1package UNIVERSAL; 2 3our $VERSION = '1.17'; 4 5# UNIVERSAL.pm should not contain any methods/subs, they 6# are all defined in universal.c 7 81; 9__END__ 10 11=head1 NAME 12 13UNIVERSAL - base class for ALL classes (blessed references) 14 15=head1 SYNOPSIS 16 17 my $obj_is_io = $fd->isa("IO::Handle"); 18 my $cls_is_io = Class->isa("IO::Handle"); 19 20 my $obj_does_log = $obj->DOES("Logger"); 21 my $cls_does_log = Class->DOES("Logger"); 22 23 my $obj_sub = $obj->can("print"); 24 my $cls_sub = Class->can("print"); 25 26 my $eval_sub = eval { $ref->can("fandango") }; 27 my $ver = $obj->VERSION; 28 29 # but never do this! 30 my $is_io = UNIVERSAL::isa($fd, "IO::Handle"); 31 my $sub = UNIVERSAL::can($obj, "print"); 32 33=head1 DESCRIPTION 34 35C<UNIVERSAL> is the base class from which all blessed references inherit. 36See L<perlobj>. 37 38C<UNIVERSAL> provides the following methods: 39 40=over 4 41 42=item C<< $obj->isa( TYPE ) >> 43 44=item C<< CLASS->isa( TYPE ) >> 45 46=item C<< eval { VAL->isa( TYPE ) } >> 47 48Where 49 50=over 4 51 52=item C<TYPE> 53 54is a package name 55 56=item C<$obj> 57 58is a blessed reference or a package name 59 60=item C<CLASS> 61 62is a package name 63 64=item C<VAL> 65 66is any of the above or an unblessed reference 67 68=back 69 70When used as an instance or class method (C<< $obj->isa( TYPE ) >>), 71C<isa> returns I<true> if $obj is blessed into package C<TYPE> or 72inherits from package C<TYPE>. 73 74When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes 75referred to as a static method), C<isa> returns I<true> if C<CLASS> 76inherits from (or is itself) the name of the package C<TYPE> or 77inherits from package C<TYPE>. 78 79If you're not sure what you have (the C<VAL> case), wrap the method call in an 80C<eval> block to catch the exception if C<VAL> is undefined or an unblessed 81reference. The L<C<isa> operator|perlop/"Class Instance Operator"> is an 82alternative that simply returns false in this case, so the C<eval> is not 83needed. 84 85If you want to be sure that you're calling C<isa> on an instance, not a class, 86check the invocant with C<blessed> from L<Scalar::Util> first: 87 88 use Scalar::Util 'blessed'; 89 90 if ( blessed( $obj ) && $obj->isa("Some::Class") ) { 91 ... 92 } 93 94=item C<< $obj->DOES( ROLE ) >> 95 96=item C<< CLASS->DOES( ROLE ) >> 97 98C<DOES> checks if the object or class performs the role C<ROLE>. A role is a 99named group of specific behavior (often methods of particular names and 100signatures), similar to a class, but not necessarily a complete class by 101itself. For example, logging or serialization may be roles. 102 103C<DOES> and C<isa> are similar, in that if either is true, you know that the 104object or class on which you call the method can perform specific behavior. 105However, C<DOES> is different from C<isa> in that it does not care I<how> the 106invocand performs the operations, merely that it does. (C<isa> of course 107mandates an inheritance relationship. Other relationships include aggregation, 108delegation, and mocking.) 109 110By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the 111role of all classes in their inheritance. In other words, by default C<DOES> 112responds identically to C<isa>. 113 114There is a relationship between roles and classes, as each class implies the 115existence of a role of the same name. There is also a relationship between 116inheritance and roles, in that a subclass that inherits from an ancestor class 117implicitly performs any roles its parent performs. Thus you can use C<DOES> in 118place of C<isa> safely, as it will return true in all places where C<isa> will 119return true (provided that any overridden C<DOES> I<and> C<isa> methods behave 120appropriately). 121 122=item C<< $obj->can( METHOD ) >> 123 124=item C<< CLASS->can( METHOD ) >> 125 126=item C<< eval { VAL->can( METHOD ) } >> 127 128C<can> checks if the object or class has a method called C<METHOD>. If it does, 129then it returns a reference to the sub. If it does not, then it returns 130I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or 131C<VAL>. 132 133C<can> cannot know whether an object will be able to provide a method through 134AUTOLOAD (unless the object's class has overridden C<can> appropriately), so a 135return value of I<undef> does not necessarily mean the object will not be able 136to handle the method call. To get around this some module authors use a forward 137declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For 138such 'dummy' subs, C<can> will still return a code reference, which, when 139called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided, 140calling the coderef will cause an error. 141 142You may call C<can> as a class (static) method or an object method. 143 144Again, the same rule about having a valid invocand applies -- use an C<eval> 145block or C<blessed> if you need to be extra paranoid. 146 147=item C<VERSION ( [ REQUIRE ] )> 148 149C<VERSION> will return the value of the variable C<$VERSION> in the 150package the object is blessed into. If C<REQUIRE> is given then 151it will do a comparison and die if the package version is not 152greater than or equal to C<REQUIRE>, or if either C<$VERSION> or C<REQUIRE> 153is not a "lax" version number (as defined by the L<version> module). 154 155The return from C<VERSION> will actually be the stringified version object 156using the package C<$VERSION> scalar, which is guaranteed to be equivalent 157but may not be precisely the contents of the C<$VERSION> scalar. If you want 158the actual contents of C<$VERSION>, use C<$CLASS::VERSION> instead. 159 160C<VERSION> can be called as either a class (static) method or an object 161method. 162 163=back 164 165=head1 WARNINGS 166 167B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and 168C<isa> uses a very similar method and cache-ing strategy. This may cause 169strange effects if the Perl code dynamically changes @ISA in any package. 170 171You may add other methods to the UNIVERSAL class via Perl or XS code. 172You do not need to C<use UNIVERSAL> to make these methods 173available to your program (and you should not do so). 174 175=head1 EXPORTS 176 177None. 178 179Previous versions of this documentation suggested using C<isa> as 180a function to determine the type of a reference: 181 182 $yes = UNIVERSAL::isa($h, "HASH"); 183 $yes = UNIVERSAL::isa("Foo", "Bar"); 184 185The problem is that this code would I<never> call an overridden C<isa> method in 186any class. Instead, use C<reftype> from L<Scalar::Util> for the first case: 187 188 use Scalar::Util 'reftype'; 189 190 $yes = reftype( $h ) eq "HASH"; 191 192and the method form of C<isa> for the second: 193 194 $yes = Foo->isa("Bar"); 195 196=cut 197