207 lines
6.7 KiB
Perl
207 lines
6.7 KiB
Perl
package UNIVERSAL;
|
|
|
|
our $VERSION = '1.15';
|
|
|
|
# UNIVERSAL should not contain any extra subs/methods beyond those
|
|
# that it exists to define. The existence of import() below is a historical
|
|
# accident that can't be fixed without breaking code.
|
|
|
|
# Make sure that even though the import method is called, it doesn't do
|
|
# anything unless called on UNIVERSAL.
|
|
sub import {
|
|
return unless $_[0] eq __PACKAGE__;
|
|
return unless @_ > 1;
|
|
require Carp;
|
|
Carp::croak("UNIVERSAL does not export anything");
|
|
}
|
|
|
|
1;
|
|
__END__
|
|
|
|
=head1 NAME
|
|
|
|
UNIVERSAL - base class for ALL classes (blessed references)
|
|
|
|
=head1 SYNOPSIS
|
|
|
|
my $obj_is_io = $fd->isa("IO::Handle");
|
|
my $cls_is_io = Class->isa("IO::Handle");
|
|
|
|
my $obj_does_log = $obj->DOES("Logger");
|
|
my $cls_does_log = Class->DOES("Logger");
|
|
|
|
my $obj_sub = $obj->can("print");
|
|
my $cls_sub = Class->can("print");
|
|
|
|
my $eval_sub = eval { $ref->can("fandango") };
|
|
my $ver = $obj->VERSION;
|
|
|
|
# but never do this!
|
|
my $is_io = UNIVERSAL::isa($fd, "IO::Handle");
|
|
my $sub = UNIVERSAL::can($obj, "print");
|
|
|
|
=head1 DESCRIPTION
|
|
|
|
C<UNIVERSAL> is the base class from which all blessed references inherit.
|
|
See L<perlobj>.
|
|
|
|
C<UNIVERSAL> provides the following methods:
|
|
|
|
=over 4
|
|
|
|
=item C<< $obj->isa( TYPE ) >>
|
|
|
|
=item C<< CLASS->isa( TYPE ) >>
|
|
|
|
=item C<< eval { VAL->isa( TYPE ) } >>
|
|
|
|
Where
|
|
|
|
=over 4
|
|
|
|
=item C<TYPE>
|
|
|
|
is a package name
|
|
|
|
=item C<$obj>
|
|
|
|
is a blessed reference or a package name
|
|
|
|
=item C<CLASS>
|
|
|
|
is a package name
|
|
|
|
=item C<VAL>
|
|
|
|
is any of the above or an unblessed reference
|
|
|
|
=back
|
|
|
|
When used as an instance or class method (C<< $obj->isa( TYPE ) >>),
|
|
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
|
|
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>.
|
|
|
|
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 or an unblessed
|
|
reference. The L<C<isa> operator|perlop/"Class Instance Operator"> is an
|
|
alternative that simply returns false in this case, so the C<eval> is not
|
|
needed.
|
|
|
|
If you want to be sure that you're calling C<isa> as a method, not a class,
|
|
check the invocand with C<blessed> from L<Scalar::Util> first:
|
|
|
|
use Scalar::Util 'blessed';
|
|
|
|
if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
|
|
...
|
|
}
|
|
|
|
=item C<< $obj->DOES( ROLE ) >>
|
|
|
|
=item C<< CLASS->DOES( ROLE ) >>
|
|
|
|
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
|
|
invocand 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, as well as the
|
|
role of all classes in their inheritance. In other words, by default C<DOES>
|
|
responds identically to C<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 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<< $obj->can( METHOD ) >>
|
|
|
|
=item C<< CLASS->can( METHOD ) >>
|
|
|
|
=item C<< eval { VAL->can( METHOD ) } >>
|
|
|
|
C<can> checks if the object or class has a method called C<METHOD>. If it does,
|
|
then it returns a reference to the sub. If it does not, then it returns
|
|
I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or
|
|
C<VAL>.
|
|
|
|
C<can> cannot know whether an object will be able to provide a method through
|
|
AUTOLOAD (unless the object's class has overridden 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.
|
|
|
|
You may call C<can> as a class (static) method or an object method.
|
|
|
|
Again, the same rule about having a valid invocand applies -- use an C<eval>
|
|
block or C<blessed> if you need to be extra paranoid.
|
|
|
|
=item C<VERSION ( [ REQUIRE ] )>
|
|
|
|
C<VERSION> will return the value of the variable C<$VERSION> in the
|
|
package the object is blessed into. If C<REQUIRE> is given then
|
|
it will do a comparison and die if the package version is not
|
|
greater than or equal to C<REQUIRE>, or if either C<$VERSION> or C<REQUIRE>
|
|
is not a "lax" version number (as defined by the L<version> module).
|
|
|
|
The return from C<VERSION> will actually be the stringified version object
|
|
using the package C<$VERSION> scalar, which is guaranteed to be equivalent
|
|
but may not be precisely the contents of the C<$VERSION> scalar. If you want
|
|
the actual contents of C<$VERSION>, use C<$CLASS::VERSION> instead.
|
|
|
|
C<VERSION> can be called as either a class (static) method or an object
|
|
method.
|
|
|
|
=back
|
|
|
|
=head1 WARNINGS
|
|
|
|
B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
|
|
C<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 C<use UNIVERSAL> to make these methods
|
|
available to your program (and you should not do so).
|
|
|
|
=head1 EXPORTS
|
|
|
|
None.
|
|
|
|
Previous versions of this documentation suggested using C<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 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
|