Rafaël Garcia-Suarez > perl-5.9.3 > UNIVERSAL

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Module Version: 1.03   Source   Latest Release: perl-5.21.11

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");

    $sub   = eval { $ref->can("fandango") };
    $ver   = $obj->VERSION;

    # but never do this!
    $is_io = UNIVERSAL::isa($fd, "IO::Handle");
    $sub   = UNIVERSAL::can($obj, "print");

DESCRIPTION ^

UNIVERSAL is the base class from which all blessed references inherit. See perlobj.

UNIVERSAL provides the following methods:

$obj->isa( TYPE )
CLASS->isa( TYPE )
eval { VAL->isa( 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.

If you're not sure what you have (the VAL case), wrap the method call in an eval block to catch the exception if VAL is undefined.

If you want to be sure that you're calling isa as a method, not a class, check the invocant with blessed from Scalar::Util first:

  use Scalar::Util 'blessed';

  if ( blessed( $obj ) && $obj->isa("Some::Class") {
      ...
  }
$obj->can( METHOD )
CLASS->can( METHOD )
eval { VAL->can( METHOD ) }

can checks if the object or class has a method called METHOD. If it does, then it returns a reference to the sub. If it does not, then it returns undef. 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 (unless the object's class has overriden can appropriately), 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.

You may call can as a class (static) method or an object method.

Again, the same rule about having a valid invocant applies -- use an eval block or blessed if you need to be extra paranoid.

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 or an object method.

EXPORTS ^

None by default.

You may request the import of all three functions (isa, can, and VERSION), however it is usually harmful to do so. Please don't do this in new code.

For example, previous versions of this documentation suggested using isa as a function to determine the type of a reference:

  use UNIVERSAL 'isa';

  $yes = isa $h, "HASH";
  $yes = isa "Foo", "Bar";

The problem is that this code will never call an overridden isa method in any class. Instead, use reftype from Scalar::Util for the first case:

  use Scalar::Util 'reftype';

  $yes = reftype( $h ) eq "HASH";

and the method form of isa for the second:

  $yes = Foo->isa("Bar");
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