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package UNIVERSAL; |
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our $VERSION = '1.01'; |
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# UNIVERSAL should not contain any extra subs/methods beyond those |
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# that it exists to define. The use of Exporter below is a historical |
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# accident that can't be fixed without breaking code. Note that we |
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# *don't* set @ISA here, don't want all classes/objects inheriting from |
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# Exporter. It's bad enough that all classes have a import() method |
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# whenever UNIVERSAL.pm is loaded. |
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require Exporter; |
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*import = \&Exporter::import; |
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@EXPORT_OK = qw(isa can VERSION); |
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1; |
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__END__ |
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=head1 NAME |
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UNIVERSAL - base class for ALL classes (blessed references) |
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=head1 SYNOPSIS |
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$is_io = $fd->isa("IO::Handle"); |
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$is_io = Class->isa("IO::Handle"); |
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$sub = $obj->can("print"); |
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$sub = Class->can("print"); |
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use UNIVERSAL qw( isa can VERSION ); |
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$yes = isa $ref, "HASH" ; |
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$sub = can $ref, "fandango" ; |
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$ver = VERSION $obj ; |
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=head1 DESCRIPTION |
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C<UNIVERSAL> is the base class which all bless references will inherit from, |
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see L<perlobj>. |
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C<UNIVERSAL> provides the following methods and functions: |
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=over 4 |
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=item C<< $obj->isa( TYPE ) >> |
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=item C<< CLASS->isa( TYPE ) >> |
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=item C<isa( VAL, TYPE )> |
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Where |
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=over 4 |
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=item C<TYPE> |
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is a package name |
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=item C<$obj> |
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is a blessed reference or a string containing a package name |
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=item C<CLASS> |
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is a package name |
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=item C<VAL> |
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is any of the above or an unblessed reference |
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=back |
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When used as an instance or class method (C<< $obj->isa( TYPE ) >>), |
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C<isa> returns I<true> if $obj is blessed into package C<TYPE> or |
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inherits from package C<TYPE>. |
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When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes |
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referred to as a static method), C<isa> returns I<true> if C<CLASS> |
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inherits from (or is itself) the name of the package C<TYPE> or |
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inherits from package C<TYPE>. |
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When used as a function, like |
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use UNIVERSAL qw( isa ) ; |
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$yes = isa $h, "HASH"; |
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$yes = isa "Foo", "Bar"; |
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or |
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require UNIVERSAL ; |
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$yes = UNIVERSAL::isa $a, "ARRAY"; |
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C<isa> returns I<true> in the same cases as above and also if C<VAL> is an |
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unblessed reference to a perl variable of type C<TYPE>, such as "HASH", |
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"ARRAY", or "Regexp". |
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=item C<< $obj->can( METHOD ) >> |
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=item C<< CLASS->can( METHOD ) >> |
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=item C<can( VAL, METHOD )> |
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C<can> checks if the object or class has a method called C<METHOD>. If it does |
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then a reference to the sub is returned. If it does not then I<undef> is |
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returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or |
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C<VAL>. |
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C<can> cannot know whether an object will be able to provide a method |
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through AUTOLOAD, so a return value of I<undef> does not necessarily mean |
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the object will not be able to handle the method call. To get around |
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this some module authors use a forward declaration (see L<perlsub>) |
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for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<can> |
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will still return a code reference, which, when called, will fall through |
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to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef |
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will cause an error. |
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C<can> can be called as a class (static) method, an object method, or a |
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function. |
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When used as a function, if C<VAL> is a blessed reference or package name which |
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has a method called C<METHOD>, C<can> returns a reference to the subroutine. |
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If C<VAL> is not a blessed reference, or if it does not have a method |
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C<METHOD>, I<undef> is returned. |
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=item C<VERSION ( [ REQUIRE ] )> |
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C<VERSION> will return the value of the variable C<$VERSION> in the |
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package the object is blessed into. If C<REQUIRE> is given then |
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it will do a comparison and die if the package version is not |
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greater than or equal to C<REQUIRE>. |
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C<VERSION> can be called as either a class (static) method, an object |
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method or a function. |
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=back |
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=head1 EXPORTS |
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None by default. |
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You may request the import of all three functions (C<isa>, C<can>, and |
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C<VERSION>), however it isn't usually necessary to do so. Perl magically |
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makes these functions act as methods on all objects. The one exception is |
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C<isa>, which is useful as a function when operating on non-blessed |
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references. |
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=cut |
