1package UNIVERSAL; 2 3our $VERSION = '1.14'; 4 5# UNIVERSAL should not contain any extra subs/methods beyond those 6# that it exists to define. The existence of import() below is a historical 7# accident that can't be fixed without breaking code. 8 9# Make sure that even though the import method is called, it doesn't do 10# anything unless called on UNIVERSAL. 11sub import { 12 return unless $_[0] eq __PACKAGE__; 13 return unless @_ > 1; 14 require Carp; 15 Carp::croak("UNIVERSAL does not export anything"); 16} 17 181; 19__END__ 20 21=head1 NAME 22 23UNIVERSAL - base class for ALL classes (blessed references) 24 25=head1 SYNOPSIS 26 27 $is_io = $fd->isa("IO::Handle"); 28 $is_io = Class->isa("IO::Handle"); 29 30 $does_log = $obj->DOES("Logger"); 31 $does_log = Class->DOES("Logger"); 32 33 $sub = $obj->can("print"); 34 $sub = Class->can("print"); 35 36 $sub = eval { $ref->can("fandango") }; 37 $ver = $obj->VERSION; 38 39 # but never do this! 40 $is_io = UNIVERSAL::isa($fd, "IO::Handle"); 41 $sub = UNIVERSAL::can($obj, "print"); 42 43=head1 DESCRIPTION 44 45C<UNIVERSAL> is the base class from which all blessed references inherit. 46See L<perlobj>. 47 48C<UNIVERSAL> provides the following methods: 49 50=over 4 51 52=item C<< $obj->isa( TYPE ) >> 53 54=item C<< CLASS->isa( TYPE ) >> 55 56=item C<< eval { VAL->isa( TYPE ) } >> 57 58Where 59 60=over 4 61 62=item C<TYPE> 63 64is a package name 65 66=item C<$obj> 67 68is a blessed reference or a package name 69 70=item C<CLASS> 71 72is a package name 73 74=item C<VAL> 75 76is any of the above or an unblessed reference 77 78=back 79 80When used as an instance or class method (C<< $obj->isa( TYPE ) >>), 81C<isa> returns I<true> if $obj is blessed into package C<TYPE> or 82inherits from package C<TYPE>. 83 84When used as a class method (C<< CLASS->isa( TYPE ) >>, sometimes 85referred to as a static method), C<isa> returns I<true> if C<CLASS> 86inherits from (or is itself) the name of the package C<TYPE> or 87inherits from package C<TYPE>. 88 89If you're not sure what you have (the C<VAL> case), wrap the method call in an 90C<eval> block to catch the exception if C<VAL> is undefined or an unblessed 91reference. The L<C<isa> operator|perlop/"Class Instance Operator"> is an 92alternative that simply returns false in this case, so the C<eval> is not 93needed. 94 95If you want to be sure that you're calling C<isa> as a method, not a class, 96check the invocand with C<blessed> from L<Scalar::Util> first: 97 98 use Scalar::Util 'blessed'; 99 100 if ( blessed( $obj ) && $obj->isa("Some::Class") ) { 101 ... 102 } 103 104=item C<< $obj->DOES( ROLE ) >> 105 106=item C<< CLASS->DOES( ROLE ) >> 107 108C<DOES> checks if the object or class performs the role C<ROLE>. A role is a 109named group of specific behavior (often methods of particular names and 110signatures), similar to a class, but not necessarily a complete class by 111itself. For example, logging or serialization may be roles. 112 113C<DOES> and C<isa> are similar, in that if either is true, you know that the 114object or class on which you call the method can perform specific behavior. 115However, C<DOES> is different from C<isa> in that it does not care I<how> the 116invocand performs the operations, merely that it does. (C<isa> of course 117mandates an inheritance relationship. Other relationships include aggregation, 118delegation, and mocking.) 119 120By default, classes in Perl only perform the C<UNIVERSAL> role, as well as the 121role of all classes in their inheritance. In other words, by default C<DOES> 122responds identically to C<isa>. 123 124There is a relationship between roles and classes, as each class implies the 125existence of a role of the same name. There is also a relationship between 126inheritance and roles, in that a subclass that inherits from an ancestor class 127implicitly performs any roles its parent performs. Thus you can use C<DOES> in 128place of C<isa> safely, as it will return true in all places where C<isa> will 129return true (provided that any overridden C<DOES> I<and> C<isa> methods behave 130appropriately). 131 132=item C<< $obj->can( METHOD ) >> 133 134=item C<< CLASS->can( METHOD ) >> 135 136=item C<< eval { VAL->can( METHOD ) } >> 137 138C<can> checks if the object or class has a method called C<METHOD>. If it does, 139then it returns a reference to the sub. If it does not, then it returns 140I<undef>. This includes methods inherited or imported by C<$obj>, C<CLASS>, or 141C<VAL>. 142 143C<can> cannot know whether an object will be able to provide a method through 144AUTOLOAD (unless the object's class has overridden C<can> appropriately), so a 145return value of I<undef> does not necessarily mean the object will not be able 146to handle the method call. To get around this some module authors use a forward 147declaration (see L<perlsub>) for methods they will handle via AUTOLOAD. For 148such 'dummy' subs, C<can> will still return a code reference, which, when 149called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided, 150calling the coderef will cause an error. 151 152You may call C<can> as a class (static) method or an object method. 153 154Again, the same rule about having a valid invocand applies -- use an C<eval> 155block or C<blessed> if you need to be extra paranoid. 156 157=item C<VERSION ( [ REQUIRE ] )> 158 159C<VERSION> will return the value of the variable C<$VERSION> in the 160package the object is blessed into. If C<REQUIRE> is given then 161it will do a comparison and die if the package version is not 162greater than or equal to C<REQUIRE>, or if either C<$VERSION> or C<REQUIRE> 163is not a "lax" version number (as defined by the L<version> module). 164 165The return from C<VERSION> will actually be the stringified version object 166using the package C<$VERSION> scalar, which is guaranteed to be equivalent 167but may not be precisely the contents of the C<$VERSION> scalar. If you want 168the actual contents of C<$VERSION>, use C<$CLASS::VERSION> instead. 169 170C<VERSION> can be called as either a class (static) method or an object 171method. 172 173=back 174 175=head1 WARNINGS 176 177B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and 178C<isa> uses a very similar method and cache-ing strategy. This may cause 179strange effects if the Perl code dynamically changes @ISA in any package. 180 181You may add other methods to the UNIVERSAL class via Perl or XS code. 182You do not need to C<use UNIVERSAL> to make these methods 183available to your program (and you should not do so). 184 185=head1 EXPORTS 186 187None. 188 189Previous versions of this documentation suggested using C<isa> as 190a function to determine the type of a reference: 191 192 $yes = UNIVERSAL::isa($h, "HASH"); 193 $yes = UNIVERSAL::isa("Foo", "Bar"); 194 195The problem is that this code would I<never> call an overridden C<isa> method in 196any class. Instead, use C<reftype> from L<Scalar::Util> for the first case: 197 198 use Scalar::Util 'reftype'; 199 200 $yes = reftype( $h ) eq "HASH"; 201 202and the method form of C<isa> for the second: 203 204 $yes = Foo->isa("Bar"); 205 206=cut 207