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UNIVERSAL(3perl) Perl Programmers Reference Guide UNIVERSAL(3perl)
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
$obj
is a blessed reference or a string containing a package name
"CLASS"
is a package name
"VAL"
is any of the above or an unblessed reference
When used as an instance or class method ("$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->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. To mark that your own
classes perform other roles, override "DOES" appropriately.
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.
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:
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");
perl v5.10.0 2011-06-30 UNIVERSAL(3perl)
Generated by $Id: phpMan.php,v 4.49 2006/02/26 13:18:18 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
2012-05-25 04:22 @38.107.179.239 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)