mingw/html/lib/strict.html

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   5 <title>strict - Perl pragma to restrict unsafe constructs</title>
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  13 <big><strong><span class="block">&nbsp;strict - Perl pragma to restrict unsafe constructs</span></strong></big>
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  20 <ul>
  21
  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#synopsis">SYNOPSIS</a></li>
  24         <li><a href="#description">DESCRIPTION</a></li>
  25         <li><a href="#history">HISTORY</a></li>
  26 </ul>
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  29 <hr />
  30 <p>
  31 </p>
  32 <h1><a name="name">NAME</a></h1>
  33 <p>strict - Perl pragma to restrict unsafe constructs</p>
  34 <p>
  35 </p>
  36 <hr />
  37 <h1><a name="synopsis">SYNOPSIS</a></h1>
  38 <pre>
  39     use strict;</pre>
  40 <pre>
  41     use strict &quot;vars&quot;;
  42     use strict &quot;refs&quot;;
  43     use strict &quot;subs&quot;;</pre>
  44 <pre>
  45     use strict;
  46     no strict &quot;vars&quot;;</pre>
  47 <p>
  48 </p>
  49 <hr />
  50 <h1><a name="description">DESCRIPTION</a></h1>
  51 <p>If no import list is supplied, all possible restrictions are assumed.
  52 (This is the safest mode to operate in, but is sometimes too strict for
  53 casual programming.)  Currently, there are three possible things to be
  54 strict about:  ``subs'', ``vars'', and ``refs''.</p>
  55 <dl>
  56 <dt><strong><a name="item_strict_refs"><code>strict refs</code></a></strong>
  57
  58 <dd>
  59 <p>This generates a runtime error if you
  60 use symbolic references (see <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>).</p>
  61 </dd>
  62 <dd>
  63 <pre>
  64     use strict 'refs';
  65     $ref = \$foo;
  66     print $$ref;        # ok
  67     $ref = &quot;foo&quot;;
  68     print $$ref;        # runtime error; normally ok
  69     $file = &quot;STDOUT&quot;;
  70     print $file &quot;Hi!&quot;;  # error; note: no comma after $file</pre>
  71 </dd>
  72 <dd>
  73 <p>There is one exception to this rule:</p>
  74 </dd>
  75 <dd>
  76 <pre>
  77     $bar = \&amp;{'foo'};
  78     &amp;$bar;</pre>
  79 </dd>
  80 <dd>
  81 <p>is allowed so that <code>goto &amp;$AUTOLOAD</code> would not break under stricture.</p>
  82 </dd>
  83 </li>
  84 <dt><strong><a name="item_strict_vars"><code>strict vars</code></a></strong>
  85
  86 <dd>
  87 <p>This generates a compile-time error if you access a variable that wasn't
  88 declared via <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_our"><code>our</code></a> or <code>use vars</code>,
  89 localized via <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my"><code>my()</code></a>, or wasn't fully qualified.  Because this is to avoid
  90 variable suicide problems and subtle dynamic scoping issues, a merely
  91 <code>local()</code> variable isn't good enough.  See <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my">my in the perlfunc manpage</a> and
  92 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#local">local in the perlfunc manpage</a>.</p>
  93 </dd>
  94 <dd>
  95 <pre>
  96     use strict 'vars';
  97     $X::foo = 1;         # ok, fully qualified
  98     my $foo = 10;        # ok, my() var
  99     local $foo = 9;      # blows up</pre>
 100 </dd>
 101 <dd>
 102 <pre>
 103     package Cinna;
 104     our $bar;                   # Declares $bar in current package
 105     $bar = 'HgS';               # ok, global declared via pragma</pre>
 106 </dd>
 107 <dd>
 108 <p>The <code>local()</code> generated a compile-time error because you just touched a global
 109 name without fully qualifying it.</p>
 110 </dd>
 111 <dd>
 112 <p>Because of their special use by sort(), the variables $a and $b are
 113 exempted from this check.</p>
 114 </dd>
 115 </li>
 116 <dt><strong><a name="item_strict_subs"><code>strict subs</code></a></strong>
 117
 118 <dd>
 119 <p>This disables the poetry optimization, generating a compile-time error if
 120 you try to use a bareword identifier that's not a subroutine, unless it
 121 is a simple identifier (no colons) and that it appears in curly braces or
 122 on the left hand side of the <code>=&gt;</code> symbol.</p>
 123 </dd>
 124 <dd>
 125 <pre>
 126     use strict 'subs';
 127     $SIG{PIPE} = Plumber;       # blows up
 128     $SIG{PIPE} = &quot;Plumber&quot;;     # just fine: quoted string is always ok
 129     $SIG{PIPE} = \&amp;Plumber;     # preferred form</pre>
 130 </dd>
 131 </li>
 132 </dl>
 133 <p>See <a href="file://C|\msysgit\mingw\html/pod/perlmodlib.html#pragmatic_modules">Pragmatic Modules in the perlmodlib manpage</a>.</p>
 134 <p>
 135 </p>
 136 <hr />
 137 <h1><a name="history">HISTORY</a></h1>
 138 <p><code>strict 'subs'</code>, with Perl 5.6.1, erroneously permitted to use an unquoted
 139 compound identifier (e.g. <code>Foo::Bar</code>) as a hash key (before <code>=&gt;</code> or
 140 inside curlies), but without forcing it always to a literal string.</p>
 141 <p>Starting with Perl 5.8.1 strict is strict about its restrictions:
 142 if unknown restrictions are used, the strict pragma will abort with</p>
 143 <pre>
 144     Unknown 'strict' tag(s) '...'</pre>
 145 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 146 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 147 <big><strong><span class="block">&nbsp;strict - Perl pragma to restrict unsafe constructs</span></strong></big>
 148 </td></tr>
 149 </table>
 150
 151 </body>
 152
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