package UNIVERSAL;
-our $VERSION = '1.02';
+our $VERSION = '1.04';
# UNIVERSAL should not contain any extra subs/methods beyond those
# that it exists to define. The use of Exporter below is a historical
# accident that can't be fixed without breaking code. Note that we
-# *don't* set @ISA here, don't want all classes/objects inheriting from
+# *don't* set @ISA here, as we don't want all classes/objects inheriting from
# Exporter. It's bad enough that all classes have a import() method
# whenever UNIVERSAL.pm is loaded.
require Exporter;
@EXPORT_OK = qw(isa can VERSION);
# Make sure that even though the import method is called, it doesn't do
-# anything unless its called on UNIVERSAL
+# anything unless called on UNIVERSAL.
sub import {
return unless $_[0] eq __PACKAGE__;
goto &Exporter::import;
=head1 SYNOPSIS
- $is_io = $fd->isa("IO::Handle");
- $is_io = Class->isa("IO::Handle");
+ $is_io = $fd->isa("IO::Handle");
+ $is_io = Class->isa("IO::Handle");
- $sub = $obj->can("print");
- $sub = Class->can("print");
+ $does_log = $obj->DOES("Logger");
+ $does_log = Class->DOES("Logger");
- use UNIVERSAL qw( isa can VERSION );
- $yes = isa $ref, "HASH" ;
- $sub = can $ref, "fandango" ;
- $ver = VERSION $obj ;
+ $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");
=head1 DESCRIPTION
-C<UNIVERSAL> is the base class which all bless references will inherit from,
+C<UNIVERSAL> is the base class from which all blessed references inherit.
-C<UNIVERSAL> provides the following methods and functions:
+C<UNIVERSAL> provides the following methods:
=over 4
=item C<< $obj->isa( TYPE ) >>
-=item C<< CLASS->isa( TYPE ) >>
+=item C<< CLASS->isa( TYPE ) >>
-=item C<isa( VAL, TYPE )>
+=item C<< eval { VAL->isa( TYPE ) } >>
Where
C<isa> returns I<true> if $obj is blessed into package C<TYPE> or
inherits from package C<TYPE>.
-When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes
+When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes
referred to as a static method), C<isa> returns I<true> if C<CLASS>
inherits from (or is itself) the name of the package C<TYPE> or
inherits from package C<TYPE>.
-When used as a function, like
+If you're not sure what you have (the C<VAL> case), wrap the method call in an
+C<eval> block to catch the exception if C<VAL> is undefined.
+
+If you want to be sure that you're calling C<isa> as a method, not a class,
+check the invocant with C<blessed> from L<Scalar::Util> first:
+
+ use Scalar::Util 'blessed';
- use UNIVERSAL qw( isa ) ;
- $yes = isa $h, "HASH";
- $yes = isa "Foo", "Bar";
+ if ( blessed( $obj ) && $obj->isa("Some::Class") {
+ ...
+ }
-or
+=item C<< $obj->DOES( ROLE ) >>
- require UNIVERSAL ;
- $yes = UNIVERSAL::isa $a, "ARRAY";
+=item C<< CLASS->DOES( ROLE ) >>
-C<isa> returns I<true> in the same cases as above and also if C<VAL> is an
-unblessed reference to a perl variable of type C<TYPE>, such as "HASH",
-"ARRAY", or "Regexp".
+C<DOES> checks if the object or class performs the role C<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.
+
+C<DOES> and C<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, C<DOES> is different from C<isa> in that it does not care I<how> the
+invocant performs the operations, merely that it does. (C<isa> of course
+mandates an inheritance relationship. Other relationships include aggregation,
+delegation, and mocking.)
+
+By default, classes in Perl only perform the C<UNIVERSAL> role. To mark that
+your own classes perform other roles, override C<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 C<DOES> in
+place of C<isa> safely, as it will return true in all places where C<isa> will
+return true (provided that any overridden C<DOES> I<and> C<isa> methods behave
+appropriately).
=item C<< $obj->can( METHOD ) >>
=item C<< CLASS->can( METHOD ) >>
-=item C<can( VAL, METHOD )>
+=item C<< eval { VAL->can( METHOD ) } >>
-C<can> checks if the object or class has a method called C<METHOD>. If it does
-then a reference to the sub is returned. If it does not then I<undef> is
-returned. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
+C<can> checks if the object or class has a method called C<METHOD>. If it does,
+then it returns a reference to the sub. If it does not, then it returns
+I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
C<VAL>.
-C<can> cannot know whether an object will be able to provide a method
-through AUTOLOAD, so a return value of I<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 L<perlsub>)
-for methods they will handle via AUTOLOAD. For such 'dummy' subs, C<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.
+C<can> cannot know whether an object will be able to provide a method through
+AUTOLOAD (unless the object's class has overriden C<can> appropriately), so a
+return value of I<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 L<perlsub>) for methods they will handle via AUTOLOAD. For
+such 'dummy' subs, C<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.
-C<can> can be called as a class (static) method, an object method, or a
-function.
+You may call C<can> as a class (static) method or an object method.
-When used as a function, if C<VAL> is a blessed reference or package name which
-has a method called C<METHOD>, C<can> returns a reference to the subroutine.
-If C<VAL> is not a blessed reference, or if it does not have a method
-C<METHOD>, I<undef> is returned.
+Again, the same rule about having a valid invocant applies -- use an C<eval>
+block or C<blessed> if you need to be extra paranoid.
=item C<VERSION ( [ REQUIRE ] )>
it will do a comparison and die if the package version is not
greater than or equal to C<REQUIRE>.
-C<VERSION> can be called as either a class (static) method, an object
-method or a function.
-
+C<VERSION> can be called as either a class (static) method or an object
+method.
=back
None by default.
-You may request the import of all three functions (C<isa>, C<can>, and
-C<VERSION>), however it isn't usually necessary to do so. Perl magically
-makes these functions act as methods on all objects. The one exception is
-C<isa>, which is useful as a function when operating on non-blessed
-references.
+You may request the import of three functions (C<isa>, C<can>, and C<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 C<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 I<never> call an overridden C<isa> method in
+any class. Instead, use C<reftype> from L<Scalar::Util> for the first case:
+
+ use Scalar::Util 'reftype';
+
+ $yes = reftype( $h ) eq "HASH";
+
+and the method form of C<isa> for the second:
+
+ $yes = Foo->isa("Bar");
=cut
https://perl5.git.perl.org / perl5.git / blobdiff