CINXE.COM
defined - Perldoc Browser
<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <meta name="viewport" content="width=device-width, initial-scale=1"> <title>defined - Perldoc Browser</title> <link rel="search" href="/opensearch.xml" type="application/opensearchdescription+xml" title="Perldoc Browser"> <link rel="canonical" href="https://perldoc.perl.org/functions/defined"> <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" integrity="sha384-JcKb8q3iqJ61gNV9KGb8thSsNjpSL0n8PARn9HuZOnIxN0hoP+VmmDGMN5t9UJ0Z" crossorigin="anonymous"> <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/10.5.0/styles/stackoverflow-light.min.css" integrity="sha512-cG1IdFxqipi3gqLmksLtuk13C+hBa57a6zpWxMeoY3Q9O6ooFxq50DayCdm0QrDgZjMUn23z/0PMZlgft7Yp5Q==" crossorigin="anonymous" /> <style> body { background: #f4f4f5; color: #020202; } .navbar-dark { background-image: -webkit-linear-gradient(top, #005f85 0, #002e49 100%); background-image: -o-linear-gradient(top, #005f85 0, #002e49 100%); background-image: linear-gradient(to bottom, #005f85 0, #002e49 100%); filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff005f85', endColorstr='#ff002e49', GradientType=0); background-repeat: repeat-x; } .navbar-dark .navbar-nav .nav-link, .navbar-dark .navbar-nav .nav-link:focus { color: #fff } .navbar-dark .navbar-nav .nav-link:hover { color: #ffef68 } #wrapperlicious { margin: 0 auto; font: 0.9em 'Helvetica Neue', Helvetica, sans-serif; font-weight: normal; line-height: 1.5em; margin: 0; padding: 0; } #wrapperlicious h1 { font-size: 1.5em } #wrapperlicious h2 { font-size: 1.3em } #wrapperlicious h3 { font-size: 1.1em } #wrapperlicious h4 { font-size: 0.9em } #wrapperlicious h1, #wrapperlicious h2, #wrapperlicious h3, #wrapperlicious h4, #wrapperlicious dt { color: #020202; margin-top: 1em; margin-bottom: 1em; position: relative; font-weight: bold; } #wrapperlicious a { color: inherit; text-decoration: underline } #wrapperlicious #toc { text-decoration: none } #wrapperlicious a:hover { color: #2a2a2a } #wrapperlicious a img { border: 0 } #wrapperlicious :not(pre) > code { color: inherit; background-color: rgba(0, 0, 0, 0.04); border-radius: 3px; font: 0.9em Consolas, Menlo, Monaco, monospace; padding: 0.3em; } #wrapperlicious dd { margin: 0; margin-left: 2em; } #wrapperlicious dt { color: #2a2a2a; font-weight: bold; margin-left: 0.9em; } #wrapperlicious p { margin-bottom: 1em; margin-top: 1em; } #wrapperlicious li > p { margin-bottom: 0; margin-top: 0; } #wrapperlicious pre { border: 1px solid #c1c1c1; border-radius: 3px; font: 100% Consolas, Menlo, Monaco, monospace; margin-bottom: 1em; margin-top: 1em; } #wrapperlicious pre > code { display: block; background-color: #f6f6f6; font: 0.9em Consolas, Menlo, Monaco, monospace; line-height: 1.5em; text-align: left; white-space: pre; padding: 1em; } #wrapperlicious dl, #wrapperlicious ol, #wrapperlicious ul { margin-bottom: 1em; margin-top: 1em; } #wrapperlicious ul { list-style-type: square; } #wrapperlicious ul ul { margin-bottom: 0px; margin-top: 0px; } #footer { font-size: 0.8em; padding-top: 0.5em; text-align: center; } #more { display: inline; font-size: 0.8em; } #perldocdiv { background-color: #fff; border: 1px solid #c1c1c1; border-bottom-left-radius: 5px; border-bottom-right-radius: 5px; margin-left: auto; margin-right: auto; padding: 3em; padding-top: 1em; max-width: 960px; } #moduleversion { float: right } #wrapperlicious .leading-notice { font-style: italic; padding-left: 1em; margin-top: 1em; margin-bottom: 1em; } #wrapperlicious .permalink { display: none; left: -0.75em; position: absolute; padding-right: 0.25em; text-decoration: none; } #wrapperlicious h1:hover .permalink, #wrapperlicious h2:hover .permalink, #wrapperlicious h3:hover .permalink, #wrapperlicious h4:hover .permalink, #wrapperlicious dt:hover .permalink { display: block; } </style> <!