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<HTML><HEAD><TITLE>Recipe 11.5. Taking References to Scalars (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:40:36Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch11_01.htm"TITLE="11. References and Records"><LINKREL="prev"HREF="ch11_05.htm"TITLE="11.4. Taking References to Functions"><LINKREL="next"HREF="ch11_07.htm"TITLE="11.6. Creating Arrays of Scalar References"></HEAD><BODYBGCOLOR="#FFFFFF"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_05.htm"TITLE="11.4. Taking References to Functions"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 11.4. Taking References to Functions"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch11_01.htm"TITLE="11. References and Records"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_07.htm"TITLE="11.6. Creating Arrays of Scalar References"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 11.6. Creating Arrays of Scalar References"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch11-chap11_taking_3">11.5. Taking References to Scalars</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-624">Problem<ACLASS="indexterm"NAME="ch11-idx-1000004819-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004819-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004819-2"></A><ACLASS="indexterm"NAME="ch11-idx-1000004819-3"></A></A></H3><PCLASS="para">You want to create and manipulate a reference to a scalar value.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-630">Solution</A></H3><PCLASS="para">To create a reference to a scalar variable, use the backslash operator:</P><PRECLASS="programlisting">$scalar_ref = \$scalar; # get reference to named scalar</PRE><PCLASS="para">To create a reference to an anonymous scalar value (a value that isn't in a variable), assign through a dereference of an undefined variable:</P><PRECLASS="programlisting">undef $anon_scalar_ref;$$anon_scalar_ref = 15;</PRE><PCLASS="para">This creates a reference to a constant scalar:</P><PRECLASS="programlisting">$anon_scalar_ref = \15;</PRE><PCLASS="para">Use <CODECLASS="literal">${...}</CODE> to dereference:</P><PRECLASS="programlisting">print ${ $scalar_ref }; # dereference it${ $scalar_ref } .= "string"; # alter referent's value</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-654">Discussion</A></H3><PCLASS="para">If you want to create many new anonymous scalars, use a subroutine that returns a reference to a lexical variable out of scope, as explained in the Introduction:</P><PRECLASS="programlisting">sub new_anon_scalar { my $temp; return \$temp;}</PRE><PCLASS="para">Perl almost never implicitly dereferences for you. Exceptions include references to filehandles, code references to <CODECLASS="literal">sort</CODE>, and the reference argument to <CODECLASS="literal">bless</CODE>. Because of this, you can only dereference a scalar reference by prefacing it with <CODECLASS="literal">$</CODE> to get at its contents:</P><PRECLASS="programlisting">$sref = new_anon_scalar();$$sref = 3;print "Three = $$sref\n";@array_of_srefs = ( new_anon_scalar(), new_anon_scalar() );${ $array[0] } = 6.02e23;${ $array[1] } = "avocado";print "\@array contains: ", join(", ", map { $$_ } @array ), "\n";</PRE><PCLASS="para">Notice we have to put braces around <CODECLASS="literal">$array[0]</CODE> and <CODECLASS="literal">$array[1]</CODE>. If we tried to say <CODECLASS="literal">$$array[0]</CODE>, the tight binding of dereferencing would turn it into <CODECLASS="literal">$array->[0]</CODE>. It would treat <CODECLASS="literal">$array</CODE> as an array reference and return the element at index zero.</P><PCLASS="para">Here are other examples where it is safe to omit the braces:</P><PRECLASS="programlisting">$var = `uptime`; # $var holds text$vref = \$var; # $vref "points to" $varif ($$vref =~ /load/) {} # look at $var, indirectlychomp $$vref; # alter $var, indirectly</PRE><PCLASS="para">As mentioned in the introduction, you may use the <CODECLASS="literal">ref</CODE> built-in to inspect a reference for its referent's type. Calling <CODECLASS="literal">ref</CODE> on a scalar reference returns the string "<CODECLASS="literal">SCALAR</CODE>":</P><PRECLASS="programlisting"># check whether $someref contains a simple scalar referenceif (ref($someref) ne 'SCALAR') { die "Expected a scalar reference, not $someref\n";<ACLASS="indexterm"NAME="ch11-idx-1000004821-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004821-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004821-2"></A><ACLASS="indexterm"NAME="ch11-idx-1000004821-3"></A>}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-706">See Also</A></H3><PCLASS="para">Chapter 4 of <ICLASS="filename">Programming Perl</I> and <ICLASS="filename">perlref </I>(1)</P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_05.htm"TITLE="11.4. 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Creating Arrays of Scalar References</TD></TR></TABLE><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><FONTSIZE="-1"></DIV<!-- LIBRARY NAV BAR --> <img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p> <a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font> </p> <map name="library-map"> <area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map> </BODY></HTML>⌨️ 快捷键说明
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