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UNIVERSAL

Perl 5 version 8.8 documentation
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UNIVERSAL

NAME

UNIVERSAL - base class for ALL classes (blessed references)

SYNOPSIS

  1.     $is_io = $fd->isa("IO::Handle");
  2.     $is_io = Class->isa("IO::Handle");
  4.     $sub = $obj->can("print");
  5.     $sub = Class->can("print");
  7.     use UNIVERSAL qw( isa can VERSION );
  8.     $yes = isa $ref, "HASH" ;
  9.     $sub = can $ref, "fandango" ;
  10.     $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

    1.    use UNIVERSAL qw( isa ) ;
    2.    $yes = isa $h, "HASH";
    3.    $yes = isa "Foo", "Bar";

    or

    1.    require UNIVERSAL ;
    2.    $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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