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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 _private); |
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12 | } |
13 | ... |
14 | my Foo $var = new Foo; |
15 | $var->{foo} = 42; |
16 | |
17 | # This will generate a compile-time error. |
18 | $var->{zap} = 42; |
19 | |
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20 | { |
21 | package Bar; |
22 | use base 'Foo'; |
23 | use fields 'bar'; # hides Foo->{bar} |
24 | use fields qw(baz _private); # not shared with Foo |
25 | } |
26 | |
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27 | =head1 DESCRIPTION |
28 | |
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29 | The C<fields> pragma enables compile-time verified class fields. It |
30 | does so by updating the %FIELDS hash in the calling package. |
31 | |
32 | If a typed lexical variable holding a reference is used to access a |
33 | hash element and the %FIELDS hash of the given type exists, then the |
34 | operation is turned into an array access at compile time. The %FIELDS |
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35 | hash maps from hash element names to the array indices. If the hash |
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36 | element is not present in the %FIELDS hash, then a compile-time error |
37 | is signaled. |
38 | |
39 | Since the %FIELDS hash is used at compile-time, it must be set up at |
40 | compile-time too. This is made easier with the help of the 'fields' |
41 | and the 'base' pragma modules. The 'base' pragma will copy fields |
42 | from base classes and the 'fields' pragma adds new fields. Field |
43 | names that start with an underscore character are made private to a |
44 | class and are not visible to subclasses. Inherited fields can be |
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45 | overridden but will generate a warning if used together with the C<-w> |
46 | switch. |
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47 | |
48 | The effect of all this is that you can have objects with named fields |
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49 | which are as compact and as fast arrays to access. This only works |
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50 | as long as the objects are accessed through properly typed variables. |
51 | For untyped access to work you have to make sure that a reference to |
52 | the proper %FIELDS hash is assigned to the 0'th element of the array |
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53 | object (so that the objects can be treated like an pseudo-hash). A |
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54 | constructor like this does the job: |
55 | |
56 | sub new |
57 | { |
58 | my $class = shift; |
59 | no strict 'refs'; |
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60 | my $self = bless [\%{"$class\::FIELDS"}], $class; |
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61 | $self; |
62 | } |
63 | |
64 | |
65 | =head1 SEE ALSO |
66 | |
67 | L<base>, |
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68 | L<perlref/Pseudo-hashes: Using an array as a hash> |
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69 | |
70 | =cut |
71 | |
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72 | use strict; |
73 | no strict 'refs'; |
74 | use vars qw(%attr $VERSION); |
75 | |
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76 | $VERSION = "1.01"; |
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77 | |
78 | # some constants |
79 | sub _PUBLIC () { 1 } |
80 | sub _PRIVATE () { 2 } |
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81 | |
82 | # The %attr hash holds the attributes of the currently assigned fields |
83 | # per class. The hash is indexed by class names and the hash value is |
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84 | # an array reference. The first element in the array is the lowest field |
85 | # number not belonging to a base class. The remaining elements' indices |
86 | # are the field numbers. The values are integer bit masks, or undef |
87 | # in the case of base class private fields (which occupy a slot but are |
88 | # otherwise irrelevant to the class). |
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89 | |
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90 | sub import { |
91 | my $class = shift; |
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92 | return unless @_; |
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93 | my $package = caller(0); |
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94 | my $fields = \%{"$package\::FIELDS"}; |
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95 | my $fattr = ($attr{$package} ||= [1]); |
96 | my $next = @$fattr; |
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97 | |
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98 | if ($next > $fattr->[0] |
99 | and ($fields->{$_[0]} || 0) >= $fattr->[0]) |
100 | { |
101 | # There are already fields not belonging to base classes. |
102 | # Looks like a possible module reload... |
103 | $next = $fattr->[0]; |
104 | } |
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105 | foreach my $f (@_) { |
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106 | my $fno = $fields->{$f}; |
107 | |
108 | # Allow the module to be reloaded so long as field positions |
109 | # have not changed. |
110 | if ($fno and $fno != $next) { |
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111 | require Carp; |
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112 | if ($fno < $fattr->[0]) { |
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113 | Carp::carp("Hides field '$f' in base class") if $^W; |
114 | } else { |
115 | Carp::croak("Field name '$f' already in use"); |
116 | } |
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117 | } |
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118 | $fields->{$f} = $next; |
119 | $fattr->[$next] = ($f =~ /^_/) ? _PRIVATE : _PUBLIC; |
120 | $next += 1; |
121 | } |
122 | if (@$fattr > $next) { |
123 | # Well, we gave them the benefit of the doubt by guessing the |
124 | # module was reloaded, but they appear to be declaring fields |
125 | # in more than one place. We can't be sure (without some extra |
126 | # bookkeeping) that the rest of the fields will be declared or |
127 | # have the same positions, so punt. |
128 | require Carp; |
129 | Carp::croak ("Reloaded module must declare all fields at once"); |
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130 | } |
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131 | } |
132 | |
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133 | sub inherit # called by base.pm when $base_fields is nonempty |
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134 | { |
135 | my($derived, $base) = @_; |
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136 | my $base_attr = $attr{$base}; |
137 | my $derived_attr = $attr{$derived} ||= []; |
138 | my $base_fields = \%{"$base\::FIELDS"}; |
139 | my $derived_fields = \%{"$derived\::FIELDS"}; |
140 | |
141 | $derived_attr->[0] = $base_attr ? scalar(@$base_attr) : 1; |
142 | while (my($k,$v) = each %$base_fields) { |
143 | my($fno); |
144 | if ($fno = $derived_fields->{$k} and $fno != $v) { |
145 | require Carp; |
146 | Carp::croak ("Inherited %FIELDS can't override existing %FIELDS"); |
147 | } |
148 | if ($base_attr->[$v] & _PRIVATE) { |
149 | $derived_attr->[$v] = undef; |
150 | } else { |
151 | $derived_attr->[$v] = $base_attr->[$v]; |
152 | $derived_fields->{$k} = $v; |
153 | } |
154 | } |
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155 | } |
156 | |
157 | sub _dump # sometimes useful for debugging |
158 | { |
159 | for my $pkg (sort keys %attr) { |
160 | print "\n$pkg"; |
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161 | if (@{"$pkg\::ISA"}) { |
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162 | print " (", join(", ", @{"$pkg\::ISA"}), ")"; |
163 | } |
164 | print "\n"; |
165 | my $fields = \%{"$pkg\::FIELDS"}; |
166 | for my $f (sort {$fields->{$a} <=> $fields->{$b}} keys %$fields) { |
167 | my $no = $fields->{$f}; |
168 | print " $no: $f"; |
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169 | my $fattr = $attr{$pkg}[$no]; |
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170 | if (defined $fattr) { |
171 | my @a; |
172 | push(@a, "public") if $fattr & _PUBLIC; |
173 | push(@a, "private") if $fattr & _PRIVATE; |
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174 | push(@a, "inherited") if $no < $attr{$pkg}[0]; |
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175 | print "\t(", join(", ", @a), ")"; |
176 | } |
177 | print "\n"; |
178 | } |
179 | } |
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180 | } |
181 | |
182 | 1; |