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1 | package fields; |
2 | |
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3 | =head1 NAME |
4 | |
5 | fields - compile-time class fields |
6 | |
7 | =head1 SYNOPSIS |
8 | |
9 | { |
10 | package Foo; |
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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 | } |
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22 | } |
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23 | |
24 | my Foo $var = Foo::->new; |
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25 | $var->{foo} = 42; |
26 | |
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27 | # this will generate a compile-time error |
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28 | $var->{zap} = 42; |
29 | |
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30 | # subclassing |
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31 | { |
32 | package Bar; |
33 | use base 'Foo'; |
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34 | use fields qw(baz _Bar_private); # not shared with Foo |
35 | sub new { |
36 | my $class = shift; |
37 | my $self = fields::new($class); |
38 | $self->SUPER::new(); # init base fields |
39 | $self->{baz} = 10; # init own fields |
40 | $self->{_Bar_private} = "this is Bar's secret"; |
41 | return $self; |
42 | } |
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43 | } |
44 | |
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45 | =head1 DESCRIPTION |
46 | |
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47 | The C<fields> pragma enables compile-time verified class fields. |
48 | |
49 | NOTE: The current implementation keeps the declared fields in the %FIELDS |
50 | hash of the calling package, but this may change in future versions. |
51 | Do B<not> update the %FIELDS hash directly, because it must be created |
52 | at compile-time for it to be fully useful, as is done by this pragma. |
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53 | |
54 | If a typed lexical variable holding a reference is used to access a |
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55 | hash element and a package with the same name as the type has declared |
56 | class fields using this pragma, then the operation is turned into an |
57 | array access at compile time. |
58 | |
59 | The related C<base> pragma will combine fields from base classes and any |
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60 | fields declared using the C<fields> pragma. This enables field |
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61 | inheritance to work properly. |
62 | |
63 | Field names that start with an underscore character are made private to |
64 | the class and are not visible to subclasses. Inherited fields can be |
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65 | overridden but will generate a warning if used together with the C<-w> |
66 | switch. |
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67 | |
68 | The effect of all this is that you can have objects with named fields |
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69 | which are as compact and as fast arrays to access. This only works |
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70 | as long as the objects are accessed through properly typed variables. |
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71 | If the objects are not typed, access is only checked at run time. |
72 | |
73 | The following functions are supported: |
74 | |
75 | =over 8 |
76 | |
77 | =item new |
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78 | |
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79 | fields::new() creates and blesses a pseudo-hash comprised of the fields |
80 | declared using the C<fields> pragma into the specified class. |
81 | This makes it possible to write a constructor like this: |
82 | |
83 | package Critter::Sounds; |
84 | use fields qw(cat dog bird); |
85 | |
86 | sub new { |
87 | my Critter::Sounds $self = shift; |
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88 | $self = fields::new($self) unless ref $self; |
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89 | $self->{cat} = 'meow'; # scalar element |
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90 | @$self{'dog','bird'} = ('bark','tweet'); # slice |
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91 | return $self; |
92 | } |
93 | |
94 | =item phash |
95 | |
96 | fields::phash() can be used to create and initialize a plain (unblessed) |
97 | pseudo-hash. This function should always be used instead of creating |
98 | pseudo-hashes directly. |
99 | |
100 | If the first argument is a reference to an array, the pseudo-hash will |
101 | be created with keys from that array. If a second argument is supplied, |
102 | it must also be a reference to an array whose elements will be used as |
103 | the values. If the second array contains less elements than the first, |
104 | the trailing elements of the pseudo-hash will not be initialized. |
105 | This makes it particularly useful for creating a pseudo-hash from |
106 | subroutine arguments: |
107 | |
108 | sub dogtag { |
109 | my $tag = fields::phash([qw(name rank ser_num)], [@_]); |
110 | } |
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111 | |
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112 | fields::phash() also accepts a list of key-value pairs that will |
113 | be used to construct the pseudo hash. Examples: |
114 | |
115 | my $tag = fields::phash(name => "Joe", |
116 | rank => "captain", |
117 | ser_num => 42); |
118 | |
119 | my $pseudohash = fields::phash(%args); |
120 | |
121 | =back |
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122 | |
123 | =head1 SEE ALSO |
124 | |
125 | L<base>, |
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126 | L<perlref/Pseudo-hashes: Using an array as a hash> |
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127 | |
128 | =cut |
129 | |
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130 | use 5.005_64; |
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131 | use strict; |
132 | no strict 'refs'; |
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133 | our(%attr, $VERSION); |
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134 | |
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135 | $VERSION = "1.01"; |
