X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/96f1132bd42b8b1255f98b7e95b1ed46ae0eff5c..042560a65fd56038b3116f30639cb99d98c48622:/lib/UNIVERSAL.pm diff --git a/lib/UNIVERSAL.pm b/lib/UNIVERSAL.pm index f2f1fe9..0d6fbe0 100644 --- a/lib/UNIVERSAL.pm +++ b/lib/UNIVERSAL.pm @@ -1,11 +1,22 @@ package UNIVERSAL; +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 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 +27,168 @@ 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"); + + $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; - $yes = UNIVERSAL::isa($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 + +=item C -C can be called as either a static or object method call. +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->DOES( ROLE ) >> -=item * +=item C<< CLASS->DOES( ROLE ) >> -C is a reference to a C of Perl variable (e.g. 'HASH'). +C checks if the object or class performs the role C. 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. -=item * +C and C 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 is different from C in that it does not care I the +invocant performs the operations, merely that it does. (C of course +mandates an inheritance relationship. Other relationships include aggregation, +delegation, and mocking.) -C is the name of a package that inherits from (or is itself) -package C. +By default, classes in Perl only perform the C role. To mark that +your own classes perform other roles, override C appropriately. -=back +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 in +place of C safely, as it will return true in all places where C will +return true (provided that any overridden C I C methods behave +appropriately). + +=item C<< $obj->can( METHOD ) >> + +=item C<< CLASS->can( METHOD ) >> -=item UNIVERSAL::can ( VAL, METHOD ) +=item C<< eval { VAL->can( METHOD ) } >> -If C is a blessed reference which has a method called C, -C returns a reference to the subroutine. If C is not -a blessed reference, or if it does not have a method C, -I is returned. +C checks if the object or class has a method called C. If it does, +then it returns a reference to the sub. If it does not, then it returns +I. This includes methods inherited or imported by C<$obj>, C, or +C. + +C 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. + +You may call C as a class (static) method or an object method. + +Again, the same rule about having a valid invocant applies -- use an C +block or C if you need to be extra paranoid. + +=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 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