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<h1>UNIVERSAL</h1>
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Perl 5 version 22.0 documentation
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<h1>UNIVERSAL</h1>
<!-- -->
<ul><li><a href="#NAME">NAME</a><li><a href="#SYNOPSIS">SYNOPSIS</a><li><a href="#DESCRIPTION">DESCRIPTION</a><li><a href="#WARNINGS">WARNINGS</a><li><a href="#EXPORTS">EXPORTS</a></ul><a name="NAME"></a><h1>NAME</h1>
<p>UNIVERSAL - base class for ALL classes (blessed references)</p>
<a name="SYNOPSIS"></a><h1>SYNOPSIS</h1>
<pre class="verbatim"><ol><li> <span class="i">$is_io</span> = <span class="i">$fd</span><span class="i">->isa</span><span class="s">(</span><span class="q">"IO::Handle"</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">$is_io</span> = <span class="w">Class</span><span class="w">->isa</span><span class="s">(</span><span class="q">"IO::Handle"</span><span class="s">)</span><span class="sc">;</span></li><li></li><li> <span class="i">$does_log</span> = <span class="i">$obj</span><span class="i">->DOES</span><span class="s">(</span><span class="q">"Logger"</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">$does_log</span> = <span class="w">Class</span><span class="w">->DOES</span><span class="s">(</span><span class="q">"Logger"</span><span class="s">)</span><span class="sc">;</span></li><li></li><li> <span class="i">$sub</span> = <span class="i">$obj</span><span class="i">->can</span><span class="s">(</span><span class="q">"print"</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">$sub</span> = <span class="w">Class</span><span class="w">->can</span><span class="s">(</span><span class="q">"print"</span><span class="s">)</span><span class="sc">;</span></li><li></li><li> <span class="i">$sub</span> = <a class="l_k" href="functions/eval.html">eval</a> <span class="s">{</span> <span class="i">$ref</span><span class="i">->can</span><span class="s">(</span><span class="q">"fandango"</span><span class="s">)</span> <span class="s">}</span><span class="sc">;</span></li><li> <span class="i">$ver</span> = <span class="i">$obj</span><span class="i">->VERSION</span><span class="sc">;</span></li><li></li><li> <span class="c"># but never do this!</span></li><li> <span class="i">$is_io</span> = <span class="i">UNIVERSAL::isa</span><span class="s">(</span><span class="i">$fd</span><span class="cm">,</span> <span class="q">"IO::Handle"</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">$sub</span> = <span class="i">UNIVERSAL::can</span><span class="s">(</span><span class="i">$obj</span><span class="cm">,</span> <span class="q">"print"</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p><code class="inline"><span class="w">UNIVERSAL</span></code>
is the base class from which all blessed references inherit.
See <a href="perlobj.html">perlobj</a>.</p>
<p><code class="inline"><span class="w">UNIVERSAL</span></code>
provides the following methods:</p>
<ul>
<li><a name="%24obj-%3eisa(-TYPE-)"></a><b><code class="inline"><span class="i">$obj</span><span class="i">->isa</span><span class="s">(</span> <span class="w">TYPE</span> <span class="s">)</span></code>
</b>
</li>
<li><a name="CLASS-%3eisa(-TYPE-)"></a><b><code class="inline"><span class="w">CLASS</span><span class="w">->isa</span><span class="s">(</span> <span class="w">TYPE</span> <span class="s">)</span></code>
</b>
</li>
<li><a name="eval-%7b-VAL-%3eisa(-TYPE-)-%7d"></a><b><code class="inline"><a class="l_k" href="functions/eval.html">eval</a> <span class="s">{</span> <span class="w">VAL</span><span class="w">->isa</span><span class="s">(</span> <span class="w">TYPE</span> <span class="s">)</span> <span class="s">}</span></code>
</b>
<p>Where</p>
<ul>
<li><a name="TYPE"></a><b><code class="inline"><span class="w">TYPE</span></code>
</b>
<p>is a package name</p>
</li>
<li><a name="%24obj"></a><b><code class="inline"><span class="i">$obj</span></code>
</b>
<p>is a blessed reference or a package name</p>
</li>
<li><a name="CLASS"></a><b><code class="inline"><span class="w">CLASS</span></code>
</b>
<p>is a package name</p>
</li>
<li><a name="VAL"></a><b><code class="inline"><span class="w">VAL</span></code>
</b>
<p>is any of the above or an unblessed reference</p>
</li>
</ul>
<p>When used as an instance or class method (<code class="inline"><span class="i">$obj</span><span class="i">->isa</span><span class="s">(</span> <span class="w">TYPE</span> <span class="s">)</span></code>
),
<code class="inline"><span class="w">isa</span></code>
returns <i>true</i> if $obj is blessed into package <code class="inline"><span class="w">TYPE</span></code>
or
inherits from package <code class="inline"><span class="w">TYPE</span></code>