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136 | |
137 | # some constants |
138 | sub _PUBLIC () { 1 } |
139 | sub _PRIVATE () { 2 } |
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140 | |
141 | # The %attr hash holds the attributes of the currently assigned fields |
142 | # per class. The hash is indexed by class names and the hash value is |
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143 | # an array reference. The first element in the array is the lowest field |
144 | # number not belonging to a base class. The remaining elements' indices |
145 | # are the field numbers. The values are integer bit masks, or undef |
146 | # in the case of base class private fields (which occupy a slot but are |
147 | # otherwise irrelevant to the class). |
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148 | |
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149 | sub import { |
150 | my $class = shift; |
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151 | return unless @_; |
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152 | my $package = caller(0); |
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153 | # avoid possible typo warnings |
154 | %{"$package\::FIELDS"} = () unless %{"$package\::FIELDS"}; |
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155 | my $fields = \%{"$package\::FIELDS"}; |
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156 | my $fattr = ($attr{$package} ||= [1]); |
157 | my $next = @$fattr; |
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158 | |
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159 | if ($next > $fattr->[0] |
160 | and ($fields->{$_[0]} || 0) >= $fattr->[0]) |
161 | { |
162 | # There are already fields not belonging to base classes. |
163 | # Looks like a possible module reload... |
164 | $next = $fattr->[0]; |
165 | } |
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166 | foreach my $f (@_) { |
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167 | my $fno = $fields->{$f}; |
168 | |
169 | # Allow the module to be reloaded so long as field positions |
170 | # have not changed. |
171 | if ($fno and $fno != $next) { |
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172 | require Carp; |
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173 | if ($fno < $fattr->[0]) { |
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174 | Carp::carp("Hides field '$f' in base class") if $^W; |
175 | } else { |
176 | Carp::croak("Field name '$f' already in use"); |
177 | } |
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178 | } |
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179 | $fields->{$f} = $next; |
180 | $fattr->[$next] = ($f =~ /^_/) ? _PRIVATE : _PUBLIC; |
181 | $next += 1; |
182 | } |
183 | if (@$fattr > $next) { |
184 | # Well, we gave them the benefit of the doubt by guessing the |
185 | # module was reloaded, but they appear to be declaring fields |
186 | # in more than one place. We can't be sure (without some extra |
187 | # bookkeeping) that the rest of the fields will be declared or |
188 | # have the same positions, so punt. |
189 | require Carp; |
190 | Carp::croak ("Reloaded module must declare all fields at once"); |
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191 | } |
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192 | } |
193 | |
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194 | sub inherit { # called by base.pm when $base_fields is nonempty |
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195 | my($derived, $base) = @_; |
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196 | my $base_attr = $attr{$base}; |
197 | my $derived_attr = $attr{$derived} ||= []; |
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198 | # avoid possible typo warnings |
199 | %{"$base\::FIELDS"} = () unless %{"$base\::FIELDS"}; |
200 | %{"$derived\::FIELDS"} = () unless %{"$derived\::FIELDS"}; |
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201 | my $base_fields = \%{"$base\::FIELDS"}; |
202 | my $derived_fields = \%{"$derived\::FIELDS"}; |
203 | |
204 | $derived_attr->[0] = $base_attr ? scalar(@$base_attr) : 1; |
205 | while (my($k,$v) = each %$base_fields) { |
206 | my($fno); |
207 | if ($fno = $derived_fields->{$k} and $fno != $v) { |
208 | require Carp; |
209 | Carp::croak ("Inherited %FIELDS can't override existing %FIELDS"); |
210 | } |
211 | if ($base_attr->[$v] & _PRIVATE) { |
212 | $derived_attr->[$v] = undef; |
213 | } else { |
214 | $derived_attr->[$v] = $base_attr->[$v]; |
215 | $derived_fields->{$k} = $v; |
216 | } |
217 | } |
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218 | } |
219 | |
220 | sub _dump # sometimes useful for debugging |
221 | { |
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222 | for my $pkg (sort keys %attr) { |
223 | print "\n$pkg"; |
224 | if (@{"$pkg\::ISA"}) { |
225 | print " (", join(", ", @{"$pkg\::ISA"}), ")"; |
226 | } |
227 | print "\n"; |
228 | my $fields = \%{"$pkg\::FIELDS"}; |
229 | for my $f (sort {$fields->{$a} <=> $fields->{$b}} keys %$fields) { |
230 | my $no = $fields->{$f}; |
231 | print " $no: $f"; |
232 | my $fattr = $attr{$pkg}[$no]; |
233 | if (defined $fattr) { |
234 | my @a; |
235 | push(@a, "public") if $fattr & _PUBLIC; |
236 | push(@a, "private") if $fattr & _PRIVATE; |
237 | push(@a, "inherited") if $no < $attr{$pkg}[0]; |
238 | print "\t(", join(", ", @a), ")"; |
239 | } |
240 | print "\n"; |
241 | } |
242 | } |
243 | } |
244 | |
245 | sub new { |
246 | my $class = shift; |
247 | $class = ref $class if ref $class; |
248 | return bless [\%{$class . "::FIELDS"}], $class; |
249 | } |
250 | |
251 | sub phash { |
252 | my $h; |
253 | my $v; |
254 | if (@_) { |
255 | if (ref $_[0] eq 'ARRAY') { |
256 | my $a = shift; |
257 | @$h{@$a} = 1 .. @$a; |
258 | if (@_) { |
259 | $v = shift; |
260 | unless (! @_ and ref $v eq 'ARRAY') { |
261 | require Carp; |
262 | Carp::croak ("Expected at most two array refs\n"); |
263 | } |
264 | } |
265 | } |
266 | else { |
267 | if (@_ % 2) { |
268 | require Carp; |
269 | Carp::croak ("Odd number of elements initializing pseudo-hash\n"); |
270 | } |
271 | my $i = 0; |
272 | @$h{grep ++$i % 2, @_} = 1 .. @_ / 2; |
273 | $i = 0; |
274 | $v = [grep $i++ % 2, @_]; |
275 | } |
276 | } |
277 | else { |
278 | $h = {}; |
279 | $v = []; |
280 | } |
281 | [ $h, @$v ]; |
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282 | } |
283 | |
284 | 1; |