dog','bird'} = ('bark','tweet');    # slice
95                   return $self;
96               }
97
98       phash
99           This function only works in Perl 5.8.x and earlier.  Pseudo-hashes
100           were removed from Perl as of 5.10.  Consider using restricted
101           hashes or fields::new() instead (which itself uses restricted
102           hashes under 5.10+).  See Hash::Util.  Using fields::phash() under
103           5.10 or higher will cause an error.
104
105           fields::phash() can be used to create and initialize a plain
106           (unblessed) pseudo-hash.  This function should always be used
107           instead of creating pseudo-hashes directly.
108
109           If the first argument is a reference to an array, the pseudo-hash
110           will be created with keys from that array.  If a second argument is
111           supplied, it must also be a reference to an array whose elements
112           will be used as the values.  If the second array contains less
113           elements than the first, the trailing elements of the pseudo-hash
114           will not be initialized.  This makes it particularly useful for
115           creating a pseudo-hash from subroutine arguments:
116
117               sub dogtag {
118                  my $tag = fields::phash([qw(name rank ser_num)], [@_]);
119               }
120
121           fields::phash() also accepts a list of key-value pairs that will be
122           used to construct the pseudo hash.  Examples:
123
124               my $tag = fields::phash(name => "Joe",
125                                       rank => "captain",
126                                       ser_num => 42);
127
128               my $pseudohash = fields::phash(%args);
129

NAME

SYNOPSIS

DESCRIPTION

SEE ALSO