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UNIVERSAL

Perl 5 version 10.1 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.     $does_log = $obj->DOES("Logger");
  5.     $does_log = Class->DOES("Logger");
  7.     $sub      = $obj->can("print");
  8.     $sub      = Class->can("print");
  10.     $sub      = eval { $ref->can("fandango") };
  11.     $ver      = $obj->VERSION;
  13.     # but never do this!
  14.     $is_io    = UNIVERSAL::isa($fd, "IO::Handle");
  15.     $sub      = UNIVERSAL::can($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 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:

    1.   use Scalar::Util 'blessed';
    3.   if ( blessed( $obj ) && $obj->isa("Some::Class") {
    4.       ...
    5.   }
  • $obj->DOES( ROLE )
  • CLASS->DOES( ROLE )

    DOES checks if the object or class performs the role ROLE . A role is a named group of specific behavior (often methods of particular names and signatures), similar to a class, but not necessarily a complete class by itself. For example, logging or serialization may be roles.

    DOES and isa are similar, in that if either is true, you know that the object or class on which you call the method can perform specific behavior. However, DOES is different from isa in that it does not care how the invocant performs the operations, merely that it does. (isa of course mandates an inheritance relationship. Other relationships include aggregation, delegation, and mocking.)

    By default, classes in Perl only perform the UNIVERSAL role, as well as the role of all classes in their inheritance. In other words, by default DOES responds identically to isa .

    There is a relationship between roles and classes, as each class implies the existence of a role of the same name. There is also a relationship between inheritance and roles, in that a subclass that inherits from an ancestor class implicitly performs any roles its parent performs. Thus you can use DOES in place of isa safely, as it will return true in all places where isa will return true (provided that any overridden DOES and isa methods behave appropriately).

  • $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.

WARNINGS

NOTE: can directly uses Perl's internal code for method lookup, and isa uses a very similar method and cache-ing strategy. This may cause strange effects if the Perl code dynamically changes @ISA in any package.

You may add other methods to the UNIVERSAL class via Perl or XS code. You do not need to use UNIVERSAL to make these methods available to your program (and you should not do so).

EXPORTS

None by default.

You may request the import of 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:

  1.   use UNIVERSAL 'isa';
  3.   $yes = isa $h, "HASH";
  4.   $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:

  1.   use Scalar::Util 'reftype';
  3.   $yes = reftype( $h ) eq "HASH";

and the method form of isa for the second:

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