.</p>
<p>When used as a class method (<code class="inline"><span class="w">CLASS</span><span class="w">->isa</span><span class="s">(</span> <span class="w">TYPE</span> <span class="s">)</span></code>
, sometimes
referred to as a static method), <code class="inline"><span class="w">isa</span></code>
returns <i>true</i> if <code class="inline"><span class="w">CLASS</span></code>
inherits from (or is itself) the name of the package <code class="inline"><span class="w">TYPE</span></code>
or
inherits from package <code class="inline"><span class="w">TYPE</span></code>
.</p>
<p>If you're not sure what you have (the <code class="inline"><span class="w">VAL</span></code>
case), wrap the method call in an
<code class="inline"><a class="l_k" href="functions/eval.html">eval</a></code> block to catch the exception if <code class="inline"><span class="w">VAL</span></code>
is undefined.</p>
<p>If you want to be sure that you're calling <code class="inline"><span class="w">isa</span></code>
as a method, not a class,
check the invocand with <code class="inline"><span class="w">blessed</span></code>
from <a href="Scalar/Util.html">Scalar::Util</a> first:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">Scalar::Util</span> <span class="q">'blessed'</span><span class="sc">;</span></li><li></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span> <span class="i">blessed</span><span class="s">(</span> <span class="i">$obj</span> <span class="s">)</span> && <span class="i">$obj</span><span class="i">->isa</span><span class="s">(</span><span class="q">"Some::Class"</span><span class="s">)</span> <span class="s">)</span> <span class="s">{</span></li><li> ...</li><li> <span class="s">}</span></li></ol></pre></li>
<li><a name="%24obj-%3eDOES(-ROLE-)"></a><b><code class="inline"><span class="i">$obj</span><span class="i">->DOES</span><span class="s">(</span> <span class="w">ROLE</span> <span class="s">)</span></code>
</b>
</li>
<li><a name="CLASS-%3eDOES(-ROLE-)"></a><b><code class="inline"><span class="w">CLASS</span><span class="w">->DOES</span><span class="s">(</span> <span class="w">ROLE</span> <span class="s">)</span></code>
</b>
<p><code class="inline"><span class="w">DOES</span></code>
checks if the object or class performs the role <code class="inline"><span class="w">ROLE</span></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 class="inline"><span class="w">DOES</span></code>
and <code class="inline"><span class="w">isa</span></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 class="inline"><span class="w">DOES</span></code>
is different from <code class="inline"><span class="w">isa</span></code>
in that it does not care <i>how</i> the
invocand performs the operations, merely that it does. (<code class="inline"><span class="w">isa</span></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 class="inline"><span class="w">UNIVERSAL</span></code>
role, as well as the
role of all classes in their inheritance. In other words, by default <code class="inline"><span class="w">DOES</span></code>
responds identically to <code class="inline"><span class="w">isa</span></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 class="inline"><span class="w">DOES</span></code>
in
place of <code class="inline"><span class="w">isa</span></code>
safely, as it will return true in all places where <code class="inline"><span class="w">isa</span></code>
will
return true (provided that any overridden <code class="inline"><span class="w">DOES</span></code>
<i>and</i> <code class="inline"><span class="w">isa</span></code>
methods behave
appropriately).</p>
</li>
<li><a name="%24obj-%3ecan(-METHOD-)"></a><b><code class="inline"><span class="i">$obj</span><span class="i">->can</span><span class="s">(</span> <span class="w">METHOD</span> <span class="s">)</span></code>
</b>
</li>
<li><a name="CLASS-%3ecan(-METHOD-)"></a><b><code class="inline"><span class="w">CLASS</span><span class="w">->can</span><span class="s">(</span> <span class="w">METHOD</span> <span class="s">)</span></code>
</b>
</li>
<li><a name="eval-%7b-VAL-%3ecan(-METHOD-)-%7d"></a><b><code class="inline"><a class="l_k" href="functions/eval.html">eval</a> <span class="s">{</span> <span class="w">VAL</span><span class="w">->can</span><span class="s">(</span> <span class="w">METHOD</span> <span class="s">)</span> <span class="s">}</span></code>
</b>
<p><code class="inline"><span class="w">can</span></code>
checks if the object or class has a method called <code class="inline"><span class="w">METHOD</span></code>
. If it does,
then it returns a reference to the sub. If it does not, then it returns
<i>undef</i>. This includes methods inherited or imported by <code class="inline"><span class="i">$obj</span></code>