-- Global site tag (gtag.js) - Google Analytics --> <script async src="https://www.googletagmanager.com/gtag/js?id=G-KVNWBNT5FB"></script> <script> window.dataLayer = window.dataLayer || []; function gtag(){dataLayer.push(arguments);} gtag('js', new Date()); gtag('config', 'G-KVNWBNT5FB'); gtag('config', 'UA-50555-3'); </script> </head> <body> <nav class="navbar navbar-expand-md navbar-dark bg-dark justify-content-between"> <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation"> <span class="navbar-toggler-icon"></span> </button> <a class="navbar-brand" href="/"><img src="/images/perl_camel_30.png" width="30" height="30" class="d-inline-block align-top" alt="Perl Camel Logo"> Perldoc Browser</a> <div class="collapse navbar-collapse" id="navbarNav"> <ul class="navbar-nav mr-auto"> <li class="nav-item dropdown text-nowrap"> <a class="nav-link dropdown-toggle" href="#" id="dropdownlink-stable" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">5.8.8</a> <div class="dropdown-menu" aria-labelledby="dropdownlink-stable"> <a class="dropdown-item" href="/functions/defined">Latest</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.40.1/functions/defined">5.40.1</a> <a class="dropdown-item" href="/5.40.0/functions/defined">5.40.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.38.3/functions/defined">5.38.3</a> <a class="dropdown-item" href="/5.38.2/functions/defined">5.38.2</a> <a class="dropdown-item" href="/5.38.1/functions/defined">5.38.1</a> <a class="dropdown-item" href="/5.38.0/functions/defined">5.38.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.36.3/functions/defined">5.36.3</a> <a class="dropdown-item" href="/5.36.2/functions/defined">5.36.2</a> <a class="dropdown-item" href="/5.36.1/functions/defined">5.36.1</a> <a class="dropdown-item" href="/5.36.0/functions/defined">5.36.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.34.3/functions/defined">5.34.3</a> <a class="dropdown-item" href="/5.34.2/functions/defined">5.34.2</a> <a class="dropdown-item" href="/5.34.1/functions/defined">5.34.1</a> <a class="dropdown-item" href="/5.34.0/functions/defined">5.34.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.32.1/functions/defined">5.32.1</a> <a class="dropdown-item" href="/5.32.0/functions/defined">5.32.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.30.3/functions/defined">5.30.3</a> <a class="dropdown-item" href="/5.30.2/functions/defined">5.30.2</a> <a class="dropdown-item" href="/5.30.1/functions/defined">5.30.1</a> <a class="dropdown-item" href="/5.30.0/functions/defined">5.30.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.28.3/functions/defined">5.28.3</a> <a class="dropdown-item" href="/5.28.2/functions/defined">5.28.2</a> <a class="dropdown-item" href="/5.28.1/functions/defined">5.28.1</a> <a class="dropdown-item" href="/5.28.0/functions/defined">5.28.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.26.3/functions/defined">5.26.3</a> <a class="dropdown-item" href="/5.26.2/functions/defined">5.26.2</a> <a class="dropdown-item" href="/5.26.1/functions/defined">5.26.1</a> <a class="dropdown-item" href="/5.26.0/functions/defined">5.26.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.24.4/functions/defined">5.24.4</a> <a class="dropdown-item" href="/5.24.3/functions/defined">5.24.3</a> <a class="dropdown-item" href="/5.24.2/functions/defined">5.24.2</a> <a class="dropdown-item" href="/5.24.1/functions/defined">5.24.1</a> <a class="dropdown-item" href="/5.24.0/functions/defined">5.24.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.22.4/functions/defined">5.22.4</a> <a class="dropdown-item" href="/5.22.3/functions/defined">5.22.3</a> <a class="dropdown-item" href="/5.22.2/functions/defined">5.22.2</a> <a class="dropdown-item" href="/5.22.1/functions/defined">5.22.1</a> <a class="dropdown-item" href="/5.22.0/functions/defined">5.22.