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YAPC Brasil 2010

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r1 - 26 May 2008 - TWikiGuest

NAME

UNIVERSAL - base class for ALL classes (blessed references)

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SYNOPSIS 

    $is_io = $fd->isa("IO::Handle");
    $is_io = Class->isa("IO::Handle");
    $sub = $obj->can("print");
    $sub = Class->can("print");
    use UNIVERSAL qw( isa can VERSION );
    $yes = isa $ref, "HASH" ;
    $sub = can $ref, "fandango" ;
    $ver = VERSION $obj ;

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DESCRIPTION

UNIVERSAL is the base class which all bless references will inherit from, see the perlobj manpage. UNIVERSAL provides the following methods and functions:

$obj->isa( TYPE )

CLASS->isa( TYPE )

isa( VAL, TYPE )
Where

TYPE
is a package name

$obj
is a blessed reference or a string containing a package name

CLASS
is a package name

VAL
is any of the above or an unblessed reference
When used as an instance or class method ($obj->isa( TYPE )), isa returns true if $obj is blessed into package TYPE or inherits from package TYPE. When used as a class method (CLASS->isa( TYPE ): sometimes referred to as a static method), isa returns true if CLASS inherits from (or is itself) the name of the package TYPE or inherits from package TYPE. When used as a function, like 
   use UNIVERSAL qw( isa ) ;
   $yes = isa $h, "HASH";
   $yes = isa "Foo", "Bar";
or 
   require UNIVERSAL ;
   $yes = UNIVERSAL::isa $a, "ARRAY";
isa returns true in the same cases as above and also if VAL is an unblessed reference to a perl variable of type TYPE, such as ``HASH'', ``ARRAY'', or ``Regexp''.

$obj->can( METHOD )

CLASS->can( METHOD )

can( VAL, METHOD )
can checks if the object or class has a method called METHOD. If it does
then a reference to the sub is returned. If it does not then undef is
returned. This includes methods inherited or imported by $obj, CLASS, or
VAL.
can cannot know whether an object will be able to provide a method
through AUTOLOAD, so a return value of undef does not necessarily mean
the object will not be able to handle the method call. To get around
this some module authors use a forward declaration (see the perlsub manpage)
for methods they will handle via AUTOLOAD. For such 'dummy' subs, can
will still return a code reference, which, when called, will fall through
to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
will cause an error.
can can be called as a class (static) method, an object method, or a
function.
When used as a function, if VAL is a blessed reference or package name which
has a method called METHOD, can returns a reference to the subroutine.
If VAL is not a blessed reference, or if it does not have a method
METHOD, undef is returned.

VERSION ( [ REQUIRE ] )
VERSION will return the value of the variable $VERSION in the
package the object is blessed into. If REQUIRE is given then
it will do a comparison and die if the package version is not
greater than or equal to REQUIRE.
VERSION can be called as either a class (static) method, an object
method or a function.

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EXPORTS

None by default. You may request the import of all three functions (isa, can, and VERSION), however it isn't usually necessary to do so. Perl magically makes these functions act as methods on all objects. The one exception is isa, which is useful as a function when operating on non-blessed references.

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