Nicholas Clark > perl-5.8.3 > UNIVERSAL

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Module Version: 1.01   Source   Latest Release: perl-5.21.6

NAME ^

UNIVERSAL - base class for ALL classes (blessed references)

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 ;

DESCRIPTION ^

UNIVERSAL is the base class which all bless references will inherit from, see perlobj.

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 perlsub) 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.

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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