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. 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 )),isareturns true if $obj is blessed into packageTYPEor inherits from packageTYPE.When used as a class method (
CLASS->isa( TYPE ), sometimes referred to as a static method),isareturns true ifCLASSinherits from (or is itself) the name of the packageTYPEor inherits from packageTYPE.If you're not sure what you have (the
VALcase), wrap the method call in anevalblock to catch the exception ifVALis undefined.If you want to be sure that you're calling
isaas a method, not a class, check the invocand withblessedfrom Scalar::Util first:use Scalar::Util 'blessed'; if ( blessed( $obj ) && $obj->isa("Some::Class") { ... } $obj->DOES( ROLE )CLASS->DOES( ROLE )-
DOESchecks 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.DOESandisaare 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,DOESis different fromisain that it does not care how the invocand performs the operations, merely that it does. (isaof course mandates an inheritance relationship. Other relationships include aggregation, delegation, and mocking.)By default, classes in Perl only perform the
UNIVERSALrole, as well as the role of all classes in their inheritance. In other words, by defaultDOESresponds 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 use
DOESin place ofisasafely, as it will return true in all places whereisawill return true (provided that any overriddenDOESandisamethods behave appropriately). $obj->can( METHOD )CLASS->can( METHOD )eval { VAL->can( METHOD ) }-
canchecks if the object or class has a method calledMETHOD. If it does, then it returns a reference to the sub. If it does not, then it returns undef. This includes methods inherited or imported by$obj,CLASS, orVAL.cancannot know whether an object will be able to provide a method through AUTOLOAD (unless the object's class has overriddencanappropriately), 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,canwill 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
canas a class (static) method or an object method.Again, the same rule about having a valid invocand applies -- use an
evalblock orblessedif you need to be extra paranoid. VERSION ( [ REQUIRE ] )-
VERSIONwill return the value of the variable$VERSIONin the package the object is blessed into. IfREQUIREis given then it will do a comparison and die if the package version is not greater than or equal toREQUIRE. Both$VERSIONorREQUIREmust be "lax" version numbers (as defined by the version module) orVERSIONwill die with an error.VERSIONcan 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), but this feature is deprecated and will be removed. 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");