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1 | package UNIVERSAL; |
2 | |
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3 | our $VERSION = '1.03'; |
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4 | |
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5 | # UNIVERSAL should not contain any extra subs/methods beyond those |
6 | # that it exists to define. The use of Exporter below is a historical |
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7 | # accident that can't be fixed without breaking code. Note that we |
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8 | # *don't* set @ISA here, as we don't want all classes/objects inheriting from |
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9 | # Exporter. It's bad enough that all classes have a import() method |
10 | # whenever UNIVERSAL.pm is loaded. |
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11 | require Exporter; |
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12 | @EXPORT_OK = qw(isa can VERSION); |
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13 | |
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14 | # Make sure that even though the import method is called, it doesn't do |
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15 | # anything unless called on UNIVERSAL. |
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16 | sub import { |
17 | return unless $_[0] eq __PACKAGE__; |
18 | goto &Exporter::import; |
19 | } |
20 | |
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21 | 1; |
22 | __END__ |
23 | |
24 | =head1 NAME |
25 | |
26 | UNIVERSAL - base class for ALL classes (blessed references) |
27 | |
28 | =head1 SYNOPSIS |
29 | |
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30 | $is_io = $fd->isa("IO::Handle"); |
31 | $is_io = Class->isa("IO::Handle"); |
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32 | |
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33 | $sub = $obj->can("print"); |
34 | $sub = Class->can("print"); |
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35 | |
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36 | $sub = eval { $ref->can("fandango") }; |
37 | $ver = $obj->VERSION; |
38 | |
39 | # but never do this! |
40 | $is_io = UNIVERSAL::isa($fd, "IO::Handle"); |
41 | $sub = UNIVERSAL::can($obj, "print"); |
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42 | |
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43 | =head1 DESCRIPTION |
44 | |
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45 | C<UNIVERSAL> is the base class from which all blessed references inherit. |
46 | See L<perlobj>. |
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47 | |
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48 | C<UNIVERSAL> provides the following methods: |
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49 | |
50 | =over 4 |
51 | |
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52 | =item C<< $obj->isa( TYPE ) >> |
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53 | |
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54 | =item C<< CLASS->isa( TYPE ) >> |
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55 | |
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56 | =item C<< eval { VAL->isa( TYPE ) } >> |
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57 | |
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58 | Where |
59 | |
60 | =over 4 |
61 | |
62 | =item C<TYPE> |
63 | |
64 | is a package name |
65 | |
66 | =item C<$obj> |
67 | |
68 | is a blessed reference or a string containing a package name |
69 | |
70 | =item C<CLASS> |
71 | |
72 | is a package name |
73 | |
74 | =item C<VAL> |
75 | |
76 | is any of the above or an unblessed reference |
77 | |
78 | =back |
79 | |
80 | When used as an instance or class method (C<< $obj->isa( TYPE ) >>), |
81 | C<isa> returns I<true> if $obj is blessed into package C<TYPE> or |
82 | inherits from package C<TYPE>. |
83 | |
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84 | When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes |
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85 | referred to as a static method), C<isa> returns I<true> if C<CLASS> |
86 | inherits from (or is itself) the name of the package C<TYPE> or |
87 | inherits from package C<TYPE>. |
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88 | |
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89 | If you're not sure what you have (the C<VAL> case), wrap the method call in an |
90 | C<eval> block to catch the exception if C<VAL> is undefined. |
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91 | |
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92 | If you want to be sure that you're calling C<isa> as a method, not a class, |
93 | check the invocant with C<blessed> from L<Scalar::Util> first: |
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94 | |
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95 | use Scalar::Util 'blessed'; |
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96 | |
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97 | if ( blessed( $obj ) && $obj->isa("Some::Class") { |
98 | ... |
99 | } |
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100 | |
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101 | =item C<< $obj->can( METHOD ) >> |
102 | |
103 | =item C<< CLASS->can( METHOD ) >> |
104 | |
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105 | =item C<< eval { VAL->can( METHOD ) } >> |
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106 | |
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107 | C<can> checks if the object or class has a method called C<METHOD>. If it does, |
108 | then it returns a reference to the sub. If it does not, then it returns |
109 | I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or |
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110 | C<VAL>. |
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111 | |
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112 | C<can> cannot know whether an object will be able to provide a method through |
113 | AUTOLOAD (unless the object's class has overriden C<can> appropriately), so a |
114 | return value of I<undef> does not necessarily mean the object will not be able |
115 | to handle the method call. To get around this some module authors use a forward |
116 | declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For |
117 | such 'dummy' subs, C<can> will still return a code reference, which, when |
118 | called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided, |
119 | calling the coderef will cause an error. |
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120 | |
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121 | You may call C<can> as a class (static) method or an object method. |
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122 | |
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123 | Again, the same rule about having a valid invocant applies -- use an C<eval> |
124 | block or C<blessed> if you need to be extra paranoid. |
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125 | |
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126 | =item C<VERSION ( [ REQUIRE ] )> |
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127 | |
128 | C<VERSION> will return the value of the variable C<$VERSION> in the |
129 | package the object is blessed into. If C<REQUIRE> is given then |
130 | it will do a comparison and die if the package version is not |
131 | greater than or equal to C<REQUIRE>. |
132 | |
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133 | C<VERSION> can be called as either a class (static) method or an object |
134 | method. |
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135 | |
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136 | =back |
137 | |
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138 | =head1 EXPORTS |
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139 | |
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140 | None by default. |
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141 | |
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142 | You may request the import of all three functions (C<isa>, C<can>, and |
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143 | C<VERSION>), however it is usually harmful to do so. Please don't do this in |
144 | new code. |
145 | |
146 | For example, previous versions of this documentation suggested using C<isa> as |
147 | a function to determine the type of a reference: |
148 | |
149 | use UNIVERSAL 'isa'; |
150 | |
151 | $yes = isa $h, "HASH"; |
152 | $yes = isa "Foo", "Bar"; |
153 | |
154 | The problem is that this code will I<never> call an overridden C<isa> method in |
155 | any class. Instead, use C<reftype> from L<Scalar::Util> for the first case: |
156 | |
157 | use Scalar::Util 'reftype'; |
158 | |
159 | $yes = reftype( $h ) eq "HASH"; |
160 | |
161 | and the method form of C<isa> for the second: |
162 | |
163 | $yes = Foo->isa("Bar"); |
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164 | |
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165 | =cut |