| 1 | package UNIVERSAL; |
| 2 | |
| 3 | our $VERSION = '1.01'; |
| 4 | |
| 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 |
| 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. |
| 11 | require Exporter; |
| 12 | *import = \&Exporter::import; |
| 13 | @EXPORT_OK = qw(isa can VERSION); |
| 14 | |
| 15 | 1; |
| 16 | __END__ |
| 17 | |
| 18 | =head1 NAME |
| 19 | |
| 20 | UNIVERSAL - base class for ALL classes (blessed references) |
| 21 | |
| 22 | =head1 SYNOPSIS |
| 23 | |
| 24 | $is_io = $fd->isa("IO::Handle"); |
| 25 | $is_io = Class->isa("IO::Handle"); |
| 26 | |
| 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 ; |
| 34 | |
| 35 | =head1 DESCRIPTION |
| 36 | |
| 37 | C<UNIVERSAL> is the base class which all bless references will inherit from, |
| 38 | see L<perlobj>. |
| 39 | |
| 40 | C<UNIVERSAL> provides the following methods and functions: |
| 41 | |
| 42 | =over 4 |
| 43 | |
| 44 | =item C<< $obj->isa( TYPE ) >> |
| 45 | |
| 46 | =item C<< CLASS->isa( TYPE ) >> |
| 47 | |
| 48 | =item C<isa( VAL, TYPE )> |
| 49 | |
| 50 | Where |
| 51 | |
| 52 | =over 4 |
| 53 | |
| 54 | =item C<TYPE> |
| 55 | |
| 56 | is a package name |
| 57 | |
| 58 | =item C<$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>. |
| 80 | |
| 81 | When used as a function, like |
| 82 | |
| 83 | use UNIVERSAL qw( isa ) ; |
| 84 | $yes = isa $h, "HASH"; |
| 85 | $yes = isa "Foo", "Bar"; |
| 86 | |
| 87 | or |
| 88 | |
| 89 | require UNIVERSAL ; |
| 90 | $yes = UNIVERSAL::isa $a, "ARRAY"; |
| 91 | |
| 92 | C<isa> returns I<true> in the same cases as above and also if C<VAL> is an |
| 93 | unblessed reference to a perl variable of type C<TYPE>, such as "HASH", |
| 94 | "ARRAY", or "Regexp". |
| 95 | |
| 96 | =item C<< $obj->can( METHOD ) >> |
| 97 | |
| 98 | =item C<< CLASS->can( METHOD ) >> |
| 99 | |
| 100 | =item C<can( VAL, METHOD )> |
| 101 | |
| 102 | C<can> checks if the object or class has a method called C<METHOD>. If it does |
| 103 | then a reference to the sub is returned. If it does not then I<undef> is |
| 104 | returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or |
| 105 | C<VAL>. |
| 106 | |
| 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 | |
| 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. |
| 123 | |
| 124 | =item C<VERSION ( [ REQUIRE ] )> |
| 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 | |
| 131 | C<VERSION> can be called as either a class (static) method, an object |
| 132 | method or a function. |
| 133 | |
| 134 | |
| 135 | =back |
| 136 | |
| 137 | =head1 EXPORTS |
| 138 | |
| 139 | None by default. |
| 140 | |
| 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. |
| 146 | |
| 147 | =cut |