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[perl5.git] / lib / UNIVERSAL.pm
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1package UNIVERSAL;
2
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3our $VERSION = '1.00';
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.
def3c102 11require Exporter;
84902520 12*import = \&Exporter::import;
ea8fae29 13@EXPORT_OK = qw(isa can VERSION);
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14
151;
16__END__
17
18=head1 NAME
19
20UNIVERSAL - 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");
def3c102 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 ;
84902520 34
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35=head1 DESCRIPTION
36
37C<UNIVERSAL> is the base class which all bless references will inherit from,
ea8fae29 38see L<perlobj>.
def3c102 39
ea8fae29 40C<UNIVERSAL> provides the following methods and functions:
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41
42=over 4
43
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44=item $obj->isa( TYPE ), CLASS->isa( TYPE ), isa( VAL, TYPE )
45
46 C<TYPE> is a package name
47 $obj is a blessed reference or a string containing a package name
48 C<CLASS> is a package name
49 C<VAL> is any of the above or an unblessed reference
50
51When used as an instance or class method (C<$obj->isa( TYPE )>), C<isa>
52returns I<true> if $obj is blessed into package C<TYPE> or inherits from
53package C<TYPE>.
54
55When used as a class method (C<CLASS->isa( TYPE )>; sometimes referred to as a
56static method), C<isa> returns I<true> if C<CLASS> inherits from (or is itself)
57the name of the package C<TYPE> or inherits from package C<TYPE>.
58
59When used as a function, like
60
61 use UNIVERSAL qw( isa ) ;
62 $yes = isa $h, "HASH";
63 $yes = isa "Foo", "Bar";
def3c102 64
ea8fae29 65or
def3c102 66
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67 require UNIVERSAL ;
68 $yes = UNIVERSAL::isa $a, "ARRAY";
def3c102 69
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70, C<isa> returns I<true> in the same cases as above and also if C<VAL> is an
71unblessed reference to a perl variable of type C<TYPE>, such as "HASH",
72"ARRAY", or "Regexp".
def3c102 73
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74=item $obj->can( METHOD ), CLASS->can( METHOD ), can( VAL, METHOD )
75
76C<can> checks if the object or class has a method called C<METHOD>. If it does
77then a reference to the sub is returned. If it does not then I<undef> is
78returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
79C<VAL>.
def3c102 80
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81C<can> cannot know whether an object will be able to provide a method
82through AUTOLOAD, so a return value of I<undef> does not necessarily mean
83the object will not be able to handle the method call. To get around
84this some module authors use a forward declaration (see L<perlsub>)
85for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can>
86will still return a code reference, which, when called, will fall through
87to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
88will cause an error.
89
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90C<can> can be called as a class (static) method, an object method, or a
91function.
92
93When used as a function, if C<VAL> is a blessed reference or package name which
94has a method called C<METHOD>, C<can> returns a reference to the subroutine.
95If C<VAL> is not a blessed reference, or if it does not have a method
96C<METHOD>, I<undef> is returned.
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97
98=item VERSION ( [ REQUIRE ] )
99
100C<VERSION> will return the value of the variable C<$VERSION> in the
101package the object is blessed into. If C<REQUIRE> is given then
102it will do a comparison and die if the package version is not
103greater than or equal to C<REQUIRE>.
104
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105C<VERSION> can be called as either a class (static) method, an object method or
106or a function.
a66bc3b0 107
a66bc3b0 108
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109=back
110
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111These subroutines should I<not> be imported via S<C<use UNIVERSAL qw(...)>>.
112If you want simple local access to them you can do
113
114 *isa = \&UNIVERSAL::isa;
115
116to import isa into your package.
117
def3c102 118=cut