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.20.3/functions/defined">5.20.3</a> <a class="dropdown-item" href="/5.20.2/functions/defined">5.20.2</a> <a class="dropdown-item" href="/5.20.1/functions/defined">5.20.1</a> <a class="dropdown-item" href="/5.20.0/functions/defined">5.20.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.18.4/functions/defined">5.18.4</a> <a class="dropdown-item" href="/5.18.3/functions/defined">5.18.3</a> <a class="dropdown-item" href="/5.18.2/functions/defined">5.18.2</a> <a class="dropdown-item" href="/5.18.1/functions/defined">5.18.1</a> <a class="dropdown-item" href="/5.18.0/functions/defined">5.18.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.16.3/functions/defined">5.16.3</a> <a class="dropdown-item" href="/5.16.2/functions/defined">5.16.2</a> <a class="dropdown-item" href="/5.16.1/functions/defined">5.16.1</a> <a class="dropdown-item" href="/5.16.0/functions/defined">5.16.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.14.4/functions/defined">5.14.4</a> <a class="dropdown-item" href="/5.14.3/functions/defined">5.14.3</a> <a class="dropdown-item" href="/5.14.2/functions/defined">5.14.2</a> <a class="dropdown-item" href="/5.14.1/functions/defined">5.14.1</a> <a class="dropdown-item" href="/5.14.0/functions/defined">5.14.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.12.5/functions/defined">5.12.5</a> <a class="dropdown-item" href="/5.12.4/functions/defined">5.12.4</a> <a class="dropdown-item" href="/5.12.3/functions/defined">5.12.3</a> <a class="dropdown-item" href="/5.12.2/functions/defined">5.12.2</a> <a class="dropdown-item" href="/5.12.1/functions/defined">5.12.1</a> <a class="dropdown-item" href="/5.12.0/functions/defined">5.12.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.10.1/functions/defined">5.10.1</a> <a class="dropdown-item" href="/5.10.0/functions/defined">5.10.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.8.9/functions/defined">5.8.9</a> <a class="dropdown-item active" href="/5.8.8/functions/defined">5.8.8</a> <a class="dropdown-item" href="/5.8.7/functions/defined">5.8.7</a> <a class="dropdown-item" href="/5.8.6/functions/defined">5.8.6</a> <a class="dropdown-item" href="/5.8.5/functions/defined">5.8.5</a> <a class="dropdown-item" href="/5.8.4/functions/defined">5.8.4</a> <a class="dropdown-item" href="/5.8.3/functions/defined">5.8.3</a> <a class="dropdown-item" href="/5.8.2/functions/defined">5.8.2</a> <a class="dropdown-item" href="/5.8.1/functions/defined">5.8.1</a> <a class="dropdown-item" href="/5.8.0/functions/defined">5.8.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.6.2/functions/defined">5.6.2</a> <a class="dropdown-item" href="/5.6.1/functions/defined">5.6.1</a> <a class="dropdown-item" href="/5.6.0/functions/defined">5.6.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.005_04/functions/defined">5.005_04</a> <a class="dropdown-item" href="/5.005_03/functions/defined">5.005_03</a> <a class="dropdown-item" href="/5.005_02/functions/defined">5.005_02</a> <a class="dropdown-item" href="/5.005_01/functions/defined">5.005_01</a> <a class="dropdown-item" href="/5.005/functions/defined">5.005</a> </div> </li> <li class="nav-item dropdown text-nowrap"> <a class="nav-link dropdown-toggle" href="#" id="dropdownlink-dev" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dev</a> <div class="dropdown-menu" aria-labelledby="dropdownlink-dev"> <a class="dropdown-item" href="/blead/functions/defined">blead</a> <a class="dropdown-item" href="/5.41.9/functions/defined">5.41.9</a> <a class="dropdown-item" href="/5.41.8/functions/defined">5.41.8</a> <a class="dropdown-item" href="/5.41.7/functions/defined">5.41.7</a> <a