UNIVERSAL - base class for ALL classes (blessed references)
$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");
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 invocand with blessed
from Scalar::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 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 invocand 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 overridden 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 invocand 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
, or if either $VERSION
or REQUIRE
is not a "lax" version number (as defined by the version module).
The return from VERSION
will actually be the stringified version object using the package $VERSION
scalar, which is guaranteed to be equivalent but may not be precisely the contents of the $VERSION
scalar. If you want the actual contents of $VERSION
, use $CLASS::VERSION
instead.
VERSION
can be called as either a class (static) method or an object method.
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).
None.
Previous versions of this documentation suggested using isa
as a function to determine the type of a reference:
$yes = UNIVERSAL::isa($h, "HASH");
$yes = UNIVERSAL::isa("Foo", "Bar");
The problem is that this code would 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");