UNIVERSAL::isa(3) User Contributed Perl Documentation UNIVERSAL::isa(3)NAME
UNIVERSAL::isa - recover from people calling UNIVERSAL::isa as a function
SYNOPSIS
# from the shell
echo 'export PERL5OPT=-MUNIVERSAL::isa' >> /etc/profile
# within your program
use UNIVERSAL::isa;
# enable warnings for all dodgy uses of UNIVERSAL::isa
use UNIVERSAL::isa 'verbose';
DESCRIPTION
Whenever you use "isa" in UNIVERSAL as a function, a kitten using Test::MockObject dies. Normally, the kittens would be helpless, but if
they use UNIVERSAL::isa (the module whose docs you are reading), the kittens can live long and prosper.
This module replaces "UNIVERSAL::isa" with a version that makes sure that, when called as a function on objects which override "isa", "isa"
will call the appropriate method on those objects
In all other cases, the real "UNIVERSAL::isa" gets called directly.
NOTE: You should use this module only for debugging purposes. It does not belong as a dependency in running code.
WARNINGS
If the lexical warnings pragma is available, this module will emit a warning for each naughty invocation of "UNIVERSAL::isa". Silence these
warnings by saying:
no warnings 'UNIVERSAL::isa';
in the lexical scope of the naughty code.
After version 1.00, warnings only appear when naughty code calls UNIVERSAL::isa() as a function on an invocant for which there is an
overridden isa(). These are really truly active bugs, and you should fix them rather than relying on this module to find them.
To get warnings for all potentially dangerous uses of UNIVERSAL::isa() as a function, not a method (that is, for all uses of the method as
a function, which are latent bugs, if not bugs that will break your code as it exists now), pass the "verbose" flag when using the module.
This can generate many extra warnings, but they're more specific as to the actual wrong practice and they usually suggest proper fixes.
SEE ALSO
Perl::Critic::Policy::BuiltinFunctions::ProhibitUniversalIsa
UNIVERSAL::can for another discussion of the problem at hand.
Test::MockObject for one example of a module that really needs to override "isa()".
Any decent explanation of OO to understand why calling methods as functions is a staggeringly bad idea.
AUTHORS
Audrey Tang <cpan@audreyt.org>
chromatic <chromatic@wgz.org>
Yuval Kogman <nothingmuch@woobling.org>
COPYRIGHT &; LICENSE
Copyright (c) 2005 - 2011, chromatic. This module is made available under the same terms as Perl 5.12.
perl v5.16.3 2014-06-09 UNIVERSAL::isa(3)
Check Out this Related Man Page
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
$obj
is a blessed reference or a package name
"CLASS"
is a package name
"VAL"
is any of the above or an unblessed reference
When used as an instance or class method ("$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". Both $VERSION or "REQUIRE" must be "lax"
version numbers (as defined by the version module) or "VERSION" will die with an error.
"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"), 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");
perl v5.14.2 2011-09-26 UNIVERSAL(3perl)