class="dropdown-item" href="/5.41.6/functions/defined">5.41.6</a> <a class="dropdown-item" href="/5.41.5/functions/defined">5.41.5</a> <a class="dropdown-item" href="/5.41.4/functions/defined">5.41.4</a> <a class="dropdown-item" href="/5.41.3/functions/defined">5.41.3</a> <a class="dropdown-item" href="/5.41.2/functions/defined">5.41.2</a> <a class="dropdown-item" href="/5.41.1/functions/defined">5.41.1</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.40.1-RC1/functions/defined">5.40.1-RC1</a> <a class="dropdown-item" href="/5.40.0-RC2/functions/defined">5.40.0-RC2</a> <a class="dropdown-item" href="/5.40.0-RC1/functions/defined">5.40.0-RC1</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.39.10/functions/defined">5.39.10</a> <a class="dropdown-item" href="/5.39.9/functions/defined">5.39.9</a> <a class="dropdown-item" href="/5.39.8/functions/defined">5.39.8</a> <a class="dropdown-item" href="/5.39.7/functions/defined">5.39.7</a> <a class="dropdown-item" href="/5.39.6/functions/defined">5.39.6</a> <a class="dropdown-item" href="/5.39.5/functions/defined">5.39.5</a> <a class="dropdown-item" href="/5.39.4/functions/defined">5.39.4</a> <a class="dropdown-item" href="/5.39.3/functions/defined">5.39.3</a> <a class="dropdown-item" href="/5.39.2/functions/defined">5.39.2</a> <a class="dropdown-item" href="/5.39.1/functions/defined">5.39.1</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.38.3-RC1/functions/defined">5.38.3-RC1</a> </div> </li> <li class="nav-item dropdown text-nowrap"> <a class="nav-link dropdown-toggle" href="#" id="dropdownlink-nav" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Documentation</a> <div class="dropdown-menu" aria-labelledby="dropdownlink-nav"> <a class="dropdown-item" href="/5.8.8/perl">Perl</a> <a class="dropdown-item" href="/5.8.8/perlintro">Intro</a> <a class="dropdown-item" href="/5.8.8/perl#Tutorials">Tutorials</a> <a class="dropdown-item" href="/5.8.8/perlfaq">FAQs</a> <a class="dropdown-item" href="/5.8.8/perl#Reference-Manual">Reference</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.8.8/perlop">Operators</a> <a class="dropdown-item" href="/5.8.8/functions">Functions</a> <a class="dropdown-item" href="/5.8.8/variables">Variables</a> <a class="dropdown-item" href="/5.8.8/modules">Modules</a> <a class="dropdown-item" href="/5.8.8/perlutil">Utilities</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.8.8/perldelta">Release Notes</a> <a class="dropdown-item" href="/5.8.8/perlcommunity">Community</a> <a class="dropdown-item" href="/5.8.8/perlhist">History</a> </div> </li> </ul> <ul class="navbar-nav"> <script> function set_expand (expand) { var perldocdiv = document.getElementById('perldocdiv'); var width = window.getComputedStyle(perldocdiv).getPropertyValue('max-width'); var expanded = (width == '' || width == 'none') ? true : false; if (expand === null) { expand = !expanded; } if ((expand && !expanded) || (!expand && expanded)) { perldocdiv.style.setProperty('max-width', expand ? 'none' : '960px'); var button_classlist = document.getElementById('content-expand-button').classList; if (expand) { button_classlist.add('btn-light'); button_classlist.remove('btn-outline-light'); } else { button_classlist.add('btn-outline-light'); button_classlist.remove('btn-light'); } } return expand; } function toggle_expand () { var expand = set_expand(null); document.cookie = 'perldoc_expand=' + (expand ? 