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UNIVERSAL.pm
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package UNIVERSAL; our $VERSION = '1.13'; # 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 $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. 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
