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<HTML><HEAD><TITLE>Recipe 10.3. Creating Persistent Private Variables (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="1999-07-02T01:39:37Z"><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="ch10_01.htm"TITLE="10. Subroutines"><LINKREL="prev"HREF="ch10_03.htm"TITLE="10.2. Making Variables Private to a Function"><LINKREL="next"HREF="ch10_05.htm"TITLE="10.4. Determining Current Function Name"></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="ch10_03.htm"TITLE="10.2. Making Variables Private to a Function"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 10.2. Making Variables Private to a Function"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch10_01.htm"TITLE="10. Subroutines"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch10_05.htm"TITLE="10.4. Determining Current Function Name"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 10.4. Determining Current Function Name"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch10-20398">10.3. Creating Persistent Private Variables</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-246">Problem</A></H3><PCLASS="para"><ACLASS="indexterm"NAME="ch10-idx-1000004664-0"></A>You want a variable to retain its value between calls to a subroutine but not be visible outside that routine. For instance, you'd like your function to keep track of how many times it was called.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-252">Solution</A></H3><PCLASS="para">Wrap the function in another block, and declare <CODECLASS="literal">my</CODE> variables in that block's scope rather than the function's:</P><PRECLASS="programlisting">{ my $variable; sub mysub { # ... accessing $variable }}</PRE><PCLASS="para">If the variables require initialization, make that block a BEGIN so the variable is guaranteed to be set before the main program starts running:</P><PRECLASS="programlisting">BEGIN { my $variable = 1; # initial value sub othersub { # ... accessing $variable }}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-284">Discussion</A></H3><PCLASS="para">Unlike local variables in C or C++, Perl's lexical variables don't necessarily get recycled just because their scope has exited. If something more permanent is still aware of the lexical, it will stick around. In this case, <CODECLASS="literal">mysub</CODE> uses $<CODECLASS="literal">variable</CODE>, so Perl doesn't reclaim the variable when the block around the definition of <CODECLASS="literal">mysub</CODE> ends.</P><PCLASS="para">Here's how to write a counter:</P><PRECLASS="programlisting">{ my $counter; sub next_counter { return ++$counter }}</PRE><PCLASS="para">Each time <CODECLASS="literal">next_counter</CODE> is called, it increments and returns the <CODECLASS="literal">$counter</CODE> variable. The first time <CODECLASS="literal">next_counter</CODE> is called, <CODECLASS="literal">$counter</CODE> is undefined, so it behaves as though it were 0 for the <CODECLASS="literal">++</CODE>. The variable is not part of <CODECLASS="literal">next_counter</CODE>'s scope, but rather part of the block surrounding it. No code from outside can change <CODECLASS="literal">$counter</CODE> except by calling <CODECLASS="literal">next_counter</CODE>.</P><PCLASS="para">Generally, you should use a BEGIN for the extra scope. Otherwise, you could call the function before its variable were initialized.</P><PRECLASS="programlisting">BEGIN { my $counter = 42; sub next_counter { return ++$counter } sub prev_counter { return --$counter }}</PRE><PCLASS="para">This technique creates the Perl equivalent of C's static variables. Actually, it's a little better. Rather than being limited to just one function, both functions share their private variable.<ACLASS="indexterm"NAME="ch10-idx-1000004660-0"></A><ACLASS="indexterm"NAME="ch10-idx-1000004660-1"></A><ACLASS="indexterm"NAME="ch10-idx-1000004660-2"></A><ACLASS="indexterm"NAME="ch10-idx-1000004660-3"></A><ACLASS="indexterm"NAME="ch10-idx-1000004660-4"></A><ACLASS="indexterm"NAME="ch10-idx-1000004660-5"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-316">See Also</A></H3><PCLASS="para">The section on <ACLASS="olink"HREF="../prog/ch04_03.htm#PERL2-CH-4-SECT-3.6">"Closures"</A> in <ACLASS="olink"HREF="../prog/ch04_01.htm">Chapter 4</A>and on <ACLASS="olink"HREF="../prog/ch05_01.htm#PERL2-CH-5-SECT-1.2">"Package Constructors and Destructors: BEGIN and END"</A> in <ACLASS="olink"HREF="../prog/ch05_01.htm">Chapter 5</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the section on "Private Variables via my( )" in <ICLASS="filename">perlsub </I>(1); the section on "Package Constructors and Destructors" in <ICLASS="filename">perlmod </I>(1); <ACLASS="xref"HREF="ch11_05.htm"TITLE="Taking References to Functions">Recipe 11.4</A></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="ch10_03.htm"TITLE="10.2. 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