, <code class="inline"><span class="w">CLASS</span></code>
, or
<code class="inline"><span class="w">VAL</span></code>
.</p>
<p><code class="inline"><span class="w">can</span></code>
cannot know whether an object will be able to provide a method through
AUTOLOAD (unless the object's class has overridden <code class="inline"><span class="w">can</span></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="perlsub.html">perlsub</a>) for methods they will handle via AUTOLOAD. For
such 'dummy' subs, <code class="inline"><span class="w">can</span></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 class="inline"><span class="w">can</span></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 class="inline"><a class="l_k" href="functions/eval.html">eval</a></code>
block or <code class="inline"><span class="w">blessed</span></code>
if you need to be extra paranoid.</p>
</li>
<li><a name="VERSION-(-%5b-REQUIRE-%5d-)"></a><b><code class="inline"><span class="w">VERSION</span> <span class="s">(</span> <span class="s">[</span> <span class="w">REQUIRE</span> <span class="s">]</span> <span class="s">)</span></code>
</b>
<p><code class="inline"><span class="w">VERSION</span></code>
will return the value of the variable <code class="inline"><span class="i">$VERSION</span></code>
in the
package the object is blessed into. If <code class="inline"><span class="w">REQUIRE</span></code>
is given then
it will do a comparison and die if the package version is not
greater than or equal to <code class="inline"><span class="w">REQUIRE</span></code>
, or if either <code class="inline"><span class="i">$VERSION</span></code>
or <code class="inline"><span class="w">REQUIRE</span></code>
is not a "lax" version number (as defined by the <a href="version.html">version</a> module).</p>
<p>The return from <code class="inline"><span class="w">VERSION</span></code>
will actually be the stringified version object
using the package <code class="inline"><span class="i">$VERSION</span></code>
scalar, which is guaranteed to be equivalent
but may not be precisely the contents of the <code class="inline"><span class="i">$VERSION</span></code>
scalar. If you want
the actual contents of <code class="inline"><span class="i">$VERSION</span></code>
, use <code class="inline"><span class="i">$CLASS::VERSION</span></code>
instead.</p>
<p><code class="inline"><span class="w">VERSION</span></code>
can be called as either a class (static) method or an object
method.</p>
</li>
</ul>
<a name="WARNINGS"></a><h1>WARNINGS</h1>
<p><b>NOTE:</b> <code class="inline"><span class="w">can</span></code>
directly uses Perl's internal code for method lookup, and
<code class="inline"><span class="w">isa</span></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 class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">UNIVERSAL</span></code>
to make these methods
available to your program (and you should not do so).</p>
<a name="EXPORTS"></a><h1>EXPORTS</h1>
<p>None.</p>
<p>Previous versions of this documentation suggested using <code class="inline"><span class="w">isa</span></code>
as
a function to determine the type of a reference:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">UNIVERSAL</span> <span class="q">'isa'</span><span class="sc">;</span></li><li></li><li> <span class="i">$yes</span> = <span class="w">isa</span> <span class="i">$h</span><span class="cm">,</span> <span class="q">"HASH"</span><span class="sc">;</span></li><li> <span class="i">$yes</span> = <span class="w">isa</span> <span class="q">"Foo"</span><span class="cm">,</span> <span class="q">"Bar"</span><span class="sc">;</span></li></ol></pre><p>The problem is that this code would <i>never</i> call an overridden <code class="inline"><span class="w">isa</span></code>
method in
any class. Instead, use <code class="inline"><span class="w">reftype</span></code>
from <a href="Scalar/Util.html">Scalar::Util</a> for the first case:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">Scalar::Util</span> <span class="q">'reftype'</span><span class="sc">;</span></li><li></li><li> <span class="i">$yes</span> = <span class="i">reftype</span><span class="s">(</span> <span class="i">$h</span> <span class="s">)</span> eq <span class="q">"HASH"</span><span class="sc">;</span></li></ol></pre><p>and the method form of <code class="inline"><span class="w">isa</span></code>
for the second:</p>
<pre class="verbatim"><ol><li> <span class="i">$yes</span> = <span class="w">Foo</span><span class="w">->isa</span><span class="s">(</span><span class="q">"Bar"</span><span class="s">)</span><span class="sc">;</span></li></ol></pre>
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