CONTENTS

• NAME
• SYNOPSIS
• DESCRIPTION
• WARNINGS
• EXPORTS

#NAME

UNIVERSAL - base class for ALL classes (blessed references)

#SYNOPSIS

$is_io    = $fd->isa("IO::Handle");$is_io    = Class->isa("IO::Handle");$does_log = $obj->DOES("Logger");$does_log = Class->DOES("Logger");$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. Seeperlobj.

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 returnstrue if $obj is blessed into packageTYPE or inherits from packageTYPE.

When used as a class method (CLASS->isa( TYPE ), sometimes referred to as a static method),isa returnstrue ifCLASS inherits from (or is itself) the name of the packageTYPE or inherits from packageTYPE.

If you're not sure what you have (theVAL case), wrap the method call in aneval block to catch the exception ifVAL is undefined.

If you want to be sure that you're callingisa as a method, not a class, check the invocant withblessed fromScalar::Util first:

use Scalar::Util 'blessed';if ( blessed( $obj ) && $obj->isa("Some::Class") {    ...}

#$obj->DOES( ROLE )

#CLASS->DOES( ROLE )

DOES checks if the object or class performs the roleROLE. 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 andisa 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 fromisa in that it does not carehow 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 theUNIVERSAL role, as well as the role of all classes in their inheritance. In other words, by defaultDOES responds identically toisa.

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 useDOES in place ofisa safely, as it will return true in all places whereisa will return true (provided that any overriddenDOESandisa methods behave appropriately).

#$obj->can( METHOD )

#CLASS->can( METHOD )

#eval { VAL->can( METHOD ) }

can checks if the object or class has a method calledMETHOD. If it does, then it returns a reference to the sub. If it does not, then it returnsundef. This includes methods inherited or imported by$obj,CLASS, orVAL.

can cannot know whether an object will be able to provide a method through AUTOLOAD (unless the object's class has overridencan appropriately), so a return value ofundef 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 (seeperlsub) 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 callcan as a class (static) method or an object method.

Again, the same rule about having a valid invocant applies -- use aneval block orblessed 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. IfREQUIRE is given then it will do a comparison and die if the package version is not greater than or equal toREQUIRE.

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, andisa 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 touse 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, andVERSION), however it is usually harmful to do so. Please don't do this in new code.

For example, previous versions of this documentation suggested usingisa 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 willnever call an overriddenisa method in any class. Instead, usereftype fromScalar::Util for the first case:

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

and the method form ofisa for the second:

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

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