| package UNIVERSAL; |
| |
| our $VERSION = '1.11'; |
| |
| # 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, 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 called on UNIVERSAL. |
| sub import { |
| return unless $_[0] eq __PACKAGE__; |
| return unless @_ > 1; |
| require warnings; |
| warnings::warnif( |
| 'deprecated', |
| 'UNIVERSAL->import is deprecated and will be removed in a future perl', |
| ); |
| goto &Exporter::import; |
| } |
| |
| 1; |
| __END__ |
| |
| =head1 NAME |
| |
| UNIVERSAL - base class for ALL classes (blessed references) |
| |
| =head1 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"); |
| |
| =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. |
| |
| 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 by default. |
| |
| You may request the import of three functions (C<isa>, C<can>, and C<VERSION>), |
| B<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 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 |