UNIVERSAL - base class for ALL classes (blessed references)

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See <a href="/5.14.4/perlobj">perlobj</a>.</p> <p><code>UNIVERSAL</code> provides the following methods:</p> <dl> <dt id="$obj->isa(-TYPE-)"><a class="permalink" href="#$obj-%3Eisa(-TYPE-)">#</a><a id="isa"></a><a id="obj-isa-TYPE"></a><code>$obj->isa( TYPE )</code></dt> <dd> </dd> <dt id="CLASS->isa(-TYPE-)"><a class="permalink" href="#CLASS-%3Eisa(-TYPE-)">#</a><a id="CLASS1"></a><a id="CLASS-isa-TYPE"></a><code>CLASS->isa( TYPE )</code></dt> <dd> </dd> <dt id="eval-{-VAL->isa(-TYPE-)-}"><a class="permalink" href="#eval-%7B-VAL-%3Eisa(-TYPE-)-%7D">#</a><a id="eval"></a><a id="eval-VAL-isa-TYPE"></a><code>eval { VAL->isa( TYPE ) }</code></dt> <dd> <p>Where</p> <dl> <dt id="TYPE"><a class="permalink" href="#TYPE">#</a><code>TYPE</code></dt> <dd> <p>is a package name</p> </dd> <dt id="$obj"><a class="permalink" href="#$obj">#</a><a id="obj"></a><code>$obj</code></dt> <dd> <p>is a blessed reference or a package name</p> </dd> <dt id="CLASS"><a class="permalink" href="#CLASS">#</a><code>CLASS</code></dt> <dd> <p>is a package name</p> </dd> <dt id="VAL"><a class="permalink" href="#VAL">#</a><code>VAL</code></dt> <dd> <p>is any of the above or an unblessed reference</p> </dd> </dl> <p>When used as an instance or class method (<code>$obj->isa( TYPE )</code>), <code>isa</code> returns <i>true</i> if $obj is blessed into package <code>TYPE</code> or inherits from package <code>TYPE</code>.</p> <p>When used as a class method (<code>CLASS->isa( TYPE )</code>, sometimes referred to as a static method), <code>isa</code> returns <i>true</i> if <code>CLASS</code> inherits from (or is itself) the name of the package <code>TYPE</code> or inherits from package <code>TYPE</code>.</p> <p>If you're not sure what you have (the <code>VAL</code> case), wrap the method call in an <code>eval</code> block to catch the exception if <code>VAL</code> is undefined.</p> <p>If you want to be sure that you're calling <code>isa</code> as a method, not a class, check the invocand with <code>blessed</code> from <a href="/5.14.4/Scalar::Util">Scalar::Util</a> first:</p> <pre><code>use Scalar::Util 'blessed'; if ( blessed( $obj ) && $obj->isa("Some::Class") { ... }</code></pre> </dd> <dt id="$obj->DOES(-ROLE-)"><a class="permalink" href="#$obj-%3EDOES(-ROLE-)">#</a><a id="DOES"></a><a id="obj-DOES-ROLE"></a><code>$obj->DOES( ROLE )</code></dt> <dd> </dd> <dt id="CLASS->DOES(-ROLE-)"><a class="permalink" href="#CLASS-%3EDOES(-ROLE-)">#</a><a id="CLASS2"></a><a id="CLASS-DOES-ROLE"></a><code>CLASS->DOES( ROLE )</code></dt> <dd> <p><code>DOES</code> checks if the object or class performs the role <code>ROLE</code>. 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.</p> <p><code>DOES</code> and <code>isa</code> 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, <code>DOES</code> is different from <code>isa</code> in that it does not care <i>how</i> the invocand performs the operations, merely that it does. (<code>isa</code> of course mandates an inheritance relationship. Other relationships include aggregation, delegation, and mocking.)</p> <p>By default, classes in Perl only perform the <code>UNIVERSAL</code> role, as well as the role of all classes in their inheritance. In other words, by default <code>DOES</code> responds identically to <code>isa</code>.</p> <p>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 <code>DOES</code> in place of <code>isa</code> safely, as it will return true in all places where <code>isa</code> will return true (provided that any overridden <code>DOES</code> <i>and</i> <code>isa</code> methods behave appropriately).</p> </dd> <dt id="$obj->can(-METHOD-)"><a class="permalink" href="#$obj-%3Ecan(-METHOD-)">#</a><a id="can"></a><a id="obj-can-METHOD"></a><code>$obj->can( METHOD )</code></dt> <dd> </dd> <dt id="CLASS->can(-METHOD-)"><a class="permalink" href="#CLASS-%3Ecan(-METHOD-)">#</a><a id="CLASS3"></a><a id="CLASS-can-METHOD"></a><code>CLASS->can( METHOD )</code></dt> <dd> </dd> <dt id="eval-{-VAL->can(-METHOD-)-}"><a class="permalink" href="#eval-%7B-VAL-%3Ecan(-METHOD-)-%7D">#</a><a id="eval1"></a><a id="eval-VAL-can-METHOD"></a><code>eval { VAL->can( METHOD ) }</code></dt> <dd> <p><code>can</code> checks if the object or class has a method called <code>METHOD</code>. If it does, then it returns a reference to the sub. If it does not, then it returns <i>undef</i>. This includes methods inherited or imported by <code>$obj</code>, <code>CLASS</code>, or <code>VAL</code>.