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1 | package UNIVERSAL; |
2 | |
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3 | our $VERSION = '1.04'; |
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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 | $does_log = $obj->DOES("Logger"); |
34 | $does_log = Class->DOES("Logger"); |
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35 | |
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36 | $sub = $obj->can("print"); |
37 | $sub = Class->can("print"); |
38 | |
39 | $sub = eval { $ref->can("fandango") }; |
40 | $ver = $obj->VERSION; |
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41 | |
42 | # but never do this! |
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43 | $is_io = UNIVERSAL::isa($fd, "IO::Handle"); |
44 | $sub = UNIVERSAL::can($obj, "print"); |
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45 | |
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46 | =head1 DESCRIPTION |
47 | |
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48 | C<UNIVERSAL> is the base class from which all blessed references inherit. |
49 | See L<perlobj>. |
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50 | |
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51 | C<UNIVERSAL> provides the following methods: |
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52 | |
53 | =over 4 |
54 | |
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55 | =item C<< $obj->isa( TYPE ) >> |
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56 | |
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57 | =item C<< CLASS->isa( TYPE ) >> |
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58 | |
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59 | =item C<< eval { VAL->isa( TYPE ) } >> |
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60 | |
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61 | Where |
62 | |
63 | =over 4 |
64 | |
65 | =item C<TYPE> |
66 | |
67 | is a package name |
68 | |
69 | =item C<$obj> |
70 | |
71 | is a blessed reference or a string containing a package name |
72 | |
73 | =item C<CLASS> |
74 | |
75 | is a package name |
76 | |
77 | =item C<VAL> |
78 | |
79 | is any of the above or an unblessed reference |
80 | |
81 | =back |
82 | |
83 | When used as an instance or class method (C<< $obj->isa( TYPE ) >>), |
84 | C<isa> returns I<true> if $obj is blessed into package C<TYPE> or |
85 | inherits from package C<TYPE>. |
86 | |
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87 | When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes |
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88 | referred to as a static method), C<isa> returns I<true> if C<CLASS> |
89 | inherits from (or is itself) the name of the package C<TYPE> or |
90 | inherits from package C<TYPE>. |
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91 | |
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92 | If you're not sure what you have (the C<VAL> case), wrap the method call in an |
93 | C<eval> block to catch the exception if C<VAL> is undefined. |
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94 | |
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95 | If you want to be sure that you're calling C<isa> as a method, not a class, |
96 | check the invocant with C<blessed> from L<Scalar::Util> first: |
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97 | |
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98 | use Scalar::Util 'blessed'; |
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99 | |
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100 | if ( blessed( $obj ) && $obj->isa("Some::Class") { |
101 | ... |
102 | } |
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103 | |
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104 | =item C<< $obj->DOES( ROLE ) >> |
105 | |
106 | =item C<< CLASS->DOES( ROLE ) >> |
107 | |
108 | C<DOES> checks if the object or class performs the role C<ROLE>. A role is a |
109 | named group of specific behavior (often methods of particular names and |
110 | signatures), similar to a class, but not necessarily a complete class by |
111 | itself. For example, logging or serialization may be roles. |
112 | |
113 | C<DOES> and C<isa> are similar, in that if either is true, you know that the |
114 | object or class on which you call the method can perform specific behavior. |
115 | However, C<DOES> is different from C<isa> in that it does not care I<how> the |
116 | invocant performs the operations, merely that it does. (C<isa> of course |
117 | mandates an inheritance relationship. Other relationships include aggregation, |
118 | delegation, and mocking.) |
119 | |
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120 | By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the |
121 | role of all classes in their inheritance. In other words, by default C<DOES> |
122 | responds identically to C<isa>. |
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123 | |
124 | There is a relationship between roles and classes, as each class implies the |
125 | existence of a role of the same name. There is also a relationship between |
126 | inheritance and roles, in that a subclass that inherits from an ancestor class |
127 | implicitly performs any roles its parent performs. Thus you can use C<DOES> in |
128 | place of C<isa> safely, as it will return true in all places where C<isa> will |
129 | return true (provided that any overridden C<DOES> I<and> C<isa> methods behave |
130 | appropriately). |
131 | |
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132 | =item C<< $obj->can( METHOD ) >> |
133 | |
134 | =item C<< CLASS->can( METHOD ) >> |
135 | |
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136 | =item C<< eval { VAL->can( METHOD ) } >> |
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137 | |
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138 | C<can> checks if the object or class has a method called C<METHOD>. If it does, |
139 | then it returns a reference to the sub. If it does not, then it returns |
140 | I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or |
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141 | C<VAL>. |
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142 | |
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143 | C<can> cannot know whether an object will be able to provide a method through |
144 | AUTOLOAD (unless the object's class has overriden C<can> appropriately), so a |
145 | return value of I<undef> does not necessarily mean the object will not be able |
146 | to handle the method call. To get around this some module authors use a forward |
147 | declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For |
148 | such 'dummy' subs, C<can> will still return a code reference, which, when |
149 | called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided, |
150 | calling the coderef will cause an error. |
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151 | |
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152 | You may call C<can> as a class (static) method or an object method. |
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153 | |
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154 | Again, the same rule about having a valid invocant applies -- use an C<eval> |
155 | block or C<blessed> if you need to be extra paranoid. |
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156 | |
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157 | =item C<VERSION ( [ REQUIRE ] )> |
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158 | |
159 | C<VERSION> will return the value of the variable C<$VERSION> in the |
160 | package the object is blessed into. If C<REQUIRE> is given then |
161 | it will do a comparison and die if the package version is not |
162 | greater than or equal to C<REQUIRE>. |
163 | |
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164 | C<VERSION> can be called as either a class (static) method or an object |
165 | method. |
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166 | |
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167 | =back |
168 | |
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169 | =head1 EXPORTS |
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170 | |
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171 | None by default. |
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172 | |
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173 | You may request the import of three functions (C<isa>, C<can>, and C<VERSION>), |
174 | however it is usually harmful to do so. Please don't do this in new code. |
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175 | |
176 | For example, previous versions of this documentation suggested using C<isa> as |
177 | a function to determine the type of a reference: |
178 | |
179 | use UNIVERSAL 'isa'; |
180 | |
181 | $yes = isa $h, "HASH"; |
182 | $yes = isa "Foo", "Bar"; |
183 | |
184 | The problem is that this code will I<never> call an overridden C<isa> method in |
185 | any class. Instead, use C<reftype> from L<Scalar::Util> for the first case: |
186 | |
187 | use Scalar::Util 'reftype'; |
188 | |
189 | $yes = reftype( $h ) eq "HASH"; |
190 | |
191 | and the method form of C<isa> for the second: |
192 | |
193 | $yes = Foo->isa("Bar"); |
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194 | |
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195 | =cut |