1 | package UNIVERSAL;
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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
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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, 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
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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 |
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15 | 1;
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16 | __END__
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17 |
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18 | =head1 NAME
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19 |
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20 | UNIVERSAL - base class for ALL classes (blessed references)
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21 |
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22 | =head1 SYNOPSIS
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23 |
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24 | $is_io = $fd->isa("IO::Handle");
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25 | $is_io = Class->isa("IO::Handle");
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26 |
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27 | $sub = $obj->can("print");
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28 | $sub = Class->can("print");
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29 |
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30 | use UNIVERSAL qw( isa can VERSION );
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31 | $yes = isa $ref, "HASH" ;
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32 | $sub = can $ref, "fandango" ;
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33 | $ver = VERSION $obj ;
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34 |
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35 | =head1 DESCRIPTION
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36 |
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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 |
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42 | =over 4
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43 |
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44 | =item C<< $obj->isa( TYPE ) >>
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45 |
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46 | =item C<< CLASS->isa( TYPE ) >>
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47 |
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48 | =item C<isa( VAL, TYPE )>
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49 |
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50 | Where
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51 |
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52 | =over 4
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53 |
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54 | =item C<TYPE>
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55 |
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56 | is a package name
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57 |
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58 | =item C<$obj>
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59 |
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60 | is a blessed reference or a string containing a package name
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61 |
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62 | =item C<CLASS>
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63 |
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64 | is a package name
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65 |
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66 | =item C<VAL>
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67 |
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68 | is any of the above or an unblessed reference
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69 |
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70 | =back
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71 |
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72 | When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
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73 | C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
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74 | inherits from package C<TYPE>.
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75 |
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76 | When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes
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77 | referred to as a static method), C<isa> returns I<true> if C<CLASS>
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78 | inherits from (or is itself) the name of the package C<TYPE> or
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79 | inherits from package C<TYPE>.
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80 |
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81 | When used as a function, like
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82 |
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83 | use UNIVERSAL qw( isa ) ;
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84 | $yes = isa $h, "HASH";
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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 ;
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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",
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94 | "ARRAY", or "Regexp".
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95 |
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96 | =item C<< $obj->can( METHOD ) >>
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97 |
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98 | =item C<< CLASS->can( METHOD ) >>
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99 |
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100 | =item C<can( VAL, METHOD )>
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101 |
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102 | C<can> checks if the object or class has a method called C<METHOD>. If it does
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103 | then a reference to the sub is returned. If it does not then I<undef> is
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104 | returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
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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
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108 | through AUTOLOAD, so a return value of I<undef> does not necessarily mean
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109 | the object will not be able to handle the method call. To get around
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110 | this some module authors use a forward declaration (see L<perlsub>)
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111 | for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can>
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112 | will still return a code reference, which, when called, will fall through
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113 | to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
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114 | will cause an error.
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115 |
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116 | C<can> can be called as a class (static) method, an object method, or a
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117 | function.
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118 |
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119 | When used as a function, if C<VAL> is a blessed reference or package name which
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120 | has a method called C<METHOD>, C<can> returns a reference to the subroutine.
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121 | If C<VAL> is not a blessed reference, or if it does not have a method
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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 |
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126 | C<VERSION> will return the value of the variable C<$VERSION> in the
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127 | package the object is blessed into. If C<REQUIRE> is given then
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128 | it will do a comparison and die if the package version is not
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129 | greater than or equal to C<REQUIRE>.
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130 |
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131 | C<VERSION> can be called as either a class (static) method, an object
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132 | method or a function.
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133 |
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134 |
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135 | =back
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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
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142 | C<VERSION>), however it isn't usually necessary to do so. Perl magically
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143 | makes these functions act as methods on all objects. The one exception is
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144 | C<isa>, which is useful as a function when operating on non-blessed
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145 | references.
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146 |
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147 | =cut
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