X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/1761cee512762c09b2a848d3c6cbd5a3b4232ffa..5dfe062f6f5bff2cd3d8cf5de68c1a69d6de6f1d:/lib/UNIVERSAL.pm diff --git a/lib/UNIVERSAL.pm b/lib/UNIVERSAL.pm index f2f1fe9..eee8306 100644 --- a/lib/UNIVERSAL.pm +++ b/lib/UNIVERSAL.pm @@ -1,11 +1,22 @@ package UNIVERSAL; +our $VERSION = '1.03'; + # 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 should be fixed sometime. +# 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; -*import = \&Exporter::import; -@EXPORT_OK = qw(isa can); +@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__; + goto &Exporter::import; +} 1; __END__ @@ -16,86 +27,139 @@ UNIVERSAL - base class for ALL classes (blessed references) =head1 SYNOPSIS - $io = $fd->isa("IO::Handle"); - $sub = $obj->can('print'); + $is_io = $fd->isa("IO::Handle"); + $is_io = Class->isa("IO::Handle"); + + $sub = $obj->can("print"); + $sub = Class->can("print"); + + $sub = eval { $ref->can("fandango") }; + $ver = $obj->VERSION; - $yes = UNIVERSAL::isa($ref, "HASH"); + # but never do this! + $is_io = UNIVERSAL::isa($fd, "IO::Handle"); + $sub = UNIVERSAL::can($obj, "print"); =head1 DESCRIPTION -C is the base class which all bless references will inherit from, -see L +C is the base class from which all blessed references inherit. +See L. -C provides the following methods +C provides the following methods: =over 4 -=item isa ( TYPE ) +=item C<< $obj->isa( TYPE ) >> -C returns I if C is blessed into package C -or inherits from package C. +=item C<< CLASS->isa( TYPE ) >> -C can be called as either a static or object method call. +=item C<< eval { VAL->isa( TYPE ) } >> -=item can ( METHOD ) +Where -C checks if the object has a method called C. If it does -then a reference to the sub is returned. If it does not then I -is returned. +=over 4 -C can be called as either a static or object method call. +=item C -=item VERSION ( [ REQUIRE ] ) +is a package name -C will return the value of the variable C<$VERSION> in the -package the object is blessed into. If C is given then -it will do a comparison and die if the package version is not -greater than or equal to C. +=item C<$obj> + +is a blessed reference or a string containing a package name + +=item C + +is a package name -C can be called as either a static or object method call. +=item C + +is any of the above or an unblessed reference =back -The C and C methods can also be called as subroutines +When used as an instance or class method (C<< $obj->isa( TYPE ) >>), +C returns I if $obj is blessed into package C or +inherits from package C. -=over 4 +When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes +referred to as a static method), C returns I if C +inherits from (or is itself) the name of the package C or +inherits from package C. -=item UNIVERSAL::isa ( VAL, TYPE ) +If you're not sure what you have (the C case), wrap the method call in an +C block to catch the exception if C is undefined. -C returns I if one of the following statements is true. +If you want to be sure that you're calling C as a method, not a class, +check the invocant with C from L first: -=over 8 + use Scalar::Util 'blessed'; -=item * + if ( blessed( $obj ) && $obj->isa("Some::Class") { + ... + } -C is a reference blessed into either package C or a package -which inherits from package C. +=item C<< $obj->can( METHOD ) >> -=item * +=item C<< CLASS->can( METHOD ) >> -C is a reference to a C of Perl variable (e.g. 'HASH'). +=item C<< eval { VAL->can( METHOD ) } >> -=item * +C checks if the object or class has a method called C. If it does, +then it returns a reference to the sub. If it does not, then it returns +I. This includes methods inherited or imported by C<$obj>, C, or +C. -C is the name of a package that inherits from (or is itself) -package C. +C cannot know whether an object will be able to provide a method through +AUTOLOAD (unless the object's class has overriden C appropriately), so a +return value of I 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) for methods they will handle via AUTOLOAD. For +such 'dummy' subs, C 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. -=back +You may call C as a class (static) method or an object method. -=item UNIVERSAL::can ( VAL, METHOD ) +Again, the same rule about having a valid invocant applies -- use an C +block or C if you need to be extra paranoid. -If C is a blessed reference which has a method called C, -C returns a reference to the subroutine. If C is not -a blessed reference, or if it does not have a method C, -I is returned. +=item C + +C will return the value of the variable C<$VERSION> in the +package the object is blessed into. If C is given then +it will do a comparison and die if the package version is not +greater than or equal to C. + +C can be called as either a class (static) method or an object +method. =back -These subroutines should I be imported via S>. -If you want simple local access to them you can do +=head1 EXPORTS + +None by default. + +You may request the import of all three functions (C, C, and +C), 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 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 call an overridden C method in +any class. Instead, use C from L for the first case: + + use Scalar::Util 'reftype'; + + $yes = reftype( $h ) eq "HASH"; - *isa = \&UNIVERSAL::isa; +and the method form of C for the second: -to import isa into your package. + $yes = Foo->isa("Bar"); =cut