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<html><head><title>use strict (Programming Perl)</title><!-- STYLESHEET --><link rel="stylesheet" type="text/css" href="../style/style1.css"><!-- METADATA --><!--Dublin Core Metadata--><meta name="DC.Creator" content=""><meta name="DC.Date" content=""><meta name="DC.Format" content="text/xml" scheme="MIME"><meta name="DC.Generator" content="XSLT stylesheet, xt by James Clark"><meta name="DC.Identifier" content=""><meta name="DC.Language" content="en-US"><meta name="DC.Publisher" content="O'Reilly & Associates, Inc."><meta name="DC.Source" content="" scheme="ISBN"><meta name="DC.Subject.Keyword" content=""><meta name="DC.Title" content="use strict"><meta name="DC.Type" content="Text.Monograph"></head><body><!-- START OF BODY --><!-- TOP BANNER --><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home"><map name="banner-map"><AREA SHAPE="RECT" COORDS="0,0,466,71" HREF="index.htm" ALT="Programming Perl"><AREA SHAPE="RECT" COORDS="467,0,514,18" HREF="jobjects/fsearch.htm" ALT="Search this book"></map><!-- TOP NAV BAR --><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch31_18.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="ch31_01.htm">Chapter 31: Pragmatic Modules</a></td><td align="right" valign="top" width="172"><a href="ch31_20.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr></table></div><hr width="515" align="left"><!-- SECTION BODY --><h2 class="sect1">31.19. use strict</h2><blockquote><pre class="programlisting">use strict; # Install all three strictures.use strict "vars"; # Variables must be predeclared.use strict "refs"; # Can't use symbolic references.use strict "subs"; # Bareword strings must be quoted.use strict; # Install all...no strict "vars"; # ...then renege on one.</pre></blockquote><p>This lexically scoped pragma changes some basic rules about what Perlconsiders to be legal code. Sometimes these restrictions seem toostrict for casual programming, such as when you're just trying to whipup a five-line filter program. The larger your program, the more youneed to be strict about it.</p><p>Currently, there are three possible things to be strict about:<tt class="literal">subs</tt>, <tt class="literal">vars</tt>, and <tt class="literal">refs</tt>. If no import list is supplied,all three restrictions are assumed.</p><h3 class="sect2">31.19.1. strict 'refs'</h3><p>This generates a run-time error if you use symbolic references,intentionally or otherwise. See <a href="ch08_01.htm">Chapter 8, "References"</a>for more about these.<blockquote><pre class="programlisting">use strict 'refs';$ref = \$foo; # Store "real" (hard) reference.print $$ref; # Dereferencing is ok.$ref = "foo"; # Store name of global (package) variable.print $$ref; # WRONG, run-time error under strict refs.</pre></blockquote>Symbolic references are suspect for various reasons. It'ssurprisingly easy for even well-meaning programmers to invoke themaccidentally; <tt class="literal">strict 'refs'</tt> guards against that.Unlike real references, symbolic references can only refer to globalvariables. They aren't reference-counted. And there's often a better wayto do what you're doing: instead of referencing a symbol in a globalsymbol table, use a hash as its own little mini-symbol table. It'smore efficient, more readable, and less error prone.</p><p>Nevertheless, some sorts of valid manipulation really do requiredirect access to the package's global symbol table of variables andfunction names. For example, you might want to examine the <tt class="literal">@EXPORT</tt>list or the <tt class="literal">@ISA</tt> superclass of a given package whose name you don't know in advance. Or you might want to install a wholeslew of function calls that are all aliases to the same closure. This is just what symbolic references are best at, but to use themwhile <tt class="literal">use strict</tt> is in effect, you must first undo the "<tt class="literal">refs</tt>"stricture:<blockquote><pre class="programlisting"># make a bunch of attribute accessorsfor my $methname (qw/name rank serno/) { no strict 'refs'; *$methname = sub { $_[0]->{ __PACKAGE__ . $methname };}</pre></blockquote></p><h3 class="sect2">31.19.2. strict 'vars'</h3><p>Under this stricture, a compile-time error is triggered if youattempt to access a variable that hasn't met at leastone of the following criteria:</p><ul><li><p>Predefined by Perl itself, such as <tt class="literal">@ARGV</tt>, <tt class="literal">%ENV</tt>, and all theglobal punctuation variables such as <tt class="literal">$.</tt> or <tt class="literal">$_</tt>.</p></li><li><p>Declared with <tt class="literal">our</tt> (for a global) or <tt class="literal">my</tt> (for a lexical).</p></li><li><p>Imported from another package. (The <tt class="literal">use vars</tt> pragma fakes up animport, but use <tt class="literal">our</tt> instead.)</p></li><li><p>Fully qualified using its package name and the double-colonpackageseparator.</p></li></ul><p>Just using a <tt class="literal">local</tt> operator isn't good enough tokeep <tt class="literal">use strict 'vars'</tt> happy because, despite itsname, that operator doesn't change whether the named variable isglobal or not. It just gives the variable a new, temporary value forthe duration of block at run time. You still need to use<tt class="literal">our</tt> to declare a global variable, or<tt class="literal">my</tt> to declare a lexical variable. You can,however, localize an <tt class="literal">our</tt>:<blockquote><pre class="programlisting">local our $law = "martial";</pre></blockquote>Globals predefined by Perl are exempt from these requirements. Thisapplies to program-wide globals (those forced into package <tt class="literal">main</tt> like<tt class="literal">@ARGV</tt> or <tt class="literal">$_</tt>) and to per-package variables like <tt class="literal">$a</tt> and <tt class="literal">$b</tt>,which are normally used by the <tt class="literal">sort</tt> function. Per-package variablesused by modules like <tt class="literal">Exporter</tt> still need to be declared using<tt class="literal">our</tt>:<blockquote><pre class="programlisting">our @EXPORT_OK = qw(name rank serno);</pre></blockquote></p><h3 class="sect2">31.19.3. strict 'subs'</h3><p>This stricture makes Perl treat all barewords as syntax errors. A <em class="emphasis">bareword</em>("bearword" in some dialects) is any bare name or identifier that hasno other interpretation forced by context. (Context is often forced bya nearby keyword or token, or by predeclaration of the word inquestion.) Historically, barewords were interpreted as unquotedstrings. This stricture outlaws that interpretation. If you mean to use it as a string, quote it. If you mean to use it as a function call, predeclare it or use parentheses.</p><p>As a particular case of forced context,remember that a word that appears by itself in curly braces or on thelefthand side of the <tt class="literal">=></tt> operator counts as being quoted, andso is not subject to this restriction.<blockquote><pre class="programlisting">use strict 'subs';$x = whatever; # WRONG: bareword error!$x = whatever(); # This always works, though.sub whatever; # Predeclare function.$x = whatever; # Now it's ok.# These uses are permitted, because the => quotes:%hash = (red => 1, blue => 2, green => 3);$rednum = $hash{red}; # Ok, braces quote here.# But not this one:@coolnums = @hash{blue, green}; # WRONG: bareword error.@coolnums = @hash{"blue", "green"}; # Ok, words now quoted.@coolnums = @hash{qw/blue green/}; # Likewise.</pre></blockquote></p><!-- BOTTOM NAV BAR --><hr width="515" align="left"><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch31_18.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0"></a></td><td align="right" valign="top" width="172"><a href="ch31_20.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr><tr><td align="left" valign="top" width="172">31.18. use sigtrap</td><td align="center" valign="top" width="171"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0"></a></td><td align="right" valign="top" width="172">31.20. use subs</td></tr></table></div><hr width="515" align="left"><!-- LIBRARY NAV BAR --><img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. 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