</p> <p><code>can</code> cannot know whether an object will be able to provide a method through AUTOLOAD (unless the object's class has overridden <code>can</code> appropriately), so a return value of <i>undef</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 <a href="/5.14.4/perlsub">perlsub</a>) for methods they will handle via AUTOLOAD. For such 'dummy' subs, <code>can</code> 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.</p> <p>You may call <code>can</code> as a class (static) method or an object method.</p> <p>Again, the same rule about having a valid invocand applies -- use an <code>eval</code> block or <code>blessed</code> if you need to be extra paranoid.</p> </dd> <dt id="VERSION-(-[-REQUIRE-]-)"><a class="permalink" href="#VERSION-(-%5B-REQUIRE-%5D-)">#</a><a id="VERSION"></a><a id="VERSION-REQUIRE"></a><code>VERSION ( [ REQUIRE ] )</code></dt> <dd> <p><code>VERSION</code> will return the value of the variable <code>$VERSION</code> in the package the object is blessed into. If <code>REQUIRE</code> is given then it will do a comparison and die if the package version is not greater than or equal to <code>REQUIRE</code>. Both <code>$VERSION</code> or <code>REQUIRE</code> must be "lax" version numbers (as defined by the <a href="/5.14.4/version">version</a> module) or <code>VERSION</code> will die with an error.</p> <p><code>VERSION</code> can be called as either a class (static) method or an object method.</p> </dd> </dl> <h1 id="WARNINGS"><a class="permalink" href="#WARNINGS">#</a>WARNINGS</h1> <p><b>NOTE:</b> <code>can</code> directly uses Perl's internal code for method lookup, and <code>isa</code> uses a very similar method and cache-ing strategy. This may cause strange effects if the Perl code dynamically changes @ISA in any package.</p> <p>You may add other methods to the UNIVERSAL class via Perl or XS code. You do not need to <code>use UNIVERSAL</code> to make these methods available to your program (and you should not do so).</p> <h1 id="EXPORTS"><a class="permalink" href="#EXPORTS">#</a>EXPORTS</h1> <p>None by default.</p> <p>You may request the import of three functions (<code>isa</code>, <code>can</code>, and <code>VERSION</code>), <b>but this feature is deprecated and will be removed</b>. Please don't do this in new code.</p> <p>For example, previous versions of this documentation suggested using <code>isa</code> as a function to determine the type of a reference:</p> <pre><code>use UNIVERSAL 'isa'; $yes = isa $h, "HASH"; $yes = isa "Foo", "Bar";</code></pre> <p>The problem is that this code will <i>never</i> call an overridden <code>isa</code> method in any class. Instead, use <code>reftype</code> from <a href="/5.14.4/Scalar::Util">Scalar::Util</a> for the first case:</p> <pre><code>use Scalar::Util 'reftype'; $yes = reftype( $h ) eq "HASH";</code></pre> <p>and the method form of <code>isa</code> for the second:</p> <pre><code>$yes = Foo->isa("Bar");</code></pre> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p> <p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the <a href="https://github.com/Perl/perl5/issues">Perl issue tracker</a>, the <a href="https://lists.perl.org/list/perl5-porters.html">mailing list</a>, or <a href="https://kiwiirc.com/client/irc.perl.org/p5p">IRC</a> to report any issues with the contents or format of the documentation.</p> </div> </div> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.slim.min.js" integrity="sha512-/DXTXr6nQodMUiq+IUJYCt2PPOUjrHJ9wFrqpJ3XkgPNOZVfMok7cRw6CSxyCQxXn6ozlESsSh1/sMCTF1rL/g==" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script> <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js" integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV" crossorigin="anonymous"></script> <script src="/js/highlight.pack.js"></script> <script>hljs.highlightAll();</script> </body> </html>

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UNIVERSAL - base class for ALL classes (blessed references) - Perldoc Browser