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1 | package UNIVERSAL; |
2 | |
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3 | our $VERSION = '1.01'; |
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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 |
8 | # *don't* set @ISA here, don't want all classes/objects inheriting from |
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 | *import = \&Exporter::import; |
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13 | @EXPORT_OK = qw(isa can VERSION); |
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14 | |
15 | 1; |
16 | __END__ |
17 | |
18 | =head1 NAME |
19 | |
20 | UNIVERSAL - base class for ALL classes (blessed references) |
21 | |
22 | =head1 SYNOPSIS |
23 | |
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24 | $is_io = $fd->isa("IO::Handle"); |
25 | $is_io = Class->isa("IO::Handle"); |
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26 | |
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27 | $sub = $obj->can("print"); |
28 | $sub = Class->can("print"); |
29 | |
30 | use UNIVERSAL qw( isa can VERSION ); |
31 | $yes = isa $ref, "HASH" ; |
32 | $sub = can $ref, "fandango" ; |
33 | $ver = VERSION $obj ; |
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34 | |
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35 | =head1 DESCRIPTION |
36 | |
37 | C<UNIVERSAL> is the base class which all bless references will inherit from, |
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38 | see L<perlobj>. |
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39 | |
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40 | C<UNIVERSAL> provides the following methods and functions: |
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41 | |
42 | =over 4 |
43 | |
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44 | =item C<< $obj->isa( TYPE ) >> |
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45 | |
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46 | =item C<< CLASS->isa( TYPE ) >> |
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47 | |
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48 | =item C<isa( VAL, TYPE )> |
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49 | |
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50 | Where |
51 | |
52 | =over 4 |
53 | |
54 | =item C<TYPE> |
55 | |
56 | is a package name |
57 | |
58 | =item C<$obj> |
59 | |
60 | is a blessed reference or a string containing a package name |
61 | |
62 | =item C<CLASS> |
63 | |
64 | is a package name |
65 | |
66 | =item C<VAL> |
67 | |
68 | is any of the above or an unblessed reference |
69 | |
70 | =back |
71 | |
72 | When used as an instance or class method (C<< $obj->isa( TYPE ) >>), |
73 | C<isa> returns I<true> if $obj is blessed into package C<TYPE> or |
74 | inherits from package C<TYPE>. |
75 | |
76 | When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes |
77 | referred to as a static method), C<isa> returns I<true> if C<CLASS> |
78 | inherits from (or is itself) the name of the package C<TYPE> or |
79 | inherits from package C<TYPE>. |
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80 | |
81 | When used as a function, like |
82 | |
83 | use UNIVERSAL qw( isa ) ; |
84 | $yes = isa $h, "HASH"; |
85 | $yes = isa "Foo", "Bar"; |
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86 | |
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87 | or |
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88 | |
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89 | require UNIVERSAL ; |
90 | $yes = UNIVERSAL::isa $a, "ARRAY"; |
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91 | |
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92 | C<isa> returns I<true> in the same cases as above and also if C<VAL> is an |
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93 | unblessed reference to a perl variable of type C<TYPE>, such as "HASH", |
94 | "ARRAY", or "Regexp". |
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95 | |
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96 | =item C<< $obj->can( METHOD ) >> |
97 | |
98 | =item C<< CLASS->can( METHOD ) >> |
99 | |
100 | =item C<can( VAL, METHOD )> |
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101 | |
102 | C<can> checks if the object or class has a method called C<METHOD>. If it does |
103 | then a reference to the sub is returned. If it does not then I<undef> is |
104 | returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or |
105 | C<VAL>. |
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106 | |
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107 | C<can> cannot know whether an object will be able to provide a method |
108 | through AUTOLOAD, so a return value of I<undef> does not necessarily mean |
109 | the object will not be able to handle the method call. To get around |
110 | this some module authors use a forward declaration (see L<perlsub>) |
111 | for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can> |
112 | will still return a code reference, which, when called, will fall through |
113 | to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef |
114 | will cause an error. |
115 | |
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116 | C<can> can be called as a class (static) method, an object method, or a |
117 | function. |
118 | |
119 | When used as a function, if C<VAL> is a blessed reference or package name which |
120 | has a method called C<METHOD>, C<can> returns a reference to the subroutine. |
121 | If C<VAL> is not a blessed reference, or if it does not have a method |
122 | C<METHOD>, I<undef> is returned. |
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123 | |
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124 | =item C<VERSION ( [ REQUIRE ] )> |
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125 | |
126 | C<VERSION> will return the value of the variable C<$VERSION> in the |
127 | package the object is blessed into. If C<REQUIRE> is given then |
128 | it will do a comparison and die if the package version is not |
129 | greater than or equal to C<REQUIRE>. |
130 | |
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131 | C<VERSION> can be called as either a class (static) method, an object |
132 | method or a function. |
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133 | |
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134 | |
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135 | =back |
136 | |
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137 | =head1 EXPORTS |
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138 | |
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139 | None by default. |
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140 | |
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141 | You may request the import of all three functions (C<isa>, C<can>, and |
142 | C<VERSION>), however it isn't usually necessary to do so. Perl magically |
143 | makes these functions act as methods on all objects. The one exception is |
144 | C<isa>, which is useful as a function when operating on non-blessed |
145 | references. |
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146 | |
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147 | =cut |