From: chromatic Date: Thu, 22 Dec 2005 20:58:00 +0000 (+0000) Subject: [REPATCH lib/UNIVERSAL.pm] Recommend Against Using Methods as Functions X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=71ccbdc29a66a19ac381608f56299d7a8cb4a655;p=p5sagit%2Fp5-mst-13.2.git [REPATCH lib/UNIVERSAL.pm] Recommend Against Using Methods as Functions Message-Id: <200512221158.54474.chromatic@wgz.org> p4raw-id: //depot/perl@26466 --- diff --git a/lib/UNIVERSAL.pm b/lib/UNIVERSAL.pm index 8808271..eee8306 100644 --- a/lib/UNIVERSAL.pm +++ b/lib/UNIVERSAL.pm @@ -1,18 +1,18 @@ package UNIVERSAL; -our $VERSION = '1.02'; +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 can't be fixed without breaking code. Note that we -# *don't* set @ISA here, don't want all classes/objects inheriting from +# *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 its called on UNIVERSAL +# anything unless called on UNIVERSAL. sub import { return unless $_[0] eq __PACKAGE__; goto &Exporter::import; @@ -30,28 +30,30 @@ UNIVERSAL - base class for ALL classes (blessed references) $is_io = $fd->isa("IO::Handle"); $is_io = Class->isa("IO::Handle"); - $sub = $obj->can("print"); - $sub = Class->can("print"); + $sub = $obj->can("print"); + $sub = Class->can("print"); - use UNIVERSAL qw( isa can VERSION ); - $yes = isa $ref, "HASH" ; - $sub = can $ref, "fandango" ; - $ver = VERSION $obj ; + $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 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 and functions: +C provides the following methods: =over 4 =item C<< $obj->isa( TYPE ) >> -=item C<< CLASS->isa( TYPE ) >> +=item C<< CLASS->isa( TYPE ) >> -=item C +=item C<< eval { VAL->isa( TYPE ) } >> Where @@ -79,53 +81,47 @@ 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. -When used as a class method (C<< CLASS->isa( TYPE ) >>: sometimes +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. -When used as a function, like - - use UNIVERSAL qw( isa ) ; - $yes = isa $h, "HASH"; - $yes = isa "Foo", "Bar"; +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. -or +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: - require UNIVERSAL ; - $yes = UNIVERSAL::isa $a, "ARRAY"; + use Scalar::Util 'blessed'; -C returns I in the same cases as above and also if C is an -unblessed reference to a perl variable of type C, such as "HASH", -"ARRAY", or "Regexp". + if ( blessed( $obj ) && $obj->isa("Some::Class") { + ... + } =item C<< $obj->can( METHOD ) >> =item C<< CLASS->can( METHOD ) >> -=item C +=item C<< eval { VAL->can( METHOD ) } >> -C checks if the object or class has a method called C. If it does -then a reference to the sub is returned. If it does not then I is -returned. This includes methods inherited or imported by C<$obj>, C, or +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, 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. +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. -C can be called as a class (static) method, an object method, or a -function. +You may call C as a class (static) method or an object method. -When used as a function, if C is a blessed reference or package name 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. +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 @@ -134,9 +130,8 @@ 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, an object -method or a function. - +C can be called as either a class (static) method or an object +method. =back @@ -145,9 +140,26 @@ method or a function. None by default. You may request the import of all three functions (C, C, and -C), however it isn't usually necessary to do so. Perl magically -makes these functions act as methods on all objects. The one exception is -C, which is useful as a function when operating on non-blessed -references. +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"; + +and the method form of C for the second: + + $yes = Foo->isa("Bar"); =cut