1 : 0) + '; path=/; expires=Tue, 19 Jan 2038 03:14:07 UTC'; } function read_expand () { return document.cookie.split(';').some(function (item) { return item.indexOf('perldoc_expand=1') >= 0 }); } if (document.readyState === 'loading') { document.addEventListener('DOMContentLoaded', function () { if (read_expand()) { set_expand(true); } }); } else if (read_expand()) { set_expand(true); } </script> <button id="content-expand-button" type="button" class="btn btn-outline-light d-none d-lg-inline-block mr-4" onclick="toggle_expand()">Expand</button> </ul> <form class="form-inline" method="get" action="/5.8.8/search"> <input class="form-control mr-3" type="search" name="q" placeholder="Search" aria-label="Search" value=""> </form> </div> </nav> <div id="wrapperlicious" class="container-fluid"> <div id="perldocdiv"> <div id="links"> <a href="/5.8.8/functions">functions</a> / <a href="/5.8.8/functions/defined">defined</a> <div id="more"> (<a href="/5.8.8/functions/defined.txt">source</a>, <a href="https://metacpan.org/pod/perlfunc#defined-EXPR">CPAN</a>) </div> </div> <div class="leading-notice"> You are viewing the version of this documentation from Perl 5.8.8. <a href="/functions/defined">View the latest version</a> </div> <dl> <dt id="defined-EXPR"><a class="permalink" href="#defined-EXPR">#</a><a id="defined1"></a>defined EXPR </dt> <dd> </dd> <dt id="defined"><a class="permalink" href="#defined">#</a>defined</dt> <dd> <p>Returns a Boolean value telling whether EXPR has a value other than the undefined value <code>undef</code>. If EXPR is not present, <code>$_</code> will be checked.</p> <p>Many operations return <code>undef</code> to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish <code>undef</code> from other values. (A simple Boolean test will not distinguish among <code>undef</code>, zero, the empty string, and <code>"0"</code>, which are all equally false.) Note that since <code>undef</code> is a valid scalar, its presence doesn't <i>necessarily</i> indicate an exceptional condition: <code>pop</code> returns <code>undef</code> when its argument is an empty array, <i>or</i> when the element to return happens to be <code>undef</code>.</p> <p>You may also use <code>defined(&func)</code> to check whether subroutine <code>&func</code> has ever been defined. The return value is unaffected by any forward declarations of <code>&func</code>. Note that a subroutine which is not defined may still be callable: its package may have an <code>AUTOLOAD</code> method that makes it spring into existence the first time that it is called -- see <a href="/5.8.8/perlsub">perlsub</a>.</p> <p>Use of <code>defined</code> on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size:</p> <pre><code>if (@an_array) { print "has array elements\n" } if (%a_hash) { print "has hash members\n" }</code></pre> <p>When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use <a href="/5.8.8/functions/exists">"exists"</a> for the latter purpose.</p> <p>Examples:</p> <pre><code> print if defined $switch{'D'}; print "$val\n" while defined($val = pop(@ary)); die "Can't readlink $sym: $!" unless defined($value = readlink $sym); sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } $debugging = 0 unless defined $debugging;</code></pre> <p>Note: Many folks tend to overuse <code>defined</code>, and then are surprised to discover that the number <code>0</code> and <code>""</code> (the zero-length string) are, in fact, defined values. For example, if you say</p> <pre><code>"ab" =~ /a(.*)b/;</code></pre> <p>The pattern match succeeds, and <code>$1</code> is defined, despite the fact that it matched "nothing". It didn't really fail to match anything. Rather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use <code>defined</code> only when you're questioning the integrity of what you're trying to do. At other times, a simple comparison to <code>0</code> or <code>""</code> is what you want.</p> <p>See also <a href="/5.8.8/functions/undef">"undef"</a>, <a href="/5.8.8/functions/exists">"exists"</a>, <a href="/5.8.8/functions/ref">"ref"</a>.</p> </dd> </